Omc 25h.p.

[DonChichoJr.]

I JUST BOUGHT AN OMC 25 H.P., 1998, SHORT SHAFT, IN A+ CONDITION.
HAS ALREADY DONE AN EXHAUST RELIEVE HOLES IN THE L/U .

I WOULD LIKE TO IMPROVE THE TOP END OF THAT PONNY .

THE CARB # IS : 439377 . WITH #58D INTERMEDIATE JET AND #69D H.S. JET.

THE PACK IS A RAPAIR NON-LIMITER . AND USING A SST 10 1/2x11 PROP .
I'M USING A 95 OCTANE GAS WITH PENNZOIL 100% SYNTHETIC OIL (40-1)

CAN I BE ABLE TO MILL THE HEAD ???
HOW ABOUT TO INSTALL A BIGGER CARB ????

P.S.: I BOUGHT A SET OF BOYESEN REEDS. BUT, LOOKING THAT SMALL PIECES, I'M SURE IS A "PIN IN A..."!, TO INSTALL IT .
I JUST KEEP THE STEEL ONES .


THANKS,
DON

[Fast Fred]

if you got fiber reeds put them in.:cool: see if a 30hp carb is biger , or the next bigest size that will fit, compression, see ware it is at now, at 125 psi or lower,
you don't need high octain, prob' make more power on reg:cool:

[DonChichoJr.]

The 25 H.p. & 30 H.p., Carb Are The Same. It's Only Change The Medium Jet And The Hi-speed Jet , That I Already Installed It ( 30 H.p.) . I Dodn't Know If Using Hi-octane Gas, Can I Go Up With The Timing ( 32 Degrees ???).

The Boyesen Reeds Are Coming In A Small Pieces, It's Not Coming In A Solid Piece. You Have To Align The Pieces !, Waohhh., A Lot Of Trouble To Align That Small Pieces !!!!!!!!!!

Thanks,
Don

[DonChichoJr.]

.................or using a SPLIT FIRE 264D PLUGS ??!!

REGARDS,
DON

[DonChichoJr.]

Thanks, By The Way......, I Will Try To Install Those Fancy Small Plastic Reeds (boyesen) .

Don

[17W]

There is a difference in port timing from the 25 to the 30. They also do not run the same intake manifold. You can get the larger carb but performance increase will not meet your expectations.

Split Fire Plugs......every engine I seen running Split Fire Plugs was torn apart for rebuild.

[adila]

,,I WOULD LIKE TO IMPROVE THE TOP END OF THAT PONNY" ...with SST 10 1/2x11 PROP ???!!! Too small pitch! What gear ratio is your 25 hp? 2.15/1?
You can try one SST prop 10x17 (omc#390731)...or an aluminium 391064(10x17)...

[Jeff Akers]

17[/B] (omc#390731)...or an aluminium 391064(10x17)...

That omc 390731 is NLA .... I'm looking for one myself;)

Just a note on the carb thing...... That engine can use any OMC (SV) carb,
SV meening Side Vent. So that gives you a range of carb options from 20hp through 75hp models;) :cool:

[QUINAULTRACING]

Looking for what would be best for use as marathon or sport C engine. 1980-2005, 20hp Thur 35hp, two cylinder cross-flow or 1996-2001, 25,30, & 35hp, three cylinder loop-charged ? Correct me if I am wrong but I think both are two stroke. THANKS..steve:confused:

[17W]

Looking for what would be best for use as marathon or sport C engine. 1980-2005, 20hp Thur 35hp, two cylinder cross-flow or 1996-2001, 25,30, & 35hp, three cylinder loop-charged ? Correct me if I am wrong but I think both are two stroke. THANKS..steve:confused:
Use the 3cyl 35hp.....if you can obtain a powerpack w/out a rev limiter.

[adila]

[QUOTE=Jeff Akers]That omc 390731 is NLA .... I'm looking for one myself;)
I have one sst 390731 and an 391o64. I'm looking 4 an cleaver 11x18...19 or 20" with 14 splines( Johnson 25-35hp).

[Dago Fast]

Looking for what would be best for use as marathon or sport C engine. 1980-2005, 20hp Thur 35hp, two cylinder cross-flow or 1996-2001, 25,30, & 35hp, three cylinder loop-charged ? Correct me if I am wrong but I think both are two stroke. THANKS..steve:confused:

I have one for sale. Also have 40mc Nissan 2 cyl short shaft for sale.

Dago Fast

[DonChichoJr.]

DOES SOMEBODY HAVE TESTED A "HOTSHOT" PROP ? (MADE BY TURBO PROPS ), FOR THOSE SMALL PONNIES .

ARE THOSE PROPS BETTER THAN THE OMC SST's???

REGARDS,
DON

[adila]

ARE THOSE PROPS BETTER THAN THE OMC SST's???
I didn't test hotshot props ...I was jumping directly at cleavers and more pitch.

[JohnsonM50]

The 25 H.p. & 30 H.p., Carb Are The Same. It's Only Change The Medium Jet And The Hi-speed Jet , That I Already Installed It ( 30 H.p.) . I Dodn't Know If Using Hi-octane Gas, Can I Go Up With The Timing ( 32 Degrees ???).

The Boyesen Reeds Are Coming In A Small Pieces, It's Not Coming In A Solid Piece. You Have To Align The Pieces !, Waohhh., A Lot Of Trouble To Align That Small Pieces !!!!!!!!!!

Thanks,
Don
does this carb have a hi speed adjust needle? Im more used to 70s engines and have installed hi speed needles thru the drain hole using a brass plumbing fitting with the center threaded to the omc lo speed needle size, a drilled over jet and an 1/8 in cut of fuel hose in place of the compression collar to seal it. Ive run these up to 6700 Rs. Also when you take the reeds apart hold the manifold over the reed plate [w/o the reeds] look thru backwards and see how well or NOT they match up. the machined surface of the manifold covers up to 3/16in. of potential flow at the outer edge of the reeds. Between all that and getting more exhaust out, [relief holes might help] but im still working on that:confused: .

[DonChichoJr.]

THE CARB DOESN'T HAVE A HI-SPEED ADJ. THE CARB USE A FIXED HI-SPEED JET . THE STOCK ONE IS A 67d AND I INSTALLED A 69d .

THANKS,
DON

[JohnsonM50]

Use the 3cyl 35hp.....if you can obtain a powerpack w/out a rev limiter.

Ive heard that a rev limited power pack would have an R in its #s Ive also heard the 2 cyls arent limited, dont know for sure. Im running an OMC 31.8 2 cyl crossflow at up to 6900 Rs on a C hydro, the powerheads factory, the carbs worked, the time is locked. a 3/4in relief hole is drilled in the sorta stock lower and Im playin w/ props to get it to go :D

[JohnsonM50]

Ive heard that a rev limited power pack would have an R in its #s Ive also heard the 2 cyls arent limited, dont know for sure. Im running an OMC 31.8 2 cyl crossflow at up to 6900 Rs on a C hydro, the powerheads factory, the carbs worked, the time is locked. a 3/4in relief hole is drilled in the sorta stock lower and Im playin w/ props to get it to go :D

Oh, forgot to mention, I lightened the crap out of the flywheel, if you look at it ,its made to start 3 ways I got rid of em all and put a MK25 spool on and took a ton of junk off the motor too.

[Mark75H]

Oh, forgot to mention, I lightened the crap out of the flywheel, if you look at it ,its made to start 3 ways I got rid of em all and put a MK25 spool on and took a ton of junk off the motor too. Be careful, some flywheel lightening is dangerous; there are sections on some flywheels that look like dead weight, but are actually part of the flywheel's strength

[JohnsonM50]

Be careful, some flywheel lightening is dangerous; there are sections on some flywheels that look like dead weight, but are actually part of the flywheel's strength

Thanks Sam, Ive had that in mind and looked it over before and after running, I also bench spun it at around 9 - 9,5 g before putting it on a motor. I probably should test with more rpm but thats all Ive got for now.
I will look it over under a magnifying glass. Mike: eek:

[Skoontz]

If you want to shave the head, you can safely take .060 from it without giving the pistons a kiss. We have taken .100 from one, and we had to plane two small sections inside the chamber for the pistons to clear.

The easiest way to find what you can take is use some modeling clay on top of the head, run the engine to TDC, then let the clay set up a bit, then mike it. If you have to plane any parts inside the chamber you should really CC the head again to make sure you have equal spaces in each.

The big question is what will higher compression get you? I would have to ask how tight are you going to wind the engine? If you keep close to factory R's shaving the head might get you a little more punch out of the hole but it won't show any significant numbers on the high end.

Until we added dual carbs and opened the exhaust above the cavitation plate, the head did not really show much of anything. I.E. not worth the headache of high octane fuel unless you use it with a combination of other ingredients.

Another trick you might try is to machine the crank hole of a 15HP flywheel to fit the 25 crank. That way you run a lighter wheel without running the risk of damaged structure.

[JohnsonM50]

If you want to shave the head, you can safely take .060 from it without giving the pistons a kiss. We have taken .100 from one, and we had to plane two small sections inside the chamber for the pistons to clear.

The easiest way to find what you can take is use some modeling clay on top of the head, run the engine to TDC, then let the clay set up a bit, then mike it. If you have to plane any parts inside the chamber you should really CC the head again to make sure you have equal spaces in each.

The big question is what will higher compression get you? I would have to ask how tight are you going to wind the engine? If you keep close to factory R's shaving the head might get you a little more punch out of the hole but it won't show any significant numbers on the high end.

Until we added dual carbs and opened the exhaust above the cavitation plate, the head did not really show much of anything. I.E. not worth the headache of high octane fuel unless you use it with a combination of other ingredients.

Another trick you might try is to machine the crank hole of a 15HP flywheel to fit the 25 crank. That way you run a lighter wheel without running the risk of damaged structure.

Thanks Skoontz, The 2 carb thing Ive thought about, but instead tried oversized and or carbs with a hi speed adj adapted w/ some improve. Did you make a new intake ? and I guess you had to experiment carb size? If you made a new one side mount carbs could be tried...

[Skoontz]

To calculate carb size and do it without winging it, ( it all requires testing) you first need to know the target RPM you will want to achieve. And, realistically, the tighter you wind the motor, the more internal work it will need. If memory serves me, the 25HP rods and crank for example, can run into the high 6 range with little or no work. If you dance in to the 7K plus range, you might want to look into a set of Carillo I or H beam rods. They will set you back $800.00.

You know the drill, speed is money how fast would you like to go?

Once you set a target RPM, you can use a formula to calculate CFM/CIM needed to "feed" it. I'll use the example I use when selling an irrigation system. WE can feed the lawn valves using a 3/4" pipe just as well until the lawn needs more volume of flow. Then, by simply stepping up to a 1" diamter pipe, we can nearly double the capacity. Air/gas works the same way. A large single carb will work just as well as a set of duals with alot less mousing around if your RPM is kept under the design limitations of the carb. As soon as you want to make your motor sound like a heard of bumble bees, and dance into the high RPM range, a dual set of carbs properly sinked will feed to motor better more responsively. At least in dad's theory.

We took 3 intake manifolds , plugged the center hole, then cut off the carb mounts from the others. Then we took a piece of aluminum tubing the same diamter as the carb bore and wleded it to ther manifold, then attached the carb mounts to the other end of the pipe. We got some stainless steel stock and extended the butterfly shafts then clamped them together with the timing roller welded to the center space.

After that came the arduous task of calibration. Both carbs were old adjustable needle type, high and low speed. Until we added a vaccum fitting on each intake with a connecting hose, we could never get them to sync.

So, ask yourself what you want to achieve with the motor, and, what's the target budget. With enough work, you could turn that sucker up into the 14,000 RPM range. I cannot stress enough how important CFM, displacement, and high end RPM work hand in hand.

[JohnsonM50]

To calculate carb size and do it without winging it, ( it all requires testing) you first need to know the target RPM you will want to achieve. And, realistically, the tighter you wind the motor, the more internal work it will need. If memory serves me, the 25HP rods and crank for example, can run into the high 6 range with little or no work. If you dance in to the 7K plus range, you might want to look into a set of Carillo I or H beam rods. They will set you back $800.00.

You know the drill, speed is money how fast would you like to go?

Once you set a target RPM, you can use a formula to calculate CFM/CIM needed to "feed" it. I'll use the example I use when selling an irrigation system. WE can feed the lawn valves using a 3/4" pipe just as well until the lawn needs more volume of flow. Then, by simply stepping up to a 1" diamter pipe, we can nearly double the capacity. Air/gas works the same way. A large single carb will work just as well as a set of duals with alot less mousing around if your RPM is kept under the design limitations of the carb. As soon as you want to make your motor sound like a heard of bumble bees, and dance into the high RPM range, a dual set of carbs properly sinked will feed to motor better more responsively. At least in dad's theory.

We took 3 intake manifolds , plugged the center hole, then cut off the carb mounts from the others. Then we took a piece of aluminum tubing the same diamter as the carb bore and wleded it to ther manifold, then attached the carb mounts to the other end of the pipe. We got some stainless steel stock and extended the butterfly shafts then clamped them together with the timing roller welded to the center space.

After that came the arduous task of calibration. Both carbs were old adjustable needle type, high and low speed. Until we added a vaccum fitting on each intake with a connecting hose, we could never get them to sync.

So, ask yourself what you want to achieve with the motor, and, what's the target budget. With enough work, you could turn that sucker up into the 14,000 RPM range. I cannot stress enough how important CFM, displacement, and high end RPM work hand in hand.

Cool, The side by side idea works with the OMC de-zine. Was this a 31.8 ? The carbs sound like older 50s 60s era. I guess the connect hose worked to even out the flow. Did you separate the intake area before the reeds?
I just finished a rebuild on a 31.8-35hp and used 3 holer rods and Wisco pistons this should bring up the capabilities a bit [I hope]. What your sayin here has got my attention, however my need for speed is budgeted. This could be experimented with on a nearly part free re- assemble though...

[Skoontz]

My dads was a 1952 31 cubic inch motor with dual carbs from that same engine. We morphed a 40HP Merc Quicksilver gearcase to a 15" midsection and sawed off the end braket bolt on that section and welded a hunk of
1.25" wide by 3" high aluminum rectangluar tubing to open the exhaust. We thought at one time about running stacks, and the exhaust manifold housing on that motor would be a great fit, but on the river we were at it would be a short lived ride for the noise. The near water line exhaust was actually tuned better and since water passed through it, there was some muffling.

We ran that on a 10' GW Invader glass boat, and before it was auctioned off, Dad ran it on his Rocholt.

We did a similar thing on a '72 Johnson 22 cubic inch motor and it would wind alot tighter just did not have the grunt out of the hole as the 31.8 cubic incher.

If my memory serves me right, you should be able to connect a 40HP block to that mid section with little to no mods. The only thing with the 40 is the rods were junk, and many of them used a manual assist compression release on the cylinder head, so you would want to rid the head if that apparatus.

If you can figure out a way to oil the block, I always thought the way to run carbs on a 25 would be directly from the transfer port similarly to how a Stinger GP is aspirated. Two shot gun tubes from each port, siamese in configuration with about a 5" ram to boost the charge going into the engine. Run a tight net screen behind the carb so the fuel would vaporize better. But, that's a project for a different day. You realize of course if we keep talking like this you will force me to find an old engine and start spending money if for anything to see if the idea would work:):):)

[adila]

,,I just finished a rebuild on a 31.8-35hp "
I wish ask something...What props can you use 4 your 35 hp ...I'm asking because my Selva shaft is with 14 splines...and most cleavers are with 13 splines. Hwo knows some such props???

[JohnsonM50]

My dads was a 1952 31 cubic inch motor with dual carbs from that same engine. We morphed a 40HP Merc Quicksilver gearcase to a 15" midsection and sawed off the end braket bolt on that section and welded a hunk of
1.25" wide by 3" high aluminum rectangluar tubing to open the exhaust. We thought at one time about running stacks, and the exhaust manifold housing on that motor would be a great fit, but on the river we were at it would be a short lived ride for the noise. The near water line exhaust was actually tuned better and since water passed through it, there was some muffling.

We ran that on a 10' GW Invader glass boat, and before it was auctioned off, Dad ran it on his Rocholt.

We did a similar thing on a '72 Johnson 22 cubic inch motor and it would wind alot tighter just did not have the grunt out of the hole as the 31.8 cubic incher.

If my memory serves me right, you should be able to connect a 40HP block to that mid section with little to no mods. The only thing with the 40 is the rods were junk, and many of them used a manual assist compression release on the cylinder head, so you would want to rid the head if that apparatus.

If you can figure out a way to oil the block, I always thought the way to run carbs on a 25 would be directly from the transfer port similarly to how a Stinger GP is aspirated. Two shot gun tubes from each port, siamese in configuration with about a 5" ram to boost the charge going into the engine. Run a tight net screen behind the carb so the fuel would vaporize better. But, that's a project for a different day. You realize of course if we keep talking like this you will force me to find an old engine and start spending money if for anything to see if the idea would work:):):)

Yeah no kiddn ha ha Im not retired yet so time is limited. For all the playin with stuff that would be I wonder about findin a 3 cyl loopy 25-35, know if 1 would bolt onto the 2x mid and fit the D shaft?

[JohnsonM50]

,,I just finished a rebuild on a 31.8-35hp "
I wish ask something...What props can you use 4 your 35 hp ...I'm asking because my Selva shaft is with 14 splines...and most cleavers are with 13 splines. Hwo knows some such props???

I use one with a stock lower unit from a 79- 25hp, I changed the propshaft assembly to it from a 22ci OMC 72- 25hp, It has a shear pin set up. I adapt different props by inserting the center hub of old wrecked props.
If your 35 is a thru-hub that wont help but you might be able to get a prop made. The 35 powerhead will bolt onto a late 70s midsection but it might need exhaust holes and the shear pin will need to be drilled to 1/4in.

[NERSTROM]

JohnsonM50, the flywheels used on the OMC 25/30 are die castings which get most of their strength from the outermost layer of material. By cutting the wheel you took that material off. It always scares me when I read that someone cut a production flywheel to make it lighter. When we developed flywheels we spun them to burst speed to make sure there was a safety factor based on the highest possible speed which could be obtained. This would be the speed of the powerhead without gearcase or prop load - i.e. if the driveshaft broke this would be the highest runaway speed condition. We tested in a 3" thick steel drum and it still made you jump when the flywheel burst. We were never able to spin the billet aluminum racing flywheels fast enough to make them burst. You have created a very dangerous situation, my advice would be to machine the magnet section out and epoxy it into a new billet housing - Please!

[JohnsonM50]

JohnsonM50, the flywheels used on the OMC 25/30 are die castings which get most of their strength from the outermost layer of material. By cutting the wheel you took that material off. It always scares me when I read that someone cut a production flywheel to make it lighter. When we developed flywheels we spun them to burst speed to make sure there was a safety factor based on the highest possible speed which could be obtained. This would be the speed of the powerhead without gearcase or prop load - i.e. if the driveshaft broke this would be the highest runaway speed condition. We tested in a 3" thick steel drum and it still made you jump when the flywheel burst. We were never able to spin the billet aluminum racing flywheels fast enough to make them burst. You have created a very dangerous situation, my advice would be to machine the magnet section out and epoxy it into a new billet housing - Please!
Thanks, It has made a an acceleration improvement but not much actual speed, Ill put it aside for now. I also have run 1 for a long time with the steel start ring removed, do you think thats a bad idea too? These wheels have the steel ring pressed on and a cast on ring with a rope notch, since the rope notch is a weakend point removing the rope ring seemed ok. I didnt go much deeper than that.

[adila]

[QUOTE=JohnsonM50]I use one with a stock lower unit from a 79- 25hp, I changed the propshaft assembly to it from a 22ci OMC 72- 25hp, It has a shear pin set up. I adapt different props by inserting the center hub of old wrecked props.
If your 35 is a thru-hub that wont help but you might be able to get a prop made. The 35 powerhead will bolt onto a late 70s midsection but it might need exhaust holes and the shear pin will need to be drilled to 1/4in.


[/QUOTE
Yes ,my 35 is thru-hub with 14 splines shaft. Thanks 4 answer ...I need splined prop...The old propshaft assembly 4 omc it is a good solution 4 increase the gear ratio (at thru-hubs is 1/2.15 ...).

[NERSTROM]

No problem with removing the ring gear.

[Skoontz]

If you did not know this little cheap trick, lemme say....So lets say you don't have the coin for a hot dog gear case. You are running the old "Club Foot"

You can add a nose cone, and if you remove the reverse gear and shift dog, grind all the teeth off the gear and take the tittie wompers off the reverse side of the shift dog, it's good for 4-700 RPM. You can run ATF as lube but honestly, I never saw any real changes from that.

Back in the day, OMC had a part number for a spacer and one sided shift dog for the V-4 75 HP engines, thus, where we stole the idea from. The trick will work in any gearbox with a reverse gear.

[JohnsonM50]

If you did not know this little cheap trick, lemme say....So lets say you don't have the coin for a hot dog gear case. You are running the old "Club Foot"

You can add a nose cone, and if you remove the reverse gear and shift dog, grind all the teeth off the gear and take the tittie wompers off the reverse side of the shift dog, it's good for 4-700 RPM. You can run ATF as lube but honestly, I never saw any real changes from that.

Back in the day, OMC had a part number for a spacer and one sided shift dog for the V-4 75 HP engines, thus, where we stole the idea from. The trick will work in any gearbox with a reverse gear.

Yep Ive put a cone on long ago- a blob of epoxy, when time came to disassemble I cut it carefully with a fine hacksaw [so now I admit bein a hack] Ive got the dog pinned forward with a drill bit stem [tired of wearin out dogs] Ive though about running it in reverse with a lefty race prop but was advised negatively. And I wondered about makin the R gear a spacer. Q- bein how will the pinion gear and bearing hold up? Its a hydro reasonably lite load.
The good news is its comin apart for a rear bush-seal anyway. Thanks for all your input. Mike

[Skoontz]

There may be others with negative response, but honestly, we had a 24" clever with a mean cup on the motor and ran all summer hard.

The thrust on the pinion is going to be on the gear teeth from the direction the engine is engaged. The bearing surface on that pinion is about 1.5" long, maybe more and it's really not going to be any different than a rear end pinon thrust on a truck.

As far as running in reverse with a left hand prop, I wonder why that would be any harder than forward. Reverse is the same gear, same part number as forward, just in a different spot in the carrier. Prop shaft thrust maybe? I dunno....Curiousity has arisen though.

[JohnsonM50]

On this gearcase the thrust washer/ flat needle bearing setup is much more substantial on forward than the relatively small thrust bushing of reverse and the bush- seal assembly is held in by a locater pin, It would probably self destruct.
That reverse gear has an appointment w/ an axle and a grinder!:D
This hydro Im runnin for now while my runnabout is down for some overdue upkeep its the airborne on show yer boat pg 2. The re-b I mentiond is for that. The motor sounds very good, it has 4 very definite compressions. you could think crank comp. was cyl. comp. till ya turn it more yet you can turn it by the flywheel taper w/ 1 hand when the plugs are out. Ive only done some test wheel runnin so far. I hope to be done with the boat after next weekend. Im tryin graphite in the aft bottom epoxy.

[RichardKCMo]

OMC ,must have had a secret motor in that era, thought they were 36 , motors being slightly less.
I read all with zest even th B.S.
RichardF

[Skoontz]

Richard:

In all honesty, the 25 HP of the 50's may very well have been 36 cu in. I was 12-16 when dad had that motor, and always thought they were 31 cu in and honsetly never paid attention to that. The mods I am absolutley certain of, I welded the manifold, exhaust and gearcase mounts myself in shop class then planed the surfaces. It was a fun project.

[Roy Hodges]

check with any old timer . the stock outboards had a 36 class for 25-30 hp johnrudes ,at the last it was (i think ) won by 35 hp chryslers . somebody jump in and set us straight , please

[36run69n]

Yes, the early 50,s big twins evinjohns were 35.9ci and rated at 25 hp. the last 36ci OMC was the 1956 big twin rated at 30 hp. speeds were high 30,s to low 40,s on a 13' DU boat. The Chrysler (originally) West Bend 35hp was also 35.9ci and approved for 36 class in 1964. The speeds jumped into the mid 40's. In the early 70's the Merc 402 was approved for the class by this time the 36 speeds had reached in to the mid to upper 50's, with the Merc, I think having a slight edge. thou, both engines were very competitive. Cary wheels 9 3/4 X 15 1/2 were tops on the evinjohns and Record and Carys 10-10 1/2 X 16-17s on the Chryslers. Chrysler gear ratio was 13-21. I don't recall the evinjohns. The merc had a 2 -1 ratio and I think ran 20-22 in. pitch. The class was eliminated for '76 as APBA registration dropped below 50. We did have elimination heats at the '75 nats. in Dayton, if my recollection is right, there were 17 entries. It was a fun inexpensive class to run with parts readily available.

[JohnsonM50]

The Evinrude/Johnson stock lower units mostly used 12 to 21 gears on mid sized motors however my old book lists 18 & 33 hp motors = .571 to 1, the ha ha newer 22s & 31.8s have the same except for the thru hub types.

[RichardKCMo]

check with any old timer . the stock outboards had a 36 class for 25-30 hp johnrudes ,at the last it was (i think ) won by 35 hp chryslers . somebody jump in and set us straight , please
That i am, i don't race but spectate a bit.
I've been trying to find info on old omc motors, it seems some techs. can only relate to cc,s not cu. in. and other info without the book.
Skoontz , i like reading about things to make omcs fast, as theres not much out there except fo mercs.
RichardKCMo

[JohnsonM50]

Thats the way I find it too, dealers outlooks toward performance enhancements seem taboo, -you dont do that- or its already Hi Perf. Taking into account that a boat motor needs to rely more on tork than screemin H P cause a boats always goin up or maybe close to level but not exactly. All their accel. and speed are eek-ed out of 1 gear [forward] and they dont coast.
In comparison with motorcycle motors of similar ci. bike motors ROCK and have those 5-6 gears [hey why dont we got gears]
So its challenge to make em fly and a pleasure to hear from another who knows about it. Any one whos seen my posts knows Im a 31.8ci hack but I know a couple guys who have 22ci that smoke me! The secret... adapted AB quicky. I also saw a B mod with a 22ci OMC at Millville dominating the straightaways. Maybe some day 31.8s will run C mod, I wonder what lower unit could be used.

[RichardKCMo]

Omc 35s have already been tryed in c and d , too fast for 1 too slow for the other. Outa be somewhere to fit though.
RichardKCMo

[JohnsonM50]

Omc 35s have already been tryed in c and d , too fast for 1 too slow for the other. Outa be somewhere to fit though.
RichardKCMo

I Can guess you mean fast in C, slow in D cause of cid. What did they use for a lower unit? Were they using pipes or stacks? The 70s 50hp OMCs are I believe 43 +or- ci. and not very slow. A Bass D lower unit would be good for either.

[Skoontz]

Your best money is going to be spent below the cavitation plate reducing drag/creating transom lift first. Then after you get that dialed in, work the engine mods. The only way to make a screw together OMC gearcase work well is by removing it and adapting and Quicksilver unit, or some other reasonable unit. And when you run a stock OMC gearcase, the way to make it work is with a Merc propeller. At least that's what we did. There are also certain year OMC thru hubs hat blew water out at too slow of a speed. Post
'78 was when they worked a little harder on hydrodynamics.

[JohnsonM50]

Im gettin more than 50 from a screw together lower, ground down alot, coned, above and below the propshaft clearanced for better props, the fin thinned, the screw holes filled and I havnt removed the R teeth yet.
Bein on a Bezoats hydro and spinnin a 16P prop helps too but a set of gears would be cool. I think a 31.8s too much for an AB merc and they are too valuable to find out the hard way.

[JohnsonM50]

Now this LU is full forward, teeth removed from R gear so no more opposite spin friction or cradle- dog friction and no spacer friction because the dog is pinned forward thru the shaft, not spaced between gears + no more double gear lash. Ive been tryin a prop that would probably be pretty fast but it seems to slip excessively, maybe its because the torpedos kinda fat despite what Ive done to it. I tried some tweekin on the prop hopin to get better traction. That and going back to a non cut flywheel as advised, I want to test it this weekend. Im tryin to get an omc to be as fast as a C moda.. ha ha.. got a lonnng way to go! ;)

[Skoontz]

How much gain have you gotten from stripping the gears?

[JohnsonM50]

Im still waitn to run it, Ill let ya know, next couple days.

[JohnsonM50]

I do have a prop question before I go, I had to trim a prop to fit front to back. I trimmed away at the lower unit 1st as much as I could, then minimal y from the prop. As I mentioned it slips, you can feel it grab and release, it takes awhile to start, It does go but not with full solid traction, I tried lowering and tucking and got a small improvement. By eye it looks like its spilling off the sides too much rather than directing water to the trailing edge so I put more side cup on it and slightly curved the last 1/8 in of the trailing edge. Sometimes things are opposite to what seems right so am I on the right track is the question? :confused:

[Skoontz]

It's very difficult to fully grasp what is going on witho9ut seeing/driving/hearing the problem first hand, but to me, what you are describing sounds like blowout, which is what we called a problem the pre
'77 OMC thru hub 3 and 4 cylinder gearcases had. The design was such that at a certain speed, usually over 60, the water flatened out in front and caused an air pocket along the gear case. In '77-8 they changed housings, and their case looked more like a Merc than a Merc gearcase itself which solved that problem.

You are running a screw together housing but you epoxied all the holes and coned it, without seeing it in a test situation with colored water blown past the sides all you can do is play trial and error.

I can't imagine why a prop would slip from the hub by filing metal off a gear case. I'm not a prop guy but I';ll say that every time we ever ran any sort of side cup on a pre '77 gear case, it helped the immediate problem but did not solve what really needed to be done. The side cup as I understand has speed limitations in that it slows the rotation of the prop, thus takes valuable power. I had a hairbrained scheme for your rig that would be worth trying.

How about finding a C-D series hot dog gear case. So you don't want to drop the coin on it, thats fine. Photograph it and then shape the outside of your case with epoxy to look like the C-D housing. Then paint it and run it to see if there is a gain. You might have to cut off the skeg and heliarc a thinner one from a hunk of aludimon because you won't be able to reshape it's thickness and make it more effective.

Just a thought in the never ending quest for cheap speed. Trade dollars for labor and see what it gets you.

[JohnsonM50]

Thanks, I did copy-cat a D prop Ive got that I know works so maybe?? Labor for dollars, trial & error low cost speed.. yup

[RichardKCMo]

I hope this thread keeps going , very int. reading.
Richard

[JohnsonM50]

Me too, Ill run that hydro soon then soon after Ill run my runnabout so Ill be sure to have Q s maybe some A s. My runnabouts special to me, I built it, ran many motors on it inc. the 1 on the hydro. As Skoontz says the best $ is spent below the anticav. plate, hes right. I put a race lower on it awhile back. I just installed a balanced & mildly worked 35 OMC on it, graphite coated the bottom aft, put more roundover in the chine [used to be square for the 1st 1/4 in deep] and moved the fin back 5in [It spun once]. I doubt Ill get much speed from the speed coat but I hope it decreases the porpoise effect some and slides a little better in turns [rather than grab & hop]. My C hydro is definitly a faster hull Id bet this runnabout motor would make it fly! I need to get a flat bolt on mount, no clamp hooks cause Id have to cut the transome deep.

[Skoontz]

I had a brain fart for consideration on your 25.

Instead of epoxy in the screw holes and a cone, how about getting a hunk of aluminum tubing and welding silde brackets in the stock housing to slide it over, then attach the nose cone to the tube. Cut the top of the exhaust housing over the 5/8" bolt and add a hunk of 1 1/2 x 3" rectangular tubing just over the cavitation plate for an above water exhaust. If you need more propeller diamter you could add a inch and a half to the metal between the bullet and the bottom of the cavitation plate, then lengthen the driveshaft. Another trick would be to move the water pickup via aluminum tubing to the front bottom of the nose cone, then remove the water pump. You are going to force feed the water by using speed. Add a temp guage to see if you are heating up and if not, the removal of the pump would gain you 3-7 more usable HP (prop shaft HP).

[JohnsonM50]

Thats actually a cool set of ideas, I just got the hydro put away from a ride and it worked pretty well, GPS-ed 52.3 but not with the prop I messed with, it improved but still isnt quite there.
Now my airbornes ready so its goin on the trailer and Im going to pass this lower on loan to a pal who s got a wetback hydro and an omc 35. why would I let it go you ask... cause now I got a foot to make a real rat - heh heh.
I cant wait to go in the airborn, it can blow across the rough water of summer traffic without worrying about stuffing.:cool:

[Skoontz]

Now yer cooking. What brand lower unit and what gearing? You should still play with an open exhaust, and I think the rectangular tubing over the cav plate is the best route. If noise is an issue, find the water discharge and dump it into that opening.

[JohnsonM50]

I bought a used Y 102 hatchet 14 - 15 that I wanted for a spare and am going to see if it can be adapted to a shortened 25hp tower and a lower cut at the cav plate that will have a similar exhaust & water pickup as my 102 except this will have an omc pump. I have to ride some slow zones to play so definitly want a pump. I have a 1in relief hole in the tower maybe 1 more.
I test ran my airborne [runnabout] today man was it rough from boat wakes, from every direction - a freakin washin machine! The boat ran good, 2 hrs of break- in with the prop set as low as it can go and the timing set slightly retarded. Now I can set it up to go and try on a weekday eve when it might be calmer. After sayin I didnt have to worry about stuffin I took a splash over the bow and got hosed good. ha ha.

[Skoontz]

Heres a thought.....If you remove the water pump, through the slow zones what about using a hand held battery operated pump in the water pick up line? When done, put a clip somewhere in the boat to lock it down.

[JohnsonM50]

If when I get into the sacrificial omc foot I find that the yamato foot studs make it impossible to save the omc pump then that will have to be Plan B. Im not going to alter the Y foot, and a local pal told me he electric pumped a race lower and cooked a motor anyway. My hydro cockpits small to me at 195lb so Id rather keep it as simple as possible,, throttle, wheel & killswitch.
I think it has the potential of being pretty fast anyway though.

[JohnsonM50]

Ive mulling over how to do this gearfoot morph. The 102 foot studs clear the OMC water pump the rear 1s easy, the front 1 is close to the limit of still being in the case once drilled. The exhuast/water pick up is close to the prop area so will need to be moved back. Ill need to make an adapter plate to 2nd as a replacement anticav plate. Its alot of work but if you like that kind of thing its fun.
The biggest obstacle is aluminum welding, Im going to try an aluminum brazing system on some minor stuff to see how it does. Otherwise I can get welding done at about $30.00 an inch and a re-diculous wait time and I always do my own prep and jig work. Has anyone had any experience with these type welds? Ive got lots of braz and steel gas weld time from the autobody days, some arc too but a different brand of stuff I tried was very challenging.
I took my airborne to Lk Wallenpaupac Pa last weekend and found out you cant run altered exhaust in Pa. At least they didnt get me till I was leaving. Thats the 1st thing Im going to try the weld stuff on to divert the sound down [Id really like to figure out how to make my midsection into a tuned exhaust thats reasonably quiet]. Oh, the Ranger didnt write me up, he acknowledged that I was driving responsibly and he liked my pals and my boats. :cool: :D ;)

[Roy Hodges]

Suggestion ; i am a welder. I suggest you try and find a welder who does sheetmetal welding 40 hours a week. especially one who speciallizes in aluminum & stainless . (you don't need the stainless , but that guy will have the savvy ) and that guy can be fast. & that means more economical. or should, anyway. If he has a mig machine with a spool gun ,he can do it so fast , it'd make your head spin. it would behoove YOU to prep it first , just before welding . that means wire brush it w/a Stainless steel wire brush, then wipe it sterile with a rag saturated with acetone . nothing else . a cleaning is only good for 24 hours before welding. Aluminum is NOT like welding steel. Don't let anybody tell you it is . Cleanliness is next to godliness ! I hope this can help some of you .

[JohnsonM50]

Thanks Roy I have access to a mig welder and have found it not too hard to weld steel, judging around the end takes getting used to compared to arc. Id guess aluminum is harder or everybody would be doin it.

[JohnsonM50]

Ive mulling over how to do this gearfoot morph. The 102 foot studs clear the OMC water pump the rear 1s easy, the front 1 is close to the limit of still being in the case once drilled. The exhuast/water pick up is close to the prop area so will need to be moved back. Ill need to make an adapter plate to 2nd as a replacement anticav plate. Its alot of work but if you like that kind of thing its fun.
The biggest obstacle is aluminum welding, Im going to try an aluminum brazing system on some minor stuff to see how it does. Otherwise I can get welding done at about $30.00 an inch and a re-diculous wait time and I always do my own prep and jig work. Has anyone had any experience with these type welds? Ive got lots of braz and steel gas weld time from the autobody days, some arc too but a different brand of stuff I tried was very challenging.
I took my airborne to Lk Wallenpaupac Pa last weekend and found out you cant run altered exhaust in Pa. At least they didnt get me till I was leaving. Thats the 1st thing Im going to try the weld stuff on to divert the sound down [Id really like to figure out how to make my midsection into a tuned exhaust thats reasonably quiet]. Oh, the Ranger didnt write me up, he acknowledged that I was driving responsibly and he liked my pals and my boats. :cool: :D ;)
I tried the aluminum brazing rods on my above the water line exhaust, I angled it down at about 45 deg. to tame the noise. The weld so far held up to about 50 mph / 50 mi. so Im happy with it. Sort of difficult but w/ practice who knows. Acetylene / oxygen would be the way to go cause map and propane take a long time to heat and thick to thin welds are harder yet.

[Skoontz]

The only way to weld aluminum in my opinion is Heli-Arc. I've seen other ways work in a pinch but I feel Heli-arc will get the most penetration and control over a non ferous metal. But hey, if what you did werks, does that not say it all????

[Roy Hodges]

It depends on the thickness of the allow. mig alum. with a SPOOL gun is the way to go, if the alum is at least about 1/8th " or thicker . I know , most of you don't know what a spool gun is . ask any real pro welder .If conditions are favorable , you have to see it , to believe how good and how fast it goes . it can weld aluminum faster than steel can be welded. I KNOW what i am talking about . P S ; penetration is way OVERRATED . in a properly welded weldment, you don't need a lot of penetration, too deep is actually contra indicated , and can actually weaken the joint .

[Skoontz]

Good knowledge Roy...Never even thought to use a mig spool but I'll try it on a few scrap hunks and see what/how I do. Too many years welding moly cages and buggy bars I guess.

[JohnsonM50]

I just did some more brazing on the omc gearfoot, a filler to drill the rear stud thru. Acetone seemed to improve the tinning step, it brushed out instantly. The foot is cast, varied thickness and the filler is 3/8 thick it seemed to take well.

[DonChichoJr.]

....and How Much Can I Be Able To Shave The Head ? , For 95 Octane Gas ???


Thanks,
Don

[Skoontz]

I would strongly suggest using modeling clay on the top of the piston then run it to TDC. Mic the clay to see how much room you have. Typically, .060 was what we used to take off. We took .100 off once and ended up having to gouge out the corners of the cylinder head to match the pistons.

We ran "blue" Sunoco 260, then it turned red but still gave you a headache if you spilled it on the concrete floor. Take AS MUCH as you can then pick up some octane additive from the suto parts store....

[DonChichoJr.]

Thanks For The Reply.
What Brand Of Octane Booster ??

Don

[Skoontz]

STP had one at one point that worked fairly well. The other thing you could do is get 5 gallon cans of VP racing fuel, ($ 8.00 a gallon)or yet another trick is head to a local airport, one that has small planes and buy aviation fuel, $6.00 a gallon).

Either one worked well but I'm personally a VP guy..

[DonChichoJr.]

I Would Like To Shaved .030" , And Using 50/50 , 95 Octane Gas/ Av Gas . .030" It's Enough To Feel The Difference ???

Don

[Skoontz]

I've said this before. The biggest jump you will see is what you can do to improve hydrodynamics first, i.e., below the waterline. Gearcase, bottom of the boat etc. If you shave a head and that's all you do, you'll get a little more bottom end and that will hardly be noticable. You add the head work to anythign else necessary to getting more RPM, then it will work in sync with all the other things you have done.

As far as taking .030 from a stock 25HP head, it should be no problem at all, but as I mentioned before, check yourself the cheap way by using modeling clay, then measure how thick the clay is.

[DonChichoJr.]

Thanks A Lot !!!!!!!!!!!!!

Don

[JohnsonM50]

Heres the almost done 102-OMC lower, the 102 half remains origional but will work on a 25hp johnrude [or 35] and have a water pump.


http://i35.photobucket.com/albums/d151/mes355/Boat003.jpg

[RichardKCMo]

Not enough words, so gets reg. to 93 octane i think, but google knows for sure. Oh Skoontz is right that's the first most important place to start.
Thanks For The Reply.
What Brand Of Octane Booster ??

Don

[JohnsonM50]

Heres the almost done 102-OMC lower, the 102 half remains origional but will work on a 25hp johnrude [or 35] and have a water pump.


http://i35.photobucket.com/albums/d151/mes355/Boat003.jpg
I finally got it in the water at a teeker meet, it shows alot of promise. After only a barrel test to go on I put it on a C hydro 2in. deep to be sure of cooling. It went about 57, when I bring it to height I think itll move out.

[JohnsonM50]

Thanks David, Cone? Yup and nah, the foot will still bolt up to my I02, want to keep it stock. I ran both the OMC and the 102 Saturday on the same boat. The OMC has a 6.9 ci. advantage and is stripped of hardware and cowling so weighs near the same. I had the 102 3/4in deep and it went faster but theyre close. The real difference was in how smooth and effortless the 102 seemed compared to a screamin shakin fishin motor leavin a trail of bolts behind. Ill have to get a balanced powerhead on that.

[JohnsonM50]

Now that pretty darn neat. :cool: If you and GEEZER PRO RACING (will350 here) ever got together the PRO boys would be in trouble.
WHOA! I looked at some geezer posts , hes EONs ahead of me.

Have you considered adding a nose cone to the lower unit?

I just put that OMC back together to re-run at Lake Hopatcongs AOMCI meet. I added a water pick up to the bottoms afterplane that will feed water thru the relief plate so I can set it shallower. and I used a little lock tite where things got loose + added a few 8 grade bolts. These modas can be speedy but are not as smooth as a Yamato.