25hp 2 stroke 3 cylinder Johnson

[speedfreak]

Question, I am brand new to the forum so I hope someone may be able to help out. I have a 1996 25hp 2 stroke 3 cylinder Johnson that I converted to as 35HP this winter. They made this exact same motor in a 35hp version, except it was a long shaft. We bored it, put 35hp pistons and 25hp carburator jets in it and a set of boyseen reeds. Used all OEM Johnson (OMC) parts in the conversion (except for the reeds). I am finally through the break in period (double the oil mixture for 15 gallons of fuel) and ready to see how's it is going to do on 50:1 and a new set of plugs. I use this boat for duck hunting, 1548 seaark semi-V aluminum flat bottom. Typical Arkansas duck boat set up. I do not race per say, well I guess we do race (sort of) at 4:00am in the morning on Arkansas public duck hunting land. I am competing with the 2 stroke 3 cyl. Suzuki's and Tahatsu's. I noticed they are "higher" geared, example 1.92:1 for the Tahatsu. My Johnson is 2.15:1, is there such thing as an aftermaket gear set for this model Johnson to get it closer to the Tahatsu ratio? Sorry to be so long winded.

[BRzuki]

I don't know about the gearing, but in your mentioning of motors you are contending with, you forgot the second most important motor aside from the Tohatsu mega and that is the 3 cyl yamaha 25.

[speedfreak]

Yeap your right! Those too! Hunted out of a boat last year (1648 War Eagle) with a 2 stroke 3 cyl Yami mod'd out. Fast!!

[Fuelblendr]

If you have bored the motor, more CI, check the plugs when you switch to the 50:1,
You mentioned the 25hp jets. My .02

[speedfreak]

Fuelblndr,
I put a new set in after the break in of 25:1 fuel. Is that what you meant by check the plugs?

[proprider01us]

I'm pretty sure he meant to check to see if your too lean. You may want to consider putting bigger jets in if you increased the bore size and made the intake track more efficient with aftermarket reeds.
Just my thought.

[speedfreak]

We replaced the 25 hp jets with 35 hp jets.

[capnzee]

It is still a good thing to check the plugs soon after you have made changes to the carburetion, ignition etc. By checking the plugs you will see exactly how the engine is running in the combustion chamber. Remove the plugs and look at the gap end. If the plugs are white/gray (chalky) looking you are running too lean. If your plugs are black and oily looking, you are running too rich. If your plugs are brown, you are running pretty close to where you want to be. Look out if your plugs are chalky white!, in fact I would not run it again until you "richen" it way up.

[speedfreak]

Just checked the plugs I pulled out yesterday and they are brown in color.

[Tim Kurcz]

Just checked the plugs I pulled out yesterday and they are brown in color.

http://www.boatracingfacts.com/forums/showthread.php?t=7707&page=17

Hey Speedfreak, In case you hadn't seen this before. I built this 35HP OMC triple for my son's 11' runabout. It runs 55MPH at 5750 RPM on the A15 racing gearcase. If you need to run a shifter, contact Ron Hill for a racing prop.

Tim

BTW: You'll want to get 35HP carbs and open up the airbox.

[speedfreak]

Tim,

Sweet! This is just like my 1996 OMC triple. As stated in the opening comments of this thread, mine was originally the 25hp short shaft version. As you know, they made this little triple in a 25 (short shaft) and 35 (long shaft), so we just converted my 25, using all stock OMC parts. We bored it .030" over the stock 35 and put a set of 35HP jets in it, about the only difference between them. Stuck a set of racing boyseen reeds in it. This is strong little engine, the torque is unreal, top end speed is lacking a little. I have had the oiling system removed. I haven't been able to really see what the conversion is going to actually do performance wise yet, I just got through the break in period (25:1) after the rebuild. It feels a lot stronger. I use it for hunting so I am trying to keep it as reliable as possible, but get everything I can out of it. That 2.15:1 gear ratio hurts me running against the 2 stroke 3 cyl Tahatsu's, Suzukis and Yami's down here with higher gear ratios. I'll get some data posted as soon as I get a chance to get back out on the lake with it. I'm currently running a 13 pitch stainless stock OMC prop. Toward the end of the break in period I was turning between 6050 and 6100 RPM's WOT, 32 MPH. I was just holding it there breifly. May get a little more out of going back to 50:1 and fresh set of plugs.

[Mark75H]

More oil produces more power. Use as much as you can for power.

[biggdave92]

the tohatsu and yamaha are 1.85. the zuki is 2.09. Where a hatsu or yami would be turning a 12p, that is about equal to a 14p on a zuki. you should at least be turning a 14p in the 6-6200 range.

work on your setup. get rid of any weight you can.

[JohnsonM50]

:cool: You can go to BRP parts catalogues for part comparisons theres exploded views # lists etc. The only likely reasons for gear options would be for hi altitude &/or heavy duty or ocean going. In all cases they would be lower speed - more power gears. Your close enough to make up the difference with a 'good' prop. Usually that's where you will find the most but in light of all you've done.. sounds good. The other motors mentioned are also awesome contenders.. to even up the score would be cool, to beat-em a great accomplishment.:D Theres alot to speed gain, the weight & its distribution, motor setup, prop choice, hull type & condition, All things working together = sweet spot. Any one of them alone might be hardly noticeable. To know actual speed & rpm's is critical to testing also taking notes as to whats changed, run conditions & results. If you change too many things at once it will be hard to tell whats helping & what isn't. Good Luck

[14apache]

Tim,

Sweet! This is just like my 1996 OMC triple. As stated in the opening comments of this thread, mine was originally the 25hp short shaft version. As you know, they made this little triple in a 25 (short shaft) and 35 (long shaft), so we just converted my 25, using all stock OMC parts. We bored it .030" over the stock 35 and put a set of 35HP jets in it, about the only difference between them. Stuck a set of racing boyseen reeds in it. This is strong little engine, the torque is unreal, top end speed is lacking a little. I have had the oiling system removed. I haven't been able to really see what the conversion is going to actually do performance wise yet, I just got through the break in period (25:1) after the rebuild. It feels a lot stronger. I use it for hunting so I am trying to keep it as reliable as possible, but get everything I can out of it. That 2.15:1 gear ratio hurts me running against the 2 stroke 3 cyl Tahatsu's, Suzukis and Yami's down here with higher gear ratios. I'll get some data posted as soon as I get a chance to get back out on the lake with it. I'm currently running a 13 pitch stainless stock OMC prop. Toward the end of the break in period I was turning between 6050 and 6100 RPM's WOT, 32 MPH. I was just holding it there breifly. May get a little more out of going back to 50:1 and fresh set of plugs.
How much does your boat weigh? Thanks Eric

[speedfreak]

Eric,

About 325lbs.

[Tim Kurcz]

More oil produces more power. Use as much as you can for power.

Sam is very right about the oil...... We run 24:1 in my son's engine for general laking, 16:1 for hard running. Increasing compression will get you more power, but you need to increase octane with this mod. BTW: Don't worry about revs. The optical ignition system has a limiter set at 6750 (cannot be defeated). You can't hurt that engine so long as you're running lots of oil. Unless you're running long distances, run the short prop and spin it!

Tim

[speedfreak]

Tim,

Thought those optical ignition systems were limited at 6200 on these engines?

Shane

[Tim Kurcz]

Tim,

Thought those optical ignition systems were limited at 6200 on these engines?

Shane

Pretty sure it's 6750, but doesn't matter, you can't hurt the motor.

Tim

[speedfreak]

Tim,

Got it. Im at 6200 WOT right now with mine GPS'ing at 37.5 - 38.0 MPH. Thats with a 14P SS powertech prop. I'm not quite to my magical mark of 40 mph, getting close.

[Tim Kurcz]

Tim,

Got it. Im at 6200 WOT right now with mine GPS'ing at 37.5 - 38.0 MPH. Thats with a 14P SS powertech prop. I'm not quite to my magical mark of 40 mph, getting close.


You're pretty close. Props, bottoms, and CG are everything. Have you had custom prop work done - thinning/cupping/shaping? Jacked the motor higher on the transom? Moved ballast aft & re-trimmed the engine? Cleaned, smoothed, waxed the bottom? You might get more than 40.....

Tim

[speedfreak]

Yeah, I'm going to get after it again after hunting season is over. Should be able to get a couple mph with a few more tweaks.

[champ20B]

http://www.boatracingfacts.com/forums/showthread.php?t=7707&page=17

Hey Speedfreak, In case you hadn't seen this before. I built this 35HP OMC triple for my son's 11' runabout. It runs 55MPH at 5750 RPM on the A15 racing gearcase. If you need to run a shifter, contact Ron Hill for a racing prop.

Tim

BTW: You'll want to get 35HP carbs and open up the airbox.

Hi, I think I seen this motor or one like it on the (Quincy Looper Website). It looks great!! My brother and I have a 35 triple OMC, and we love it. It's a great motor.
My question though is about the speed to horse-power. I thought that a set up like this would be super fast, "at least like a really good Yamato 302" at 65mph or so. What I mean is that an old Mark 20H or Evinrude-15/A will run in the low to mid 50s easy. Is it because the little stock 15-20 cubic inch motors are actually making in excess of 30 actual horsepower? This is an honest question. I never seen any actual horse-power ratings on many of these little racing engines except for the Yamato 80. I happen to own a Y-80, a 1956 Champion Hotrod-20, and a super-10 mercury. Thanks.

[JohnsonM50]

Hi, I think I seen this motor or one like it on the (Quincy Looper Website). It looks great!! My brother and I have a 35 triple OMC, and we love it. It's a great motor.
My question though is about the speed to horse-power. I thought that a set up like this would be super fast, "at least like a really good Yamato 302" at 65mph or so. What I mean is that an old Mark 20H or Evinrude-15/A will run in the low to mid 50s easy. Is it because the little stock 15-20 cubic inch motors are actually making in excess of 30 actual horsepower? This is an honest question. I never seen any actual horse-power ratings on many of these little racing engines except for the Yamato 80. I happen to own a Y-80, a 1956 Champion Hotrod-20, and a super-10 mercury. Thanks.
20H's 15A's, they can run that fast but the catch is it has to be an aired out race hull, efficiency is the reason. The gearcase is geared for more prop rpm & to compensate for the need for power the prop size [dia mostly] is reduced on race gearcases. This would not work well on a normal 'wet' hull. The 35x3 has a huge gearcase & itelf a limiting factor. Getting it all right at the same time should put up 40 or so & if you got into one as full a racer with a tower & gearcase upper 60s or more on a hydro is likely possible.

[champ20B]

20H's 15A's, they can run that fast but the catch is it has to be an aired out race hull, efficiency is the reason. The gearcase is geared for more prop rpm & to compensate for the need for power the prop size [dia mostly] is reduced on race gearcases. This would not work well on a normal 'wet' hull. The 35x3 has a huge gearcase & itelf a limiting factor. Getting it all right at the same time should put up 40 or so & if you got into one as full a racer with a tower & gearcase upper 60s or more on a hydro is likely possible.

Yes, your right about that. But he built this using a 35 triple powerhead on a "class-A OMC" racing midsection and a racing gearcase thereof. As far as I know, Tim Kurkst is a highly experienced racer who put this combo together for an 11' runabout. All I can assume is that the hull would be about equivalent to a "class-C" racing hull. I thought about that motor when I saw it and estimated that it should be at least a 65 mph runner, although not a legal competition engine. This clearly indicates that the traditional 50-55 mph racing motors should be doing at least 30 HP. I know that the Y-80 is rated at 30HP, but it can do better than 55 on a runabout as just a properly set up runner, not engaging in continuous turns and race conditions.

[champ20B]

Yes, your right about that. But he built this using a 35 triple powerhead on a "class-A OMC" racing midsection and a racing gearcase thereof. As far as I know, Tim Kurkst is a highly experienced racer who put this combo together for an 11' runabout. All I can assume is that the hull would be about equivalent to a "class-C" racing hull. I thought about that motor when I saw it and estimated that it should be at least a 65 mph runner, although not a legal competition engine. This clearly indicates that the traditional 50-55 mph racing motors should be doing at least 30 HP. I know that the Y-80 is rated at 30HP, but it can do better than 55 on a runabout as just a properly set up runner, not engaging in continuous turns and race conditions.

I spelled his name as Kurkst. I apologize about that. It is Tim Kurcz.

[JohnsonM50]

I think what Tim put together has potential to go faster, a matter of decision to go there but that of course, his call. The OMC 15 is a motor of specific parts to be legal & might be slightly enhanced but totally not 30 or even 20HP its use of a hull & prop set up to do those speeds make it happen, more RPM than HP.The Mercs on the other hand I wouldn't venture a guess.

[champ20B]

I think what Tim put together has potential to go faster, a matter of decision to go there but that of course, his call. The OMC 15 is a motor of specific parts to be legal & might be slightly enhanced but totally not 30 or even 20HP its use of a hull & prop set up to do those speeds make it happen, more RPM than HP.The Mercs on the other hand I wouldn't venture a guess.

Yes. That makes sense to me. I suppose it is just rpm and not really a hp issue depending on other set up factors. A bigger or higher pitch prop would probably pick it right up. The thing is, I am strongly considering a build of this type using the 25 triple OMC powerhead. It is right at 30 cubic inches, which could be an excellent "C" mod or superstocker if it could be properly introduced and accepted as a probationary type set up, using the "A" mid and lower or some other combo thereof. This could be something of an excellent idea, as a good option of modern engine choices, in certain venues of outboard racing. We could have another Tohatsu racer "metaphorically speaking".