25 to 35hp johnson

[14apache]

Would like to know if pluging the bleed hole in the cylinder to the exhaust will cause a problem or help the performance. I did some clean up on the ports with a rotary file. I have boysen reeds would like to know what else would help the top end performance. Thanks Eric

[Fastjeff57]

No difference, for that tiny holes is open only a fraction of a second at speed. Any exhaust leaking out doesn't count.

Jeff

[14apache]

47649 The hole is .160 diameter it starts at .800 down from deck and the exhaust is 1.6 down from deck. Im not shure why they put the hole there. Compression loss at low speed. Thanks Eric

[Fastjeff57]

The hole makes pul starting a bit easier by bleeding off compression. At speed, the hole isn't open long enough to affect power.

Jeff

[speedfreak]

14apache, Did you perform a 25 to 35 conversion on your motor? I have the same 25hp johnson motor and have converted it from a 25 to a 35, I also put boyseen reeds in. I would like to know what all you did to yours to convert it. Do you know if the heads are different 25hp vs. 35hp, maybe port timing was different between them? PM me if you want. I'm not sure if I got everything during the "conversion". I was told the only difference between the 25 and 35 was larger pistons, larger carb jets, and a slight advance in the timing!

The dealership that did my conversion said plugging the bleed hole would not help performance.

[JohnsonM50]

[QUOTE=14apache;121053]Would like to know if pluging the bleed hole in the cylinder to the exhaust will cause a problem or help the performance. I did some clean up on the ports with a rotary file. I have boysen reeds would like to know what else would help the top end performance. Thanks Eric
Ive read something about pressurizing the exhaust chamber to slow the exit of new fuel . Not that motor tho so maybe unrelated & just a bleed off as written. :confused:

[Fastjeff57]

Pluging that bleed hole makes NO difference on power output; it just makes it harder to pull start.

Jeff

[14apache]

The head is not the same the chamber is wider at the deck i think for piston clearnce. I added larger carbs made a intake on bridgeport and angled the case for a straighter flow. + stuffing crank case. Will see soon.

[speedfreak]

Thanks, I thought there had to be more to a 25hp to 35hp conversion than larger pistons, larger jets and a couple degrees of timing. Thanks 14apache for the info. Any other notible differences between the two?

[14apache]

Anyone else try anything with good results? Mine is on a zodiac yl340 running 37mph needs more prop and the bigger carbs:cool:

[pdt]

the tiny exhaust relief holes being blocked give more compression = more power. (just small amount though )
got told on another part of this forum that with my triple the first thing to do is block off those little holes and it will raise compression around 3 to 5 psi.

they are put there to give a better idle it seems, but then who idles a 2 stroke ?.
if your trying for max power then block them off, if your gonna use it as recreation and troll a bit then keep the holes.

[JohnsonM50]

Its reasonable that some compression can be gained by closing them. That said it would be minimal & compression in a 31.8 is already fairly low compared to some motors. If noticable at all it would be in acceleration more than top end. The 3 cyl's are loop charged, probably have more compression & there it might mean more but hardly worth the trouble in the crossflow 31.8's.

[14apache]

the tiny exhaust relief holes being blocked give more compression = more power. (just small amount though )
got told on another part of this forum that with my triple the first thing to do is block off those little holes and it will raise compression around 3 to 5 psi.

they are put there to give a better idle it seems, but then who idles a 2 stroke ?.
if your trying for max power then block them off, if your gonna use it as recreation and troll a bit then keep the holes.

I plugged the holes up and ported the motor and filled the crank case with marine tex. I made a intake for bigger carbs but have not had the time to put it on. This thing runs like a bear going to need bigger propeller for it soon. The boat is heavy and runs 37mph will go 40mph soon. Thanks for input.

[champ20B]

I plugged the holes up and ported the motor and filled the crank case with marine tex. I made a intake for bigger carbs but have not had the time to put it on. This thing runs like a bear going to need bigger propeller for it soon. The boat is heavy and runs 37mph will go 40mph soon. Thanks for input.

My brother and I have a 1997 3 cylinder 35HP Johnson. It is on a 14 ft center console and runs very well with 2 people or 4 people aboard, nearly equal. One difference for sure, compared to the the 25, is that the cylinder bore is larger. Furthermore, I believe the carburators may have a different venturi diameter as well. All else is the same (crank, rods) except that the 35 has a longer shaft, power-tilt and trim, and mandatory electric start. The 35 is a long shaft delux model engine that is more like a 50hp outboard in power and weight as well! You cannot make a 35 from your 25 unless you get a block, with the 35 pistons, and transfer all the guts of the 25 you have, over to this new block with larger-bore cylinders. You will then need the correct carburators. You would be much better off to sell your 25 and get a 35 or, better yet, find a good surplus 35 powerhead short block. I'm sure there are some dealers around the country that may have one or more of these components thereof parted out. It would definitly be worth the time and search. You will find what you need. A "real 35" on a 25 platform would be very nice to have.

[14apache]

My brother and I have a 1997 3 cylinder 35HP Johnson. It is on a 14 ft center console and runs very well with 2 people or 4 people aboard, nearly equal. One difference for sure, compared to the the 25, is that the cylinder bore is larger. Furthermore, I believe the carburators may have a different venturi diameter as well. All else is the same (crank, rods) except that the 35 has a longer shaft, power-tilt and trim, and mandatory electric start. The 35 is a long shaft delux model engine that is more like a 50hp outboard in power and weight as well! You cannot make a 35 from your 25 unless you get a block, with the 35 pistons, and transfer all the guts of the 25 you have, over to this new block with larger-bore cylinders. You will then need the correct carburators. You would be much better off to sell your 25 and get a 35 or, better yet, find a good surplus 35 powerhead short block. I'm sure there are some dealers around the country that may have one or more of these components thereof parted out. It would definitly be worth the time and search. You will find what you need. A "real 35" on a 25 platform would be very nice to have.

I bored the engine to the bigest piston oversize that a 35hp had think it is .060. Total was .180 over a stock 25hp the head and carbs are from a 35hp also. The carbs just have biger jets,

[champ20B]

I bored the engine to the bigest piston oversize that a 35hp had think it is .060. Total was .180 over a stock 25hp the head and carbs are from a 35hp also. The carbs just have biger jets,

The 25 has a bore of 2.35" and the 35 has a bore of 2.5"...
The difference to achieve a 35 bore would be -150 thousanths- (.150) over the stock 25 to stock 35 size. It sounds like you are (.030) over the 35 bore at (.180). If this is the case, then you took out (.090) from the "all around" thickness of your cylinder walls. Hopefully, OMC used the same sleeves in raw form. Otherwise, you could compromise the structural integrity of the sleeves but "I COULD BE WRONG". One thing I STRONGLY RECOMMEND FOR THIS MOTOR IS TO REMOVE THE AUTOMATED OIL-MIX SYSTEM. We did this on our motor and pre-mix a 32-40:1 ourselves to make it last. The problem with the auto oil mixing system is that it is highly prone to failure. It can destroy your cylinders and bearings at any given time without warning.
You can look at the routing of the lines and figure out the set up/rerouting of lines quite easy.

[14apache]

The 25 has a bore of 2.35" and the 35 has a bore of 2.5"...
The difference to achieve a 35 bore would be -150 thousanths- (.150) over the stock 25 to stock 35 size. It sounds like you are (.030) over the 35 bore at (.180). If this is the case, then you took out (.090) from the "all around" thickness of your cylinder walls. Hopefully, OMC used the same sleeves in raw form. Otherwise, you could compromise the structural integrity of the sleeves but "I COULD BE WRONG". One thing I STRONGLY RECOMMEND FOR THIS MOTOR IS TO REMOVE THE AUTOMATED OIL-MIX SYSTEM. We did this on our motor and pre-mix a 32-40:1 ourselves to make it last. The problem with the auto oil mixing system is that it is highly prone to failure. It can destroy your cylinders and bearings at any given time without warning.
You can look at the routing of the lines and figure out the set up/rerouting of lines quite easy.

Automatic oiling is long gone 40to1 is my oil mix. cylinder liners where thick on the engine. It has many hours on it now.

[champ20B]

Automatic oiling is long gone 40to1 is my oil mix. cylinder liners where thick on the engine. It has many hours on it now.

Thats awsome. I actually learned something new about these particular engines as I never took mine apart. I only read the technical data on it and studied blown-up illustrations thus far.

Before I sign out, I thought you might like to see one of these engines converted into a super stock racer. It can be seen on the (Quincy Looper Website) under the "collections" section somewhere. Someone took a 3 cylinder 25 powerhead and mounted it perfectly on a OMC class "A" racing tower w/gearcase. It looks like a modern day Cresent racer. You have to look carefully to see it. Check it out!

[Fastjeff57]

Those tiny triples are sure hard to find. Shame they didn't make them for a few more years.

Jeff

[speedfreak]

What next?????????? I also have a OMC 1996 25hp 3 cylinder. I was wanting to convert my 25hp to a 35hp, (we have 25hp restriction on our WMA's) that way I have a 35hp on a short shaft lower unit. I (like 14apache) have bored mine .060" over a stock 35hp, put 35hp jets in the carbs. Also, I added a set of boyseen racing reeds, removed the air silencer, had a set of velocity stacks machined and advanced the timing a couple degrees. Last time I did a compression check I was at 160 PSI on all three cylinders. Do I need to try and locate a 35hp head? Think I'll see any performance gains from a 35hp head? Currently I'm at 37 MPH @ 6000 RPM's turning a 14p SS powertech prop (me in an empty boat, fuel tank and battery). This is on a 1548 semi V flat bottom aluminum duck boat. I also installed a CMC PT-35 trim and tilt on it, which helped. This little thing has unbelievable torque on the low end. Top end is so, so. I was really trying to get to 40 MPH. Thoughts on a 35hp head, if I can find one, or anything else I can do. Those little jacked up 3 cylinder suzuki's and tohatsu's can still smoke me in the straight-a-ways.

[pdt]

the triple suzi's and yams will be faster top end due to them being bigger engines in cubic capacity from the very start.

but with a bigger prop you'l go faster top end.
if you have a very fast pull away you could sacrifice a bit of that and go up 2 more inches in pitch for the top end your seeking.
but remember you'l lose the pull away and suffer big time if a friend comes along for the ride.

37mph is bloody fast for what you have it on anyway.
what speed gain did you get with the jackplate ?

post some pics.

[14apache]

Think a 35hp head will lose performance. I put the 35hp head on mine was afraid it would detonate with the more squeeze. I never touched the timing on mine I am a pussy. lol

[pdt]

also remember the more you try to get from it, the shorter time it will live.
better off staying a bit on the safe side and make it last longer.
anything over 160psi is putting a lot of strain on the internals, ok so its quick, but is it worth sacrificing its life for an extra 3mph ???

.

[14apache]

I would almost sacrifice my life for 3mph. lol That's the disease of racing talking.:cool:

[Fastjeff57]

The 25 hp motor is 30.5 cubes, where the 35 hp is 34.5 cubes, so you can't do a jetting or reed valve change to make ten hp.

Jeff

[speedfreak]

FastJeff57, We bored my 25hp to .060" over a stock 35hp. Same as 14Apache did.

All good points’ and information guys. I think I’ll leave well enough alone (cheaper too), guess “I want my cake and eat it too", this stuff is addictive, to 14apache point. At this point, the engine is still very reliable, starts and idles well all the time no matter how cold. I have removed the oiling system and premix 40:1 with Amsoil HP. I always burn 93 octane ethanol free fuel. Everyone that rides in it wants to buy it from me. I gained about 2.5 - 3.0 MPH from the addition of the trim and tilt, which that thing has other great advantages other than some rpm/speed gains. I don't want to get to the point where the engine is not reliable. I’m not a “racer”, so I can’t come home after the weekend and rebuild everything. I need reliability and thus far I still have that. Wasn't this OMC's "Mirage" series motor? I hear nothing but good things about the design of these engines once the oiling systems are removed. I just always wondered what it would take for a true 25 to 35 conversion other than finding a 35 horse power head and sitting on top of my 25hp short shaft lower unit. For my 25hp, I got the Cu in's (bored), jets (35hp), etc. always wondered what else internally was different (like the 35hp head, porting, exhaust, etc).

[speedfreak]

FastJeff57, check that .060" over a 35hp above. It was actually .030" over a 35hp bore. Just used standard OMC part number piston.

[speedfreak]

5610756108

Picks of my little 1997 25hp OMC 3 banger! Hope this works!

[NERSTROM]

I think the Mirage was one of the best engines I did at OMC. The 25 and 35 share the same block and lower unit castings making all port passages and exhaust tuning the same between the two versions. Increasing the bore size of a 25 and readjusting port timing makes it a 35 block. I may have posted some history on the origin of the design in an earlier post. The block is of lost foam construction and due to the small bore size there was a tendency for glue lines to partially block the posts. Unfortunately, this is the case with many of the blocks. Someone willing to take the time to grind out the glue lines, especially in the finger ports is going to be pleasantly surprised at the increased performance. The air box that came with the engine was a patented design and while reducing noise also increased power. I have never found air horns to be of much benefit, although they do look nice. Darn cold here in Chicago!

[speedfreak]

Mr. Nerstrom, Thank you for posting and sharing your information and expertise regarding this Mirage 3-cylinder. I'm not exactly sure of all the engines you designed at OMC, but you knocked it out of the park with this 25-35hp 3 cylinder Mirage. This thing has been bullet proof since the day I bought it. I have been "tinkering" with it off and on trying to get my 25hp closer to a 35hp+ to run with those peskie little 2 stroke 3 cylinder Suzuki’s, Yamaha’s and Tohatsu’s that were manufactured in that similar timeframe. As far as the air horns, I designed/drew those up on the computer one cold, rainy weekend and had a local machine shop turn them out for me. I didn't really see any gains from them, didn’t seem to hurt performance any. I can always put the original air box back on.
So far the modifications to my 25hp (in my quest to get to a 35hp+) have been:
1) Increased bore size to .030" over a 35hp specification
2) Changed the carburetor jets from a 25hp to a 35HP
3) Went with a set of Boyseen reeds
4) I think the dealer may have increased the timing a degree or 2
5) Remove the oiling system, I premix 40:1 just for some added protection.
This was all completed by a authorized Johnson/Evinrude dealer.
Looks like maybe port timing adjustment and cleaning up glue lines in the finger ports may be next on the list.
By the way, Cold in Arkansas also.

[captbone]

I think the Mirage was one of the best engines I did at OMC. The 25 and 35 share the same block and lower unit castings making all port passages and exhaust tuning the same between the two versions. Increasing the bore size of a 25 and readjusting port timing makes it a 35 block. I may have posted some history on the origin of the design in an earlier post. The block is of lost foam construction and due to the small bore size there was a tendency for glue lines to partially block the posts. Unfortunately, this is the case with many of the blocks. Someone willing to take the time to grind out the glue lines, especially in the finger ports is going to be pleasantly surprised at the increased performance. The air box that came with the engine was a patented design and while reducing noise also increased power. I have never found air horns to be of much benefit, although they do look nice. Darn cold here in Chicago!


Thank you for sharing. I would love to hear as much as you are willing to tell about the Mirage 3 cylinder or any other project you were involved with. This is like crack for an outboard junkie. Please share as much as your time can allow as we would love to hear it.

Thanks

Cheers.

[14apache]

I spent hours on the finger ports went from less than 1/8 to a match on the sleeve. This motor has so much torque have had 5 people and still goes on plane. :D
As far as port timing how much could be gained?

[Ron Hill]

I've really enjoyed these posts! Warm in California. Jim, why not take a week or two and come to California and stay with me???

[NERSTROM]

Ron, tempting offer. When I last checked with Fred he said they were in Oshkosh, I would have been really jealous if he was back in California for the winter. He must be nuts! Still too cold in Chicago.

[Ron Hill]

The APBA Convention is the end of the month (1/14), then Fred said they'd come to Kingsburg for the rest of the winter. You should fly out here in February, we could go see Fred and you could warm up. We are flying down for the Daytona 500 the end of February! I could add that Freddy has always been crazy be it has kept him from going insane!

[speedfreak]

Anyone know how I could acquire a port maps for my OMC 1997 25hp 2 stoke 3 cylinder? As Mr. Norstrom mentioned a few threads above "Increasing the bore size of a 25 and readjusting port timing makes it a 35 block" (on my quest to fully convert my 25hp to the 35hp). I need to readjust my port timing to the 35 hp specs. I've already bored the cylinders to .030 over the 35hp specs. as he mentioned a while back, along with 35hp jets in the carbs. I want to go in and clean up all the glue line in the finger ports at the same time. Any help would be appreciated.

[14apache]

Don't waist your time with the port timing. Open up the finger ports as close to the size of the sleeve as you can. Stuff the crank case if you have time. Want to buy a motor rite now mine has a blown up gear case. Runs like a bear!

[speedfreak]

So no big gains with readjusting the port timing? My machinist said he had reservations about doing that anyway (he was questioning if it would even be noticeable), even if we got the port maps of a 35hp. So it sounds like pretty good gains to be had (noticeable) by cleaning up the glue lines????? I guess stuffing the crankcase gives you a little of that compression back after you clean up the parting/glue lines which actually increases the crankcase volume by doing that? When you cleaned up the glue/parting lines on yours and stuffed the crankcase, was it pretty noticeable from a performance stand point? How about the cylinder head, I still have the 25hp head on mine. The 35hp head was different????????? Anything there?

[speedfreak]

Anyone on this thread have any recent updates on their quest to convert their OMC mirage 3 banger 25 to a 35? I took the easy way out. I bought a remanned 35hp power head, bored .030" over, cleaned all the glue lines out of the finger ports (which were over half closed off) and dropped it on my 15" short shaft. Changed the carb. jets to the 35 HP jets and stuck a set of boyseen racing reeds in it. Should have it back in a few days.

[14apache]

Any updates?

[speedfreak]

14apache,
Not yet. Been busy with kids: LL football, gymnastics, etc. they are running me to death. Also, taking advantage of fixing a few little other things on the boat while we are at it. Been running through the break in process on 25:1. I'm being patient with it. Won't be long though, and I'll post some results.

[speedfreak]

Ok, first round of results from my little OMC 3 banger Mirage 25 to 35 conversion after a proper break in. Again this is mounted on a 1548 semi V aluminum duck boat. I'm turning 6500+ RPM's (hitting the rev. limiter), 38.5 MPH, with a 14P power tech SS prop. I think if I move up to a 15P SS, I'll bust the 40 MPH mark (that was my goal) and still have enough power to spare when the boat is loaded down. So what I have now is a 35hp power head bored .030" over with all the glue lines ground out and cleaned up, boyseen reeds, set of velocity stacks, rebuilt carbs with 35hp jets. Also running a CMC PT-35 trim and tilt. This is a 15" short shaft lower unit. Motor is pretty powerful on the bottom end for what it is. It just jets out of the hole.

[speedfreak]

Did a compression check, all 3 cylinders are right at 135psi. May have to shave a few thousandths off the head. Shoot for 145 - 150psi??????? Those little engines handle that? 15psi worth it?

[champ20B]

Did a compression check, all 3 cylinders are right at 135psi. May have to shave a few thousandths off the head. Shoot for 145 - 150psi??????? Those little engines handle that? 15psi worth it?

145-150psi are racing numbers for an engine of this size or less. My Yamato 80 has that much and my Champion HR has 160+. But I have to run non ethanol 97+ in them to be safe. Your motor is built tough and can handle a good load, but if you want those numbers in compression, I strongly recommend that you back the timing off a bit to start with until you see what it does after you mill the head down. You can reset timing from there and it will keep you from blowing out a piston or head gasket.

[speedfreak]

Thanks champ20B, Probably not going to mess with it until spring time. Time to use it right now. It's running strong.

[14apache]

How is it running

[speedfreak]

14apache,

Runs strong. 39-40 mph on the 1548 flat bottom. Happy with it. Gets there quick also. A lot of low end torque in that little thing, comes out of the hole hard.

[14apache]

14apache,

Runs strong. 39-40 mph on the 1548 flat bottom. Happy with it. Gets there quick also. A lot of low end torque in that little thing, comes out of the hole hard.

Did you stuff the crank case? Sounds like it runs good.

[speedfreak]

14apache,
No we didn't stuff the crankcase. Maybe later. Pretty happy with it.