25 to 35hp johnson

[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.