Omc 25h.p.

[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.