25 OMC Mods

[88workcar]

I just got my hands on a 25 OMC. What can be done. I know that their are some of you here that don't "LIKE" to give away all the secrets, but come on now guys, someone has to carry on when you guys give it up. Yall can PM me with what don't need to be put out thier for general population.

This will be a pump gas motor, short shaft, factory lower with low water pickup. It needs to last for a little while. And needs to stay somewhat on the quiet side. Other than that no rules.

These are my plans, correct me or enlighten me, or just add to it.
1. big carb
2. fiber reeds, cut cage
3. port job, finger ports
4. polished exhast, port tuner or mod it, leave bottom closed
5. lighten rods, pistons, flywheel.
6. balance all parts
7. tune timing and jets.

Thanks for any input.

[88workcar]

Oh yes, it will all be 35 stuff to start out with.

[Powerabout]

find some new, old style high ring pistons.
Great for any crossflow

[88workcar]

can I swap from cross flow set up to a loop charged? or does the porting clash? Any one make heads or pistons for it? What about tuner specs with out going to the megaphone set up?

[Amod20HRacer]

Are you planning on racing it or just running it for fun? If you plan on racing it, sanctioning body rules regulate what you can and can't do to the engine. There's not a whole lot you can do to them if you plan on racing it.

Austin Duggan, 78M

[88workcar]

2 rules for me, pump gas, noise level. Anything eles goes.

[JohnsonM50]

2 rules for me, pump gas, noise level. Anything eles goes.
Sooo, you came to the right place but more 411 needed. What year 25 is it? 76 & under will be 22ci these are raced & can be fast but are run with a megaphone usually. 77 & up is 31.8ci, like the 22s big cousin they have alot more torque but in stock form are only a little faster. What kind of boat are you going with ?
To make a looper out of it... I dont know but the deflecter pistons would probably have to go. If it is a 31.8 the hi ring pistons will work but you need to be sure the wrist pin is the same, 2 sizes there. Ive heard the yay & nay about comp reeds but from my experience they will do well in a 31.8, dont know about the 22s tho. On a 31.8 Ive tried the biggest typical OMC carb thatll bolt up, [largest venturi & 1 1/2 bore at the manifold] will knock 2 mph off, run ya rich & rob your acceleration. The 2nd largest of the 4 Ive seen has done best for me. 1 3/8 bore at manifold & slightly smaller venturi. On the 22s my belief is the stock carb is optimal.

[88workcar]

The power head is a mid 80s 35, hull is a small light tunnel

[JohnsonM50]

The power head is a mid 80s 35, hull is a small light tunnel
Nice, have you run it yet? & if so Id get actual speed, RPMs listed & kept as a benchmark for what happens next.
One thing I did was to put a 35 powerhead on a 25 mid/gearcase w/o thru hub exhaust. Is said that the thru hub exhausts better but your advantages are a better gear ratio, more easily adapted props [by putting the propshaft assembly in from a 22ci you get a pin drive setup that will need to be drilled to 1/4 pin] & the clamp setup is alot lighter. The motor Im talkin about weighs 86 lbs & is on a pals hydro. Even tho this hydro weighs about 290- driver it does 50+.
If you keep the thru hub, performance props are made for that. Theres a race class, Mini GT thats about what you have Id guess, a prop is for sale on ebay now I think. Of the 31.8s the 35 is fairly awsome as is, If you square up the ports on top Itll net some top end power, Id avoid widening for cooling purposes & advise against changing the time as Ive never heard of a set of specs to do this motor with. Milling the head could help as well as the reeds. All said & done the real secret here is props, setup, testing & props.

[DonChichoJr.]

How Much Can Are Able To Mill The Head ??, Using 93 Octane Gas & 30 Degrees Timing ? On That 1990, 3.000 Bore Size Pistons .

.020" Or Maybe .030" ??

Regards,
Don;)

[JohnsonM50]

How Much Can Are Able To Mill The Head ??, Using 93 Octane Gas & 30 Degrees Timing ? On That 1990, 3.000 Bore Size Pistons .

.020" Or Maybe .030" ??

Regards,
Don;)
Theres is little clearance to begin with & I suspect that may vary a little so a fixed # I wouldnt venture. Best bet is to putty test the clearance you have & go from there. If it hits without the head gasket its too close. If you go oversized pistons watch for the aluminum at the bottom of the sleeve rubbing the piston skirt & the extra width of a 0.030" over piston can hit the edges of the squish band. may need some clearancing. If your compression is over 120 it wont make much difference.

[88workcar]

So these little engines don't like compression?

[JohnsonM50]

So these little engines don't like compression? Ive got 150psi out of 2 builds, havent tried to totally maximize that yet. Usually a used motor will have 120 to 135 with little difference

[88workcar]

So they will like compression

[JohnsonM50]

So they will like compression
It will, like any engine to a point, All I meant was at TDC The outer edges of the pistons are flush or slightly above the block surface & the head is shaped to the deflector without much clearance. More compression will mostly gain more acceleration, not much top end.

[JohnsonM50]

So they will like compression I sent ya a couple pms.

[M2Guy]

I am also getting ready for a project on one of these motors and have been following this thread. Just as it was getting interesting a couple private messages killed it. :mad: What is up with that? lol Can anyone else help with this? What kind of HP can I expect to see out of one of these motors?

Thanks:)

[JohnsonM50]

I am also getting ready for a project on one of these motors and have been following this thread. Just as it was getting interesting a couple private messages killed it. :mad: What is up with that? lol Can anyone else help with this? What kind of HP can I expect to see out of one of these motors?

Thanks:) Hi M2Guy, Im not tryin to be secretive its just stuff thats repeated from old posts. I have no way of measuring hp only to assume it actually started out as whats rated then figure if its 10% faster then 10% more?? For running speedboats Im more concerned with the Rs itll increase a prop to. What do ya have?

[M2Guy]

I have a 1984 OMC 35hp complete powerhead. Let's say I am a rookie and can use any good advise.