1995 OMC 25 mods 31.8

[Buddy S]

Ive been researching for a while and decided to bite the bullet because it seems to be worth the upgrade. So I found a 28hp flat top carb for $85 and order the factory 30hp jets for it because I couldn't find the specs to drill the jets out. Plans are to match bore the intake opening and open the reed side of the intake manifold to the gasket shape and size on the mill. That should make my motor a factory 30hp. Compression is exactly 120psi on each cylinder. I fish and duck hunt in temps from 15*F -100*F. No real racing, just racing to and from fishing spots :)

I also read that polishing the exhaust, changing reeds, and shaving .XXX" of the head (assuming cylinder side) would give me more but I certainly dont want to sacrifice dependability, torque, or ease of starting in those weather conditions. Is there anything other than the carb and intake manifold mod that I should do? Thanks!!!


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[Chairman]

We race these motors in GT Pro class in MN. To upgrade a 25, we just bolt on the big-bore carb and leave the 25 intake in place. In fact, the only test I'm aware of got slightly faster top with the 25 intake.

We're not allowed any other mods, so can't help you with that.

[Buddy S]

Just measured the throat at 1.5 and venturi at 1.35 of the 28 hp carb. Do you know what he 30hp carb diameters are? I couldn't cross reference the carb body but i thought on the 30hp carb it was 1.5 on both diameters. Thanks for your reply.

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[Chairman]

These are the basic measurements I've taken. Although the '70s vintage carbs are slightly smaller in some measurements, I have raced them with no noticeable ill effects.

[Buddy S]

Thanks so much for that info! My carb is a newer (1995 28hp ) carb but has the same dimensions as the 76+ w/choke. I wonder why they changed diameters between "76+ w/choke" and "80's".

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[Buddy S]

Have you guys ever messed with the high speed jets? In my 28hp flat top carb (same dimensions as choked 76+ 30/35 carb) it came with .059" (59) jet but the flat top 30hp carb come with .067" (67D).

I tried 59 jet with 25hp intake and got 27-28mph at 5200-5300 rpm loaded with 10x13 3bl alum.

I tried 67 jet with bigger 30hp intake and got 2
26-27mph at 5150-5250 loaded with 10x13 3bl alum.

Boat is 1548 mod v with Jackplate. Cavitation plate is roughly 1" above bottom of v.

Loaded=1 Battery in front and 1 battery in back with 6gal tank and 1 passenger near back of boat.


My 25hp carb and intake ran roughly 28-29 at 5000 rpm with 10.5x11 3bl alum. Slower with 10x13 prop unless it had a light load. With 28hp carb it gets on plane easier and stays on plane better the stock 25hp carb.

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[Chairman]

The standard from-the-factory jet varies over the years. I've found slight improvement going down one size from that factory jet. Two sizes smaller and the motor falls on its face from idle, but can be nursed along to plane and wot, but for no further improvement. I've just stuck with going down one size and left it at that. If you know what year your carb is, you can look up the factory jet at: http://shop2.evinrude.com/ext/index.aspx?s1=44f704bbde4fc0d0b94589c4ca6e678f

[Buddy S]

Factory jet is 59 for my carb but with 25hp intake it didn't like decelerating too fast back to idle. so based on your experience it sounds like 60 or 61 would be best?

I was going to try a 63 jet but with this 30hp intake.

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[Buddy S]

When you say one size do you mean going from 59 to 58 or from 59 down to next available size?

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[Chairman]

As far as I know, all the sizes are available, so from 59 down to 58. I don't think there's any advantage to going up to a larger jet, unless you're not getting enough fuel where you are now. More top end comes from a leaner mixture, not more fuel. But not too lean. Your motor could run too hot and damage a piston.

[champ20B]

When you say one size do you mean going from 59 to 58 or from 59 down to next available size?

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What I did once was use a large diameter carb like the one pictured above. My carb was off a bigger motor because the jet was too lean. Motor was cutting out at top end. I simply matched the jet with an equal size drill bit then tried one size larger bit at a time. I bored out the jet till it ran right. I had it on a small utility race runabout. I simply added on a 17" pitched prop I had done, and advanced the timing stop up a tad. I was able to achieve 51mph. That's all I can tell you. I think I used 1/16" or a smaller bit, but I really cant remember.

[captbone]

What is the most HP that can be had safely from these engines? I noticed Second Effort used to offer a CLE gearcase for them. Anyone ever see one?

[Chairman]

No experience with these, but downloaded this pic from BRF a couple years ago.

[captbone]

That is where I saw it. I was always hoping that someone would have a couple new in the box stashed in the corner of their barn.

I would like to raise my engine a little higher and worry about water pressure.