Help with increasing HP from 9.9 to 15 hp

Hello,
It must have been mentioned on this forum before but I cannot find it with any of my searches.

I have a 1988 Gamefisher 9.9hp, Model # 225-581990, Serial # 1398.

I cannot see what the differences are between a 9.9 and 15 hp engine of this year other than the main jet in the carb.

Can anyone verify or correct me on this?

I would love to get some more power from this engine but not at great expense.

I'm also looking for a new prop. I think the standard is 8" x 9 1/4" or 8" x 8 1/4".

I have an older 12' Blue Fin aluminum boat that I usually fish with 1 other person. I would estimate the total payload at ~500 lbs. That's 2 people, gas, anchor and fishing gear.

Any suggestions would be gretly appreciated. I see the range of the pitch on these props go all the way down to 4-6 inches. That seems pretty drastic for stock.

Thank you and happy boating,
Slimbob

kimcrwbr1 said:

It may be as simple as the jet but my guess is the venturi is larger in the 15hp carb also. All I could find is the 87 15hp #225-587150 and found both at sears parts direct. The intake manifold has similar numbers but the carb body has different numbers. Possibly you can just swap out the whole carb as long as the intake and exhaust ports on the block are the same also.

Click to expand...

Thanks for the input. I saw the same thing but can't see the difference in Carbs.

Have to look for a 15 carb i guess.

Happy boating!
Thanks again,
Slimbob

Kim, That's what I was looking for. The carb, pump and hoses were first on the list to make sure the engine is right before changing the jet.

Have you tried this? Anything similar?

Thank you,
Slimbob

What kind of difference did you see in performance? I'm not expecting alot.
50%of difference would be ok I think.
I bought a jetski run time/Tacometer to follow changes. A single wire wraps on #1 spark plug.

Gamefisher expexted:
The 9.9 WOT is 4600-5600 rpm
The 15 WOT is 4750-5750 rpm

Slimbob

Duely noted! Thanks for all the input. This will be a project for this winter. I'll post results when I start. I want to get some baseline WOT rpm readings before I change anything.

Happy Boating!
Thanks again,
Slimbob