Just thinking out loud here, but I'd "guess" this is an urban myth. It
seems easy enough to check, though.

Changing just the jets in the carburetor is not rocket science but it is
tricky - and you can end up with a useless carburetor if you make a
mistake. Instead, why not try installing a used carb from a 15hp motor
(larger jets and as you claim identical parts otherwise), seeing what
happens, and if you don't like the results, switching back to the
appropriate, original size. I wouldn't be too optimistic though.

The reason I don't think it would work is that horsepower is a function
of the amount of gasoline converted "efficiently" into smoke. It is the
pressure from the smoke that drives the pistons during the power stroke.
There is a physical limit to how much fuel a cylinder of a given size
can burn properly. Adding more than that would result in incomplete
combustion in the cylinder, fouled plugs, back firing and higher exhaust
manifold temperatures.

While it may be possible to increase the performance of your engine
slightly by using slightly larger jets then what are currently installed
(but perhaps not ones spec'd for twice the rated horsepower) my guess
is that you would also need to somehow adjust the timing as well as the
air entering the carob manifold, to maintain the proper air/gas ratio.

You could possibly achieve this by adding a blower (turbo) type
arrangement and adjusting the dwell, but I think we've now strayed in
way more effort than you had originally hoped for.

Oh, BTW, one side effect of "peaking" your engine performance,
generally, is reduced engine life. So what you gain in additional hp
tends to add stress and strain not originally intended for that hardware.

Hope this helps.

Cheers,

Robb






wrote:
O.K.: Urban myth or not?

I've heard over and over again that everything is the same on a 7.5
h.p. Johnson as on the 15 horse, except the carb. I also heard it's
not even the carb, it's just the jets.

I have a 1980-ish 7.5 seahorse that just does not have the guts to pop
my inflatable up on plane with two people and a full tank aboard. I
added those wings, which do help, but still just need a little more.

Anybody actually done this carb or jets upgade, or at least seen it
work? Every dock rat seems to have heard of it, but nobody's done it.

Failing that, any ideas on how to squeeze a bit more out of a
two-stroke (short of going 100:1 on the mix)?

Thanks!