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.

Wrong on both counts.

If my memory is correct, the 7.5hp is about 10 cubic inches
displacement, the 9.9 and 15hp are about 15 ci. The 7.5 and 15 are
completely different engines except for maybe the ignition system
and/or some pieces of rubber or plastic.

The carb differences between these three engines are more than just the
jets, there's the size of the venturi too. To be blunt, anybody who
thinks that the only secret to getting 50-100% more power from an
engine is the carb jets does not know enough about engines to be
believed on almost anything thereunto appurtaining. You have been
misled by the uninformed.

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.

The "jets upgrade" does not exist, which is why "nobody's done it."
The "carb upgrade" does work for engines for which the only difference
is indeed the carb. Otherwise you might need the "exhaust manifold
upgrade," the "reed plate upgrade," etc.

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

Going 100:1 on the mix will not give you more power. People who soup-up
"fishing" motors to race them add more oil, not less. But simply
diddling the mix won't give you more power.

Your 7.5 morphed into the 8 hp motor with some tweaks, but that's not
the kind of power increase you're looking for. You need more cubic
inches, which means you need more metal around it, which means a
different outboard.

%mod%

wrote in message
oups.com...
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!

I was reading through the replies and did not see a couple suggestions that
(I feel) could gain you *some* HP- porting/polishing and changing the prop.

When I raced the 20 mod hydro class, porting (enlarging the intake and
exhaust passages) and polishing (smoothing all the passages to a mirror
finish) added substantial power and often made enough of a difference to
leave the others 'in the dust'. Mind you, this is not a racing O/B we're
talking about here, but all motors could benefit from a little 'cleaning up
the ports'.

You can also experiment with different props to see which one gets a better
'bite' and better uses the power of the O/B to benefit your use of the boat
(loading, speed, etc).

Just a thought.

Then again, you could just buy a bigger O/B :-)

Larry

My turbine powered boat project;
http://www.turbinefun.com

a few years ago I was shown how to turn a 5hp merc into an 8hp motor
just by nbolting the exhaust manifold rotating it 180 degrees and
reinstalling it.

How did you measure the horsepower?

Roger

http://home.insightbb.com/~derbyrm

"Drew Dalgleish" wrote in message
...
a few years ago I was shown how to turn a 5hp merc into an 8hp motor
just by nbolting the exhaust manifold rotating it 180 degrees and
reinstalling it.

Drew Dalgleish wrote:
a few years ago I was shown how to turn a 5hp merc into an 8hp motor
just by nbolting the exhaust manifold rotating it 180 degrees and
reinstalling it.


For 2 and 4 stroke going back 2 decades, the 5 hp Merc is 1 cylinder,
the 8 hp is 2 cylinders.

I took the mechanics word for it. It wasn't my motor and I didn't see
it in action . He did explain how and why he was doing it.

How did you measure the horsepower?

Roger

http://home.insightbb.com/~derbyrm

"Drew Dalgleish" wrote in message
...
a few years ago I was shown how to turn a 5hp merc into an 8hp motor
just by nbolting the exhaust manifold rotating it 180 degrees and
reinstalling it.

And how long is your leg now.........


"Drew Dalgleish" wrote in message
...
I took the mechanics word for it. It wasn't my motor and I didn't see
it in action . He did explain how and why he was doing it.

How did you measure the horsepower?

Roger

http://home.insightbb.com/~derbyrm

"Drew Dalgleish" wrote in message
...
a few years ago I was shown how to turn a 5hp merc into an 8hp motor
just by nbolting the exhaust manifold rotating it 180 degrees and
reinstalling it.

whatever you want to believe I guess. Perhaps you have a better reason
for turning the manifold over.

And how long is your leg now.........


"Drew Dalgleish" wrote in message
...
I took the mechanics word for it. It wasn't my motor and I didn't see
it in action . He did explain how and why he was doing it.

How did you measure the horsepower?

Roger

http://home.insightbb.com/~derbyrm

"Drew Dalgleish" wrote in message
...
a few years ago I was shown how to turn a 5hp merc into an 8hp motor
just by nbolting the exhaust manifold rotating it 180 degrees and
reinstalling it.

Maybe an excuse to charge $75 per hour plus an additional fee for
"hotrodding" your o/b


"Drew Dalgleish" wrote in message
...
whatever you want to believe I guess. Perhaps you have a better reason
for turning the manifold over.

And how long is your leg now.........


"Drew Dalgleish" wrote in message
...
I took the mechanics word for it. It wasn't my motor and I didn't see
it in action . He did explain how and why he was doing it.

How did you measure the horsepower?

Roger

http://home.insightbb.com/~derbyrm

"Drew Dalgleish" wrote in message
.. .
a few years ago I was shown how to turn a 5hp merc into an 8hp motor
just by nbolting the exhaust manifold rotating it 180 degrees and
reinstalling it.

On Sat, 24 Jun 2006 18:24:05 -0400, "John Cassara"
wrote:

Maybe an excuse to charge $75 per hour plus an additional fee for
"hotrodding" your o/b
Maybe but this guy works for $ 25 and I imagine that if the owner
didn't need a new prop he'd be ****ed

"Drew Dalgleish" wrote in message
...
whatever you want to believe I guess. Perhaps you have a better reason
for turning the manifold over.

And how long is your leg now.........


"Drew Dalgleish" wrote in message
...
I took the mechanics word for it. It wasn't my motor and I didn't see
it in action . He did explain how and why he was doing it.

How did you measure the horsepower?

Roger

http://home.insightbb.com/~derbyrm

"Drew Dalgleish" wrote in message
. ..
a few years ago I was shown how to turn a 5hp merc into an 8hp motor
just by nbolting the exhaust manifold rotating it 180 degrees and
reinstalling it.