Thanks to all for the info. I will leave as-is for now, then add the
nitrous injection when I mount the supercharger...;-)

I got the changing the mix for added boost bit from motorcycles. Some
(amateur) racers I know cut down on the oil during time trials and
races to boost performance. I think it's only a couple of percent,
though. Could be yet another myth?

Hey, speaking of myths, maybe I'll try the cow-magnets, the Pro-lube
additive, Marvel Mystery Oil... or maybe I'll just go on a diet to
improve the weightower ratio!

Ah, who needs to plane a dink anyway?

wrote in message
oups.com...


I got the changing the mix for added boost bit from motorcycles. Some
(amateur) racers I know cut down on the oil during time trials and
races to boost performance. I think it's only a couple of percent,
though. Could be yet another myth?


There must be something to it ... although probably dangerous for the health
of the engine.

Ever notice how a chain saw engine starts to have more throttle response and
revs higher just as it is running out of gas/oil?

Eisboch

www.eisboch.com

My brother had a Kawasaki 185 Enduro once. It had an oil reservoir and an
auto-oiler system for lubing the fuel for the 2-stroke. One day we were
riding and he was really excited! "Wow! The bike's going REALLY fast
today! I wonder why?" About 5 minutes later the motor seized up ...the
tube from the auto-oiler had fallen off. So yeah, running short on oil
gives you extra power ...for awhile. Race motors are pretty well abused
anyway, so making them temporarily 'do' with a little less oil for a race
wouldn't surprise me, especially in the class races where the motors all
have the same displacement and/or the races are shorter.

Brian


wrote in message
oups.com...
Thanks to all for the info. I will leave as-is for now, then add the
nitrous injection when I mount the supercharger...;-)

I got the changing the mix for added boost bit from motorcycles. Some
(amateur) racers I know cut down on the oil during time trials and
races to boost performance. I think it's only a couple of percent,
though. Could be yet another myth?

Hey, speaking of myths, maybe I'll try the cow-magnets, the Pro-lube
additive, Marvel Mystery Oil... or maybe I'll just go on a diet to
improve the weightower ratio!

Ah, who needs to plane a dink anyway?

the idea is that these motors use the same engine block ...do the researce
if that is true then you can do it but i suspect the cost will be more than
what is resonable
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!

wrote in news:1150777597.088590.179300
@p79g2000cwp.googlegroups.com:

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.

WHOA! Look more closely at any outboard motor!

That's not a 7.5 horsepower outboard motor! It's a Johnson 7.5.

7.5 is a model number or "trade name". It doesn't say "Horsepower" on it
for the last 50 years! They USED to say HORSEPOWER on them, long ago,
until someone pointed out that was a lie....maybe the FTC?

I owned an Evinrude 55, 3-cylinder outboard on a tri-hull runabout. A
friend had an Evinrude 70, 3-cylinder outboard a few years newer.

We couldn't find any difference...

I call it "Numeric Inflation"....same phenomenon as in consumer stereo
equipment.

In article .com,
says...

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!


DO NOT screw with the jets!!! That is not the answer. You need more
AIR and FUEL!!! Not more fuel. The differance is that the bore (throat)
and throttle plate are bigger in the 15hp carb. Take note they are
not 'Twice' as big. Air flow goes up as the square of the diameter.
If the hole was twice as big it would flow four times as much air.
The differance between the 9.9 carbs and 15 carbs is remarkably small.
Remember also it takes 3 to 4 times as much power to go twice as fast.
Changing your oil mix and getting 7.7 horsepower out of a 7.5 isn't
worth the risk of a scored and ruined engine. I ski behind my 70 h.p.
and I always run 40:1 instead of 50:1 because I run my engine hard.
It's a 1981 and never been apart!! If you want to get 7.7 out of that
7.5 switch to synthetic lower unit gear grease!! It helps!

Bless and best wishes.

Try a flatter pitch prop. That will have as much or more effect than a fin
when trying to plane an inflatable.

David

The only such conversion I have heard of that is documented is the Mercury
9.9 to 15. Bass and Walleye boats magazine did a write up on it last year
soemtime. It does not put out the same as a stock 15, but its close.


--
Bob La Londe
Fishing Arizona & The Colorado River
Fishing Forums & Contests
http://www.YumaBassMan.com



--
Posted via a free Usenet account from http://www.teranews.com

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