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!

Going 100/1 on the mix won't get you anything.

All you need to do is get the parts list for both outboards and compare the
lists.
All the answers come to those that do the most homework.

-W

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 message
oups.com...
O.K.: Urban myth or not?





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

Where'd you get that BS? Changing the mix will get you a scored cylinder.

Even if the displacement is the same, there will be enough differences in
parts that the parts bill will exceed the cost of selling the one you have
and buying a 15. Additionally, your 7.5 will not have any more value at the
end of the exercise. Your call.
Steve

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!

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!

AMPowers wrote:
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



All of this is true.

Now imagine that an engine manufacturer wants to offer a range of engine
sizes, but wants to minimize the cost of doing so. Make the 15 HP
motor, and install a smaller carb on it, and voila! you have a 9.9 hp
motor. This was the tactic followed by Johnson/Evinrude for a long time
( and is a common practice in the automotive industry as well... how
many V8 engines were sold with small 2 BBL carbs?).

Changing the jets will only screw up the mixture. The ratio of gas to
air is critical, and is carefully established by the jet size. The
AMOUNT of fuel/air mix is controlled by the size of the carb.

But Steve's advice about just getting a 15 HP motor is well worth
considering...

bob
s/v Eolian
Seattle

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.


You've heard wrong. That engine is a 6/7.5/8 hp. The conversion you
are wishing for is the 9.9 - to - 15 hp, those two are nearly identical
except for the carb and, on earlier models, the exhaust tuner.

Rob

trainfan1 wrote:
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.


You've heard wrong. That engine is a 6/7.5/8 hp. The conversion you
are wishing for is the 9.9 - to - 15 hp, those two are nearly identical
except for the carb and, on earlier models, the exhaust tuner.

Rob

That is the same thing that I heard (9.9 and 15 hp nearly identical).

Bill

Don't know about the 15 hp, but I do know that incresing fuel/air flow
increases hp for car engines.
I like the idea from Robb suggesting trying a carb off a larger motor.
Changing the mix won't increase hp. Variable Ratio Oil (VRO) pumps adjust
the mix from 50:1 to 100:1 based on manufacturer speced lubrication
requirements for the operating range RPMs.
Going to a 100:1 will burn cleaner, but not properly lubricate the engine
thru the RPM range.
MMC

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!

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?