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Old 25 August 2010, 08:26   #1
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Give my engine more power ! Please..

Hello Everyone,

I have a 1982 Johnson seahorse 7.5HP Outboard, twin, My question is :

Is there anything i can do to improve the engines power ?

Are there any carb mobs that are avalible ?

I ask this because i can get my 3.2m sib to plane just with two people if i use my mate as a mobile weight ballast. I just need a little more power so im not screaming out the engine everytime.

I have done simple things like the plugs and so on. I have also played around with trim and tilt settings.

If anyone knows of anything i can do to improve the power, accept for getting a new engine, i would be grateful.

Cheers..
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Old 25 August 2010, 08:49   #2
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There may be a few things you can do. I'm not familiar back to 1982, but you will need to look and see which hp models shared the same block i.e same cu inch dissplacement. To my knowledge, and as with most manafactuers, they use the same block between 6 hp and 15 hp. Just like 20-25 ect ect. But there were a few that didn't, and from seeing pictures ect, i vaguely remember that shaft being diffrenent on the 6 hp compared to the 8, so it's possible that the 6 hp might have been the largest in that dissplacement..

Otherwise, most commonly the carbs can be either larger, or they can be the same units with the addition of a restrictor plates on the smaller hp models. You should find also that the timing would be a little more advanced on the higher hp model. Some models also have a different exhasut tube...
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Old 25 August 2010, 08:55   #3
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You beat me to it!

One thing to watch tho' as it's 2- stroke - don't advance your timing too far, otherwise you'll end up forcing your pistons into the side of the exhaust ports which is an expensive, messy business...

www.old-omc.de has a shedfull of old Ev/ Johnson brochures scanned, so you should be able to work out the block thing from there.
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Old 25 August 2010, 17:49   #4
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I have just checked out the johnson BRP parts lists and it seems that the carbs on the 7.5 HP and the 15HP are the same as they have the same model numbers for the parts...

Does this mean there could well be a restrictor plate ? If so is it situated between the carb and the manifold ?

When i was 16, a good few years ago now i had one of those annoying hair dryer sounding 50cc moto bike and we used to take of the restrictor which was situated in the exhaust manifold. Would it be similar to this ? After removing it is advised to replace the carb jet to allow more oil so a seize does not happen.

So what im saying is if the carbs are same and there is a restrictor plate, can i just add a little more oil to the fuel mix or is this not right.

If you might no where a plate might be please let me know, Also thankyou both for your first messages.
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Old 26 August 2010, 03:52   #5
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When you hear the term restrictor plate, it is typically just a gasket, usually made of rubber. Depending on the type of carb, it's most often found at the throat of the carb, basicaly limiting it's air intake.

In most cases you won't need to buy the largr gasket, you can just cut the old one so that the hole in the gasket is the same diamater as the throat of the carb. Allthough given the age of the motor, it's possible if it's the original gasket it may be perished, in which case you would need to replace it with either an OEM or aftermarket equivelant.

No need to change fuel mix, just check the timing settings of the 15 hp , and make sure yours is set to the same.

In the case of my tohatsu, the timing needed to be advanced by a further 5 degrees.

Sorry i can't be of any more assistance or offer any more detailed info, i was born in 85, and the majority of outboards i've owned have been from atlweast 90 onwards...

Best of luck
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Old 26 August 2010, 03:55   #6
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If you have found the parts lists, I'm afraid the only thing you can do is go through them component by component.

If you take the air box off the front, have a look at it when it's at "WOT" & see if the butterfly opens fully.

This is a a "high level" comment, not knowing what you will find. Assuming it is "simply" restricted, then you will have the equivalent of running a 15 at no more than, say, 3/4 throttle, so the 50:1 (or whatever the ratio is) will not change. Whether you need a bigger main jet to get more fuel in to generate the power is another discussion entirely. Assuming it;s restricted, you probably could drop the "bigger" carb in, as it would be a bit like buying a Porshe & never driving faster than 40. (i.e the power is there, but someone parked a block of wood under your acceperator pedal so you canlt use it)

IF that's the case, you may find the throttle cam is a differnet shape so the de-rated machine keeps the linear feel to the throttle. You'll probably need the likes of a Clymer to work out if the spark advance needs changed.
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Old 26 August 2010, 13:02   #7
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Try some 110 octane race fuel ... and hope the rest of the motor doesnt wear out fast
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