1972 20hp starts in driveway but not in water

gil57usa

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Jul 28, 2011
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3
When I put the muffs on the lower unit for water in the driveway, it starts on the 2nd pull every time. When I put the boat in the water, I can pull until I pass out and it won't start. The only difference I can see is that there is more exhaust back-pressure in the water due to having to push the exhaust through the prop while under water. Last year the motor got 2 new coils and new points and plugs. I'm thinking a carb problem. Any suggestions what to look for or check? The carbs have not been touched in the 5 years I have had it.
 

fishkilla22

Cadet
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Jul 20, 2011
Messages
21
Re: 1972 20hp starts in driveway but not in water

Have you checked the compression on both cylinders yet? I had this same very issue, checked compression and found they were not within tolerance. I beleive (don't hold me to it) that they have to be within 10lbs from one another. With one cylinder having low compresion the motor will run on land with muffs but not in a barrel with water or at the lake. Solution-complete rebuild.
 

gil57usa

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Jul 28, 2011
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Re: 1972 20hp starts in driveway but not in water

Have you checked the compression on both cylinders yet? I had this same very issue, checked compression and found they were not within tolerance. I beleive (don't hold me to it) that they have to be within 10lbs from one another. With one cylinder having low compresion the motor will run on land with muffs but not in a barrel with water or at the lake. Solution-complete rebuild.

I have not checked the compression. Do you recall what it should be?
 

Mi duckdown

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Joined
Apr 14, 2007
Messages
2,575
Re: 1972 20hp starts in driveway but not in water

It's in the 100 psi range. But check them both and report back..
It is the easiest check, and the least costly.
Although I don't think that is your problem. unless you put in gear and it goes no where.
 

CharlieB

Vice Admiral
Joined
Apr 10, 2007
Messages
5,617
Re: 1972 20hp starts in driveway but not in water

Dirty carb, reduced idle fuel flow.

Same symptoms of a freshly cleaned carb that doesn't have the idle mixture fat enough.

Runs great on the hose, but the additional exhaust back pressure once in the water requires more idle fuel.

Clean the carb, install new gasket kit.
 

gil57usa

Recruit
Joined
Jul 28, 2011
Messages
3
Re: 1972 20hp starts in driveway but not in water

It's in the 100 psi range. But check them both and report back..
It is the easiest check, and the least costly.
Although I don't think that is your problem. unless you put in gear and it goes no where.

I have not checked the compression with a gauge, but believe me, there is plenty of it when I pull it over. Last year just before the end of season, it had plenty of power and ran very well. I don't think the compression went bad sitting for a few months. I'm leaning towards a carb/fuel issue.
 
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