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1989 Johnson 15hp 1/2 power

B

Bnecaise74

New member
Hi guys, my 15hp runs about 1/2 power. I can take the cowl off and place my hand over air intake and it will pick up power but still doesn't run wide open. What are some things I can start checking to narrow my problem down.
 
while engine is running pull one spark plug wire off at a time and see witch cylinder isn't working, when cylinder is working you notice a big change in engine running n may even stall engine out if other cylinder isn't working at all, if cylinder isn't working engine Will run the same n not change, but doing a compression test is a good idea as well so you don't waist a lot of time or money, those two tests will narrow it down
 
Do all the above to confirm engine health. Given your description this sounds like a fuel delivery issue. Usually if the engine runs poorly or with low power but you get better performance when you restrict air flow it means the engine isn't getting enough fuel. Restricting air flow brings the air/fuel ratio closer to what the engine needs and gives you more power.

Once you confirm engine health you should check the following things:
1) Is the fuel tank venting properly?
2) Primer bulb OK?
3) Fuel line in good shape?
4) Is the fuel pump working properly?
5) Are all fuel passages in the carburetor clear?

KJ
 
When you remove the cowl and the engine revives, it can indicate an exhaust leak. Not sure what holding your hand over the air intake did but most likely was a coincidence or it restricted the exhaust from getting in and only allowed air in from another opening. Try it again and leave out the hand over the intake part and see if you get the motor to run well again. If it does, you have a leak in the exhaust which is starving the motor for air when the cowl is on and the leaked exhaust cannot get out.
 
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