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Charles, Marine Mechanic
Category: Boat
Satisfied Customers: 3475
Experience:  US Navy (Ret) Certified Mercury Marine Tech with over a decade of repair experience.
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My 40 Yamaha 4 stroke is running on 2 cyls have near new

Customer Question

My 40 Yamaha 4 stroke is running on 2 cyls have near new plugs. This happened after we had just used it and was running well. any ideas ?
Submitted: 1 year ago.
Category: Boat
Expert:  Charles replied 1 year ago.

Hello, my name is***** will assist you today.

Would you please post the full model number of the engine?

Is the fuel new?

Customer: replied 1 year ago.
Model 40BWHD Serial 67CL(###) ###-#### The fuel is new The motor had a new head gasket fitted recently compression 1 80psi 2 60psi 3 90psi After 10 hrs the head was retensioned and new oil and filter fitted by Yamaha agent
Expert:  Charles replied 1 year ago.


Is the compression numbers PSI or kPa?

Did you do the test or the agent?

Was the test done before or after the head gasket?

Customer: replied 1 year ago.
compression is psi as stated and was done by agent after head gasket replaced
Expert:  Charles replied 1 year ago.

The numbers are way low, even if the engine was a 2 stroke engine. For a 4 Stroke engine, it should be upwards of 160 PSI. this is done with the engine cold and the throttle at wide open. Also the numbers need to be with-in 15% of each other. You also need to have a leakdown test done. I feel that your rings are either carboned up or you have a valve issue.

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