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Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 11052
Experience:  40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
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My 89JohnsoN 150 hp will not accelerate. Cranks and runs

Customer Question

My 89JohnsoN 150 hp will not accelerate. Cranks and runs good. But will do better if I hold the chock in as I accelerate. Have good fuel pressure. And clean crabs.Did have too much oil in the fuel but pumped it out and refilled with proper 50/1 ratio oil. run the engine for several hours with the same results. What am I missing
Submitted: 1 year ago.
Category: Boat
Expert:  Tan replied 1 year ago.

Hello. My name is Tan.

This may take more than one reply to solve your issue.

Could I get the engine model number please?

The allowable difference between cylinders is 15% on compression. So with one cylinder at 90 there is an issue.

If the engine isn't pulling on all cylinders it will be weak.

When you used the scope on the low cylinder did you inspect the ports in the cylinders?

The piston fails at these ports and the rings can break off.

The factory spark plug for this engine is a Champion QL77JC4. What did the spark plugs look like that you replaced?

Since the scope didn't reveal anything obvious you could have a bad head gasket.

When you checked the spark how did you check the strength of the spark?

Have you checked that the timer base is not sticking?