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Tan
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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I have the same problem as the post I'm reading. 1980 70hph

Customer Question

I have the same problem as the post I'm reading. 1980 70hph Johnson idles great, starts great, put it in gear and 3 mph is it. It's been a rebuild project and today was the first day in the water
Submitted: 1 year ago.
Category: Boat
Expert:  Tan replied 1 year ago.

Hello. My name is Tan.

You have 3 things that can cause this issue. Loss of spark to one or more cylinders, low or no compression on one or more cylinders and finally poor fuel delivery.

We can eliminate compression as you have good compression. Leaving fuel delivery and spark. So we need to check spark on all cylinders with a spark gap tester. The spark must jump a 7/16" gap on the tester. Then if we have good spark that leaves carburetors.

A test you can do for the carbs is to run the engine put it in gear then as you throttle up push in on the key choking the engine momentarily. If the engine start to run better then you have dirty carbs. If no change when throttling up squeeze the primer bulb. Does engine run better? If yes then you have a bad fuel pump or a fuel restriction in the boat side fuel system. To determine which run the engine on an auxiliary fuel tank. Does engine run better? If yes then the boat side fuel system is the cause. If no change then the fuel pump is failing.

Now I know you said the fuel pump and carbs had just been done. But just been done how long ago? One of the 3 things I listed are the only things that will cause your issue.

If all of these check out you could have an issue with the rebuild. Was this rebuild done by a professional marine technician for Johnson or Evinrude motors.

Was the engine test run by the rebuilder? Since the engine was just redone why isn't the rebuilder involved with the repair? You should have gotten a perfectly running engine if it was just redone.

Every time I rebuilt an engine I took the boat for a test ride making sure the customer got a perfectly running engine. Turn key ready. They didn't have to trouble shoot why it didn't run right. They paid for a perfectly good running engine.

Post back with questions and findings.