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Jeremy R.
Jeremy R., MarineTechnician
Category: Boat
Satisfied Customers: 9020
Experience:  Multiple factory certifications 20+ years of experience.
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I have a 1978 mercury 2 stroke 40 hp marine motor. when i started

Customer Question

i have a 1978 mercury 2 stroke 40 hp marine motor. when i started it up this spring it will not run below 1/2 throttle.
Submitted: 2 years ago.
Category: Boat
Expert:  Jeremy R. replied 2 years ago.
Hi, My name is***** you for letting me try and assist you with your problem.
Typically when this happens one of the butteryfly valves isn't closing like it should.
You need to get so you can see down the throat of the carbs and with a flash light look to see if you can see which one isn't closing right.
If they are all closed then your problem is a little more indepth.
It's going to be an air leak or you're going to have to rebuild the carbs.
If it's an air leak this can happen at the flywheel, and anywhere between the carbs and the block.
When these engines have an air leak they run lean and rev up.
Same can happen if they're just running a little lean because the mixture is lean from gum or varnish in the fuel system.
One thing you might try after you verify the butterfly valves are closed is run some seafoam through it and see if it tames it down.
Use the choke if you have to to knock it down a bit.
1/2 throttle seems like it would be alot for an air leak or lean mixture.
Hopefully it's just linkage not set.

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