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Noel Schembri
Noel Schembri, Marine Technician
Category: Boat
Satisfied Customers: 1951
Experience:  Advanced technical diploma
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Noel, having problems with my old Yamaha autolube 40hp.

Customer Question

Hi Noel, having problems with my old Yamaha autolube 40hp. starts great tick over ok. when revving up onto the plane seems like a misfire problem coughing and spluttering trying to go but not if you know what i mean. stripped the carbs last week, all looked clean, installed fuel separator (genuine Yam) cleaned out fuel tank. compression checked out ok by mechanic. he says the motor is tired with a noisy crank. have owned the boat 3 months had a good few trips out in it then started with intermittent problem on and off no problem is all the time. when i take the air box off there is 2 stroke oil in the bottom of the box. do you think it could be a lube problem?
Submitted: 2 years ago.
Category: Boat
Expert:  Noel Schembri replied 2 years ago.

Hello, so from your information i am thinking of three problems that may be the culprit. The first is a spark misfire, remove the spark plugs and check weather one of them is black. If it is try replacing the spark plugs. If the problem persists you would need to check the spark while the misfire is happening using a spark gap tester. If the spark is ok, then i suggest you try removing the fuel line and see if the engine goes out for a few secs before it goes out of fuel. If yes then the problem here is the fuel pump in the engine. This would require to be rebuilt. Finally it could be a bro***** ***** valve problem. To check them you need to remove the air box and see if the carbs are sputtering back fuel out . If yes then one reed may be broken. As for the engine, if the compression is ok it should run ok irrelevant of the noise.