Any help with a Yamaha 4 stroke?

Apr 17, 2012
West Michigan
Working on the family's pontoon with an old Yamaha T50 (1996 model I think). Past few years its been running rougher and rougher and this year it would barely stay running. Did a full tune up with OE plugs and coils but reused the boots. Properly gapped. Also took the carbs down and cleaned them out- all rubber seemed intact and all passasges seemed patent when sprayed through with carb cleaner. The floats and needles moved freely. Fuel filter inspected and looked totally clean (only the basic inline filter on this). Still runs rough, looses power under load, stalls easily. It does idle much better after all that, but still not great.

Carbs seem sync'd rather well on my gauges. Fuel lines are patent and the primer bulb works well. Compression is even, about 186psi on all four. Spark is excellent when tested with a spark tester. Fuel pressure seems questionable at 2-4psi, but it doesn't help at all to pump the primer bulb. I have tried running a new fuel line straight out of a jerry can of good gas (after draining carb bowls) to eliminate contaminated gas- no change.

The only thing I can think is either the fuel pump. Or else the carb mix is all wrong somehow. Any thoughts? I hate to shotgun parts at such an old engine but not sure where else to go here.


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If it is the fuel pump it will be easy to tell. Have a passenger squeeze the primer bulb while trying to accelerate. If operation tries to improve while squeezing then you need to replace the fuel pump or at least rebuild it. The valves on many fuel pumps are thin pieces of plastic.