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My 90 hp Yamaha 2 stroke misses at idle, but will stay cranked, just shakes a little bit. When I accelerate it seems to be running on only two cylinders (misses) through mid range rpm. Then all cylinders kick in and it runs at high speed with no miss. I have changed the plugs. Drained tank and put in new gas and cleaned the carbs. checked compression and all cylinders were around 120 psi. What do I try next??
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Check for bad fuel pump diaphragm causing raw fuel to be sucked directly into that cylinder crank area causing very rich,fuel-wet spark plug condition. Remove fuel pump but leave hoses connected, then pump the primer bulb and if fuel comes out of the back of pump then it's bad, easy and fairly cheap($45) to replace. Check entire fuel system from tank pickup, fittings, primer bulb, quick disconnect on front of cowl, pump, filter, for leaks, bad O-rings, etc. Your carbs may need to be cleaned. Good Luck!
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