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misfiring 40hp two stroke twin

J

jopdegruijter

New member
Hi,

I've just bought a Mariner outboard which gives me a lot of trouble. It's an engine that is 95% equal to the Yamaha 40 hp, manufactured in 1990.

The bottom cylinder only fires at high speeds (>60% throttle). Both carbs are clean, I changed the ignition wires (1 coil for both cylinders, fires on TDC and BDC), I swopped the reed valves, tried the upper carb on the lower cylinder, measured 10 and 9,5 bar compression (145/137 PSI), I also replaced the lower crankshaft oil seals.

Still misfiring....... Any thoughts?
 
If i can remember correctly the fuel supply pipe has a tee piece in it after the fuel pump check for tar build up in the fuel supply pipe sounds as if the botton cylinder is been starved of fuel.
 
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