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Hello. My name is Tan.
If you squeeze the primer bulb will it get firm?
What do the spark plugs look like?
Have you tried adjusting the idle jets # ***** to get the engine to run better? http://www.marineengine.com/parts/yamaha-outboard-parts/img/62/03/4972/0005.gif
Ok is this with the engine in the water in gear?
Try squeezing the primer bulb when it starts to act up. Do this at idle and when you are running at full throttle.
OK. What we are doing is eliminating the possibility of fuel starvation. Chugging can be not enough fuel. The backfiring at full throttle leads me to believe we have a fuel delivery issue.
Ok this next test will require the use of a timing light to see if we have a misfire. Connect a timing light to the top cylinder run the boat watching the flash of the timing light. You should have a steady flash with no dead spots. Do this test fro each cylinder. If you find a misfire move that cylinders ignition coil to another cylinder check spark again. Does the misfire move? If yes you have a bad ignition coil if no then we have another ignition component failing. If spark is good on all cylinders then we have a fuel related issue.
Post back with findings.
Ok that's fine I will be available all week.