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I have a 2001 90 hp 2 cycle yamaha that runs great at 2k rpm and up. I've owned the motor for 10 yrs. and the problem just started. When boat is at idle or slow speed it runs rough. It appears that the top cylinder is carrying the other 2. When plug wire is removed from top cyl the motor stops. When the middle plug wire is removed, there is little to no affect. When the bottom wire is removed there is a little difference in performance. Carbs have been removed and rebuilt, and thoroughly cleaned. Changing the coil wires from 1 cylinder to another makes no difference. It seems like it's not burning all the fuel that is being put into the motor. Can smell fuel in exhaust, and can see fuel in water only at idle and slow speed.
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check the fuel pump diaphragm, they should be changed about every 3 years as maint. -
I rebuilt the diaphram. This didn't fix the problem. It seems that the problem is electrical. I switched the coils to test and it didn't change the running problem. The center cylinder doesn't fire properly until the motor reaches 1500-2000 rpm. After the motor reaches higher rpm, it blows heavier smoke then clears up as the accumlated fuel is burned off. The motor runs as it should at high rpm, buts loads up at low rpm and runs terrible. I'm also using an excessive amout of fuel at idle speed, I seem to be getting little to no spark from center cylinder at low rpm.Comment
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Test inputs to CDI like the manual states with proper test equipment.
May have problem with the inputsComment
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