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Two fishing trips ago (fifth trip of the year and second tank of fuel) my motor started a new problem. It is a ’96 Yamaha 90hp (90TLRU). After running fine all day I went to change spots. Cranked fine then went to open up and around 2500 rpm it started bogging down, like it was flooding out. Ran fine at idle and somewhat faster but not above +/-2500 rpm. Took it to the house and ran thru the fuel system from the tank to the carbs. No issues found and bowls were clean. Went the following weekend with the same issues. On the first run (cold engine) it got up to 3700rpm but when moving to the second spot never above 2500. Still idling and up to 2k fine. Went home and pulled the plugs and noted that they were all three wet and black. Appeared to be running way too rich. Since this motor has an electric choke, I checked it to see if anything apparent. Didn’t seem to be seized or other. Service manual and other sites had me wondering about other issues. I think there is a tilt switch and an oil level protection feature that grounds one cylinder if in error condition. The oil level is high enough to eliminate the actual level as the problem, but the sensor could be bad but doesn’t explain the wet plugs. Thinking that maybe the pump diaphragm may have a pinhole in it, theory being that at an elevated crankcase pressure fuel could be sucked into the crankcase and causing all cylinders to run rich. Another option could be the ‘spark advance’ but since it is a CDI I’m not sure how to check. Hoping to borrow a compression gauge tomorrow to check the cylinders for a possible issue. Am I on the right track or are there any other opinions on this?
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always good to know your compression but tear down the carbs,soak all parts overnight.have you checked all fuel lines and separator also? -
jimbob,
You could have a bad thermo switch and a bad warning buzzer - not likely, but could be. I'm not sure, but your model Yammie should have one thermo switch mounted in the head. Unplug the (pink wire on my C115) and touch it to ground with the ignition on - the warning buzzer should sound, indicating an overheat condition. If it does not, check connections and/or replace.
Sounds like your problem is temp related - do a temp test on the thermo switch. It could be faulty and closing below 180 where it should. When it closes, the alarm should sound and the RPM drastically reduce - this could be your "bogging down" that your are experiencing. The spark is reduced, but not the fuel, and this could possibly lead to "wet and black" plugs.
Hope this helps,
Ken K
Beaumont, TXComment
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