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9.9 Faulters badly after running for an hour

Beckoning

Member
In 2016, I posted a similar request and received no responses. Someone out there must know how to solve this problem. My 9.9 (presumably 2009) Mercury 4-stroke kicker motor runs good for an hour at 1200-1400rpm, then it begins to cough and sputter and run on one cylinder. I put it in neutral, rev up the engine (3000rpm) to clear it out, put it back in gear (thus, put a load on the engine), and it returns to running on 1 cylinder. It has been doing this for 4-5 years. Two years ago, I changed the prop to a shallower pitch so the motor would run at higher rpm to maintain 2.5mph, and that seemed to solve it for awhile, then it started again. Back at the hoist, I removed the plugs, clean them or replace them, allow the motor to completely cool down, and it runs fine for an hour, then loads up again. There are no carburetor adjustments...it is all preset at the factory. Most times, when it runs on one cylinder, and reduce the speed to put it in neutral, the motor dies. I crank up the throttle in neutral to restart it, and black soot covers the water behind the boat (don't tell the EPA). I use only ethanol-free gas, keep the tank full, and have replaced the fuel filter, and make sure the choke is fully open. When it has run well, I use it to drive the boat (19', 3000lbs) at nearly full throttle to burn out all the old fuel, and it still repeats the process the next time out.

I really need some help with this and hope someone has dealt with this problem and give me some good advice.
 
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