Long story short, my always reliable engine
became unreliable this weekend:
-Yanmar 2GM20F, in 2002 Hunter
-recently serviced, with new mixing exhaust
U pipe & valve adjustment
-ran perfectly for ~10 hours or so after service
Last Friday I headed out for a long weekend with friends. She ran great all day. Come Saturday morning, she just wouldn't want to keep running at low RPM
or run for more than a few seconds. The starter battery
was good at 13.4V and the engine
would turn properly with the starter, but with 0-to-1/8th throttle, it would run *very* roughly for 2-3 seconds before cutting out with a heavy shudder. With ~1/5th throttle, she'd run rough for 4-5 seconds before coming to an abrupt and bumpy stop.
After a couple dozen tries, and an attempting at bleeding the fuel
lines (and a few more rough starts), I was able to get the engine started. It would run extremely rough and want to poop out under 1/4 throttle, but when underway, and/or above 1/4 throttle, she would run perfectly and smooth as can be. We headed back home under cruising speed (and even WOT for a few moments) for the next few hours without any sign of engine trouble.
However, when I'd throttle back, she'd still run terribly rough and would cut out again - requiring ample goosing of the throttle, swearing and much luck to get fired up and back running smooth at a ~1/4 throttle.
Of course, once I cut the trip short by a day, motored home for 4-hours and got her back into her berth, she seemed to run perfectly fine at normal idle speeds and would start and stop again like there never was a problem to begin with.
I'm curious if any of you might have an idea as to what was causing this somewhat intermittent but very problematic behavior?