RevTractor 1 #1 Posted July 22, 2020 I was mowing with my 520H and when I made a sharp turn the engine began to run erratically and the RPM's started going up and down. The RPM's are still revving up and down. I pulled both plugs and one looked fouled while the other was clean. Both have less than 15 hours of run time. It cranks right up and will run but it keeps revving up and down and sounds like one cylinder is misfiring. Anybody have this experience before? Got any suggestions on where to start looking for solutions? I have the 20 HP Onan engine. Share this post Link to post Share on other sites
Ed Kennell 42,403 #2 Posted July 22, 2020 Clean the carb. The Onans are very sensitive to dirt. The sharp turn may have dislodged a speck of dirt in the bowl. Share this post Link to post Share on other sites
lynnmor 7,904 #3 Posted July 22, 2020 As above, clean the carburetor. Your engine speed should be 3600 RPM while mowing, when it acts up, did the speed climb higher than that? There is a welch plug about 3/8" in diameter just above the idle mixture screw on the exhaust side of the carburetor, is it still there? Since you have one dirty plug, check the compression. I hope that you don't have a loose valve seat, was the bad plug at the rear? Is the rubber around the oil filter there filling the gap? Are the cooling fins clean? Yes, there is a reason I am asking this. 1 Share this post Link to post Share on other sites
Bill D 2,308 #4 Posted July 22, 2020 Can someone please move this to the engines section. Share this post Link to post Share on other sites
RevTractor 1 #5 Posted July 23, 2020 Hi Bill, I'm new at this. Tell me how to move it and I will gladly do so. Bill Share this post Link to post Share on other sites
RevTractor 1 #6 Posted July 23, 2020 Ed and Lynnmor, I will grab a can of carb cleaner and dive in on Saturday morning. Thanks. Bill Share this post Link to post Share on other sites
RevTractor 1 #7 Posted July 23, 2020 Ok, I removed the air cleaner housing and sprayed the exterior of the carb. Dried everything off and replaced the housing. I took photos and the welch plug is intact. The fouling plug was the front plug. Compression is unchanged. I started the engine and sprayed inside the carb as the engine was running at about 3,000 RPM's. Every time I sprayed the engine ran smoothly once it got over the initial spray, then it went back to rough idle. I tried pouring some SeaFoam into the carb while running with the same results. I switched the engine off and sprayed directly into the jet on the carb, replaced the fuel filter and replaced the air filter and cover. This time when I cranked the engine it ran as smooth as silk. It appears problem solved. Must have been a bit of trash in the carb. Thanks! 1 Share this post Link to post Share on other sites
