WheelHorseVH 103 #1 Posted July 27 Hey guys. I am a bit stumped with this one. Onan P220G, adjusted valves very recently and brought them into spec but did not yet do a full disassemble and clean on the carb though. On hot starts I started getting a backfire. Never while idling, transitioning off idle, WOT, or under load. Just upon hot start. Its a backfire through the muffler. For what its worth, I never shut down until I am throttled back down to idle for about 5-10 seconds, then turn the key off. My thoughts for what they’re worth…leaking exhaust valve? Float level too high? Anyone have any ideas? Appreciate it. Share this post Link to post Share on other sites
lynnmor 7,218 #2 Posted July 27 Check compression, report results. While you had the intake off you should have checked it for leaks and you should have cleaned out the carburetor. Check the vacuum hose for leaks. The cylinder heads may need to come off for a thorough decarbon. Share this post Link to post Share on other sites
WheelHorseVH 103 #3 Posted July 28 @lynnmor thank you, I will do the compression test and post back. Oddly enough, I have never owned a compression gauge so I will need to run out and buy one. As for your other points, I feel like an idiot not taking the extra step of at the very least cleaning the carb while I had the intake in my hand, especially knowing it attaches from the bottom and I would need to remove everything again to get at it. I did replace the vacuum line with new so that is the only item I can likely rule out. Share this post Link to post Share on other sites
lynnmor 7,218 #4 Posted July 28 17 minutes ago, WheelHorseVH said: @lynnmor thank you, I will do the compression test and post back. Oddly enough, I have never owned a compression gauge so I will need to run out and buy one. As for your other points, I feel like an idiot not taking the extra step of at the very least cleaning the carb while I had the intake in my hand, especially knowing it attaches from the bottom and I would need to remove everything again to get at it. I did replace the vacuum line with new so that is the only item I can likely rule out. There is little clearance for a gauge at the rear spark plug hole so be sure the one you buy can get in there. I use the old fashioned type that you press in by hand and believe it or not it fits. When looking at the intake you of course need to inspect the seam where the two halves meet. A good test would be to block off the mounting holes for the carburetor, turn it upside down and fill it with gas, then watch for leaks. Also, the heat shields over the exhaust pipes can cut into the manifold so be sure there is clearance, I grind the shields back so they give about 1/8" and can't touch the intake. Share this post Link to post Share on other sites
WheelHorseVH 103 #5 Posted August 6 Working on getting a domestic version of a compression gauge instead of something off A—zon. Been thinking of this a bit, does it make sense for me to begin the disassembly of the intake in the meantime? I think the carb needs a cleaning no matter what I find. Then, I am assuming I’m going to find a burnt exhaust valve… Share this post Link to post Share on other sites
lynnmor 7,218 #6 Posted August 6 You can check the compression with the intake removed but be sure that fuel line TO the fuel pump is removed to prevent gas from spraying. When you adjust the valves something may be learned if you find them way out of adjustment. Any compression tester should be OK, a good one would be cost prohibitive for the few times it will be used. Share this post Link to post Share on other sites
Tuneup 1,433 #7 Posted August 10 Just a note on cheap gauges. May be fine, maybe not. I bought a Harbor Freight model and its low readings almost had me pulling my CB550 engine apart. The interesting thing is that all 4 cylinders read the same: 90 psi. Pulled my old push-in model from the 70s when a kid and it read 145 all across. She's fine. HF Gauge - not! 1 Share this post Link to post Share on other sites
