N875ED 98 #1 Posted May 18, 2020 Okay, searching through the forum it is easy to find MANY instances of Onans surging...with most advice suggesting a carb cleaning. But, mine has a weird issue in that it only surges after it is very warm and been running for 30 minutes or more. Until then, no problem; after that the RPM will vary by +/- 50% (3600 hi followed by 1800 RPM low, oscillating up and down) clearly controlled by the governor. Any advice by the Oracles of Red Square? Share this post Link to post Share on other sites
lynnmor 7,817 #2 Posted May 18, 2020 If you set the throttle at 3600 and it never goes faster, that rules out the flyball spacer. If you set it at 1800 and it increases when hot, probably is the flyball spacer. As an engine heats up the fuel air ratio needs change, so dirt in the carburetor is a possibility. A missing or leaking welch plug in the carburetor can cause surging, it is located just above the idle mixture screw. The intake manifold is made from two pieces and that seam might be allowing air in, but at high throttle it probably wouldn't have a great effect. Intake manifold gaskets can leak and cause your symptom. If the valve clearance is too small, engine heating will cause valve leakage, also a valve seat may loosen as the engine heats. Both of these issues need to be ruled out as major damage can happen. If you haven't adjusted the valves, checked the compression or removed carbon from the heads, now is a good time. 1 Share this post Link to post Share on other sites
Tuneup 1,452 #3 Posted May 19, 2020 You and me both. Post decarbon, new gaskets, sealed intake and spotless carb with the missing welch plug replaced, it runs great for about an hour of hard work (516H) and then begins to surge enough to have to park it in favor of the 125. Surge effect alters with RPM level setting. Maybe unlike you, I have been messing with the governor setting since the beginning to try and improve it. I'm not an Onan man so am not familiar with its behavior but it gives a hard 'snort' when you give it throttle and then settles. Much more than a Kohler or any other engine I've messed with. Drop the throttle and it lowers quickly and then snorts to recover. Seems to aggressive. If I reduce the governor effort, it goes away but then is over governed. Sticking by to hear from others. Oh, yeah, I do have those cheap cardboard intake gaskets thanks to a deal on eBay. I'll have to remove that variable... I do love the sound of that twin... Share this post Link to post Share on other sites
