JD 318 Onan Surge
Posted July 14, 2011 - 02:40 PM
I bought this 318 about three months ago and had been mowing with no major problems until bringing it back to the barn the last time. Never had any hint of a surge - all of a sudden, after backing down the throttle, it revs up then drops down. Thought it may have been hot, as the weather sure was - shut it down to cool, now a week later and time to mow again. It started up all right and seemed to idle ok with the choke out a bit, but started surging as soon as I pushed the choke in. It would run until I dropped the throttle further and then stalled out.
From what I've read, could be running lean - or could be the governor linkage - or a stuck float - or a dirty carb - or -or -or.
Anyone hazard to guess what the most likely cause would be for such an abrupt change in the way it's running. Linkages under the air filter seem free and spring tension when and where you'd expect it. Doubt any water in the gas, as the same gas used the last three mowings.
Just looking for a starting point and hoping for an easy solution. Thanks for your help!
Posted July 14, 2011 - 03:18 PM
Posted July 14, 2011 - 06:32 PM
Posted July 14, 2011 - 08:04 PM
Posted July 14, 2011 - 09:11 PM
I just dealt with the very same issue on my 318.
Have you replaced your fuel filter yet? Just asking...... Is your fuel old?
Anyway, there are two likely causes: your carb is dirty or your intake manifold is leaking. The best thing to do is clean the carb first and if that doesn't work do the manifold, which is more involved.
Take off your air filter assembly and remove the screws to take off the top of the carb (carefully...) spray out all of the orfices with carb cleaner. Pay particular attention to the low idle jet, which you need to unscrew and remove: it has several tiny holes in it; also the hole it comes out of. A clogged low idle jet will make your Onan surge.
For good measure, blow everything out with compressed air. If you are lucky, that will solve your problem. Otherwise, you have the well-known Onan leaky intake manifold problem. These manifolds are actually two halves riveted together with sealant in the seam: over time that sealant develops leaks. The fix involves removing the intake manifold and re-sealing it. It takes a few hours to do, but it's really not as hard as it sounds.
The good news is that the fix really does work: my 318 is running as good as it ever has. Good luck!
Edited by MH81, July 17, 2011 - 09:16 PM.
Posted July 14, 2011 - 09:14 PM
Posted July 14, 2011 - 10:14 PM
Posted July 15, 2011 - 06:38 AM
Hope it doesn't come to resealing the intake manifold, but whatever will be will be.
I'll start with carb cleaning and a new fuel filter, and work through the list from there. We'll let you know what works.
Nice to know there are so many of you out there familiar with the Onan!
Posted July 15, 2011 - 03:53 PM
Posted July 17, 2011 - 05:12 PM
Posted July 18, 2011 - 07:48 AM