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JD 318 Onan Surge


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#1 Pipeliner OFFLINE  

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Posted July 14, 2011 - 02:40 PM

I've been here before looking for answers and found them in response to other peoples questions - THANKS. This time I've registered so I could post a question.

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!

Pipeliner

#2 olcowhand ONLINE  

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Posted July 14, 2011 - 03:18 PM

I would suspect crud in the carb. Onan carbs seem to do this a lot. Usually with mine, a good cleaning & all is well again.

#3 grnspot110 ONLINE  

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Posted July 14, 2011 - 06:32 PM

My 420 (B48G Onan), has had some surging problems a couple of times. Took the top off the carb, cleaned the main jet, problem solved. Not a big deal on it! ~~ grnspot

#4 mikebramel OFFLINE  

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Posted July 14, 2011 - 08:04 PM

Easy fix. Replace all the fuel line. Blow out the tank. Install a new inline filter. Take the air filter off. Take the top of the carb off. Spray it all out with brake or carb cleaner

#5 pbenson OFFLINE  

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Posted July 14, 2011 - 09:11 PM

Hey Pipeliner:
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!
patrick

Edited by MH81, July 17, 2011 - 09:16 PM.


#6 Gary400 OFFLINE  

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Posted July 14, 2011 - 09:14 PM

my 318 is doing it also and it was dirt in the carb. the last time it ran like this . i'm sure its dirt this time just haven't had time to clean it .

#7 tinner OFFLINE  

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Posted July 14, 2011 - 10:14 PM

:welcome:: To GTtalk Pipeliner. Hope you've found your answer. There's always someone here that can help.

#8 Pipeliner OFFLINE  

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Posted July 15, 2011 - 06:38 AM

Thank you one and all!

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!

Pipeliner

#9 1978murray OFFLINE  

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Posted July 15, 2011 - 03:53 PM

try some mechanic in a bottle, u can buy it at the local lawnmower shop. It works great and will save u all the time of cleaning out the fuel system

#10 Gary400 OFFLINE  

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Posted July 17, 2011 - 05:12 PM

well before it got so hot this morning i got to work on the 318 . cleaned the carb , new fuel filter , new plugs and wires , new air filter and shes as good as new . now i just need to make up my mind if i want to sell it or not . kinda hard to sell something that works as good and hard as that little tractor does.:wallbanging::sensored:

#11 flip OFFLINE  

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Posted July 18, 2011 - 07:48 AM

Mine surged badly when I got it. I added Seafoam to the gas and within 20 min it ran smooth as silk.




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