Need advice on an Onan

[zeedubbya 18]

I recently picked up a 520h and the po said it needed a tune up. He also told me it flooded easily. I got it home and it Started and ran pretty rough, but intermittently it runs great. It's a beast when it runs good. So I did plugs, wires, new fuel filter, air filter, all new gas, and cleaned fuel system and carburetor out. Also ran sea foam through it. Still ran rough and started even harder I think. I am getting a little frustrated with it now. Here's my three main symptoms--gas blows out top of carb, and it will just barely stay running. Throttle plate doesn't want to seat all the way down unless forced for some reason (throttle linkage won't return it back closed). If I can get it running, after about ten min of warming up it runs pretty good. Do I have spark issues? Is it a valve issue (this is what I think it is)? Ignition? Any recommendations would be appreciated. I love this machine and want to keep it going.

[Hodge71 663]

I know that if its spitting gas out of the carb it could be a valve issue. From reading past posts, I believe theyre too tight tolerance wise when it pukes out of the carb. Also the 20 horse Onan is famous for spitting the rear valve seat, I hope that this isnt your issue as Onans are rather expensive to repair. You also might want to try taking the carb right off and throwing a rebuld kit at it and resetting the float and mixture screws back to factory specs. I know with my 18 auto that Seafoam doesnt fix everything. Sometimes it makes things worse as it loosens up all the crud in the bowl and carb. I ended up having to pull the carb off the Kohler and rebuild it because it was just too dirty to run right no matter how much seafoam I ran through it.

[MarkPalmer 80]

It sounds like you did all the standard "tune-up" procedures. I would try kitting the carb first as it can't hurt and might solve the issues. Then part of its regular high hour maintenance procedure involves cleaning the carbon buildup from the valves and combustion chambers. Knowing that, you can go in and get a good look at the general condition of the engine, checking the valves, valve seats and cylinders for any signs of problems.

-Mark-

[decksetter 50]

Adjust your valves. Had the same problem with gas spitting out the carbon till it warmed up and then running fine. Adjusted valves and that took care of it. Make sure you valve seat isn't loose, but if it runs fine warm you may just be ok adjusting valves.

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[zeedubbya 18]

Thanks for all the advice. I mowed with it on Tuesday and found if I leave the air cleaner off it runs pretty good but when pto is engaged it throws gas everywhere. I'm going to adjust the valves this weekend. My next question is this--should I just go ahead and decarbon at this point? The only issue I have with the decarbon is i think

I'll have to pull the motor to reach that back head, where on a valve adjustment I would just leave it in there. Is this a correct assumption? Again thanks for the input, I hope to get it running smooth soon!

[zeedubbya 18]

Its pobably a violation of forum etiquette because this is a duplicate post from the decarbon thread. However everyone was right! Adjusting the valves fixed the problem! I adjusted the valves and replaced the gaskets and it is a huge improvement! I was going to decarbon and adjust the valves but I didnt have time to decarbon last night. The valves were really really tight especially the rear intake valve. Adjusted them to .005 intake and .013 exhaust and it runs like a new machine! I still intend to decarbon soon, and I'll post the results of that in the decarbon thread. Thanks for the advice!!