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318 B43G backfiring

8679 Views 24 Replies 10 Participants Last post by  gzec
I've got a 318 with a B43G Onan engine. Tractor came in running very rough. Suspected the usual culprits such as worn gaskets on intake manifold, manifold seal, carb jets plugged. Pulled it apart...cleaned carb in solvent overnight. Installed rebuild kit. Split intake, cleaned halves and sealed with Permatex Motoseal. Pulled cylinder head,cleaned carbon and adjusted valves to spec--.005 on intake and .013 on exhaust (were way out of spec). Replaced head gaskets. Removed gas tank, flushed, and replaced fuel line as was very soft. Cleaned points up and reset gap to .016. Plugs are fine. Reassembled everything and started. At first would not past half throttle and not as smooth as I think it should be. Let it run a bit and RPMs came up but is backfiring through exhaust. Valves inspected OK for faces and seats. Experts, what am I missing?
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backfiring through the exhaust tells me that
the valves are not sealing.
thank you boomer ( the used onan engine parts guy, also NOS and new )

Thanks to 12 hours away from the machine, but mostly from the advice here, backfiring is resolved!! Valves needed readjustment. It helps to have the correct tappet wrenches in your box (thanks K-D Tools for USA made goodies!!). Got everything back together and it fired right up. Very small miss in engine; think the points need a little tweaking. Ran it about 45 minutes test mowing. Started to struggle a bit and I smelled what I think is a belt burning. Shut it down and it tries to start but no luck. Towed it back into shop with four wheeler SLOWLY. Letting it rest overnight before I tackle this next challenge. This thing has been a real test of my patience since taking on the repair job. Sad, it's got about 980 hours on it but someone at one point changed the fuel line to a single, bottom feed outlet in tank and plugged the top, removed the fuel valve. Along with some suspect wiring, I don't think I got the whole story from the owner when he dropped it for the repairs. But, then again, he may not know everything about it...:unkown:
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Well, reset the valves..Definitely a problem there. Still a miss and occasional backfire not through the carb. Looking at points they're pretty burnt. Going to replace them tomorrow. Having a bit of an issue setting the gap, but I'm thinking the contact surfaces are crap; that may be the problem. REALLY want to get this running well and out of the shop!!
If points are burnt, you should also replace the condenser and of course make sure it is on the correct side of the coil. ( same as the points negative)
Well, replaced the points and condenser. Started it and it was slow to respond to the throttle. Would not initially go the WOT, ran a little rough then finally went to WOT with a slight miss. Idled back down and shut off. Went to another project and came back a little later...doggone thing started popping through carb...started to run but quit, again popping through carb. I know valves are correctly set, idle mixture is correct. Plugs are clean, wires are good. Made new point lead to go to coil as it was suspect. I'm starting to go crazy with this thing now...HELP, please!!!
The hair pullers are the most interesting, especially when they are not yours. Lol.

The throttle response is interesting, can you describe it better, or make a short video?

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I have attached a YouTube link for three videos of this 318 and it's current status. I would appreciate folks looking and listening and giving me feedback, please. Valves set to spec, carb rebuilt, new points and condenser, new wire to coil, new fuel line and filter, plugs good. HELP!!!


when you set the valves,
did you have one cylinder at a time on TDC of the compression stroke?
MANY people have set the valves with one cylinder at TDC of the compression stroke,
and the other one at TDC of the exhaust stroke.
thank you. boomer
You sure the float and needle valve are correct? looks like a lot of gas in the carb??

I followed CTM #2 instructions to the letter. I hate to pull it all down again...It initially ran ok with just a small miss after setting the valves and reassembling everything...I'm not sure where to go now...

I followed CTM #2 instructions to the letter. I hate to pull it all down again...It initially ran ok with just a small miss after setting the valves and reassembling everything...I'm not sure where to go now...
Carb kit was correct for the Nikki. Thinking I may have to pull it all down again???
Sorry I was not clear. I meant is the float set up correctly.

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Smalljob...float is set correctly. I've been working on small engines for the last 40 years and have never come up on one that eventually kicked my rear end. This one is a true candidate for that. Not sure where to go next except to start over completely...
I've been thinking this through today as I try to observe a day of rest to recharge my tired brain. Thanks to the folks that responded to my quest for expert assistance. At this point I guess there's no option but to start all over again and redo everything. Unless...someone else out there feels differently...

I just re-read this thread from the beginning and before you changed out the condenser I was going to name that as the most likely cause of no high speed running, etc. Aftermarket Chinese condensers are pretty poor quality and some are bad right out of the package, so that may still be a factor.

But the backfiring through the carburetor can only be from an open intake valve -- which may be due to faulty timing, grossly out of spec clearances or perhaps a broken/cracked valve spring. Timing is pretty much fixed by the gearing and only slightly dependent on point gap (but do check that the points leaf spring is in good shape and the points are not bouncing just as they close.)

Since it has started to backfire this way, have you checked the compression on both cylinders? Might be a quick way to verify that the valves are closing correctly, at least at cranking speed...

Let us know what you find if you have to go back in there. From what you have posted I don't see how it can still be fuel related without being timing/valving as well to some degree.

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Sleeping on this, I agree. Am heading out to shop now to sort this out and will advise. Thank you, sir!!!
OK all...

Pulled this thing back down this morning. Found the right side cylinder valves out of adjustment...reset same. Double and triple checked all clearances. Checked float level and ensured no crud in carb to include idle jet. Checked point gap (.016) and plugs...both good; set at .025. Followed everything to the letter as called for in CTM #2. Put back together and fired up. Started right up; slight miss in carb. Loses power under load. Exhaust temp at about 710 F. Thinking it may well be an internal issue of as yet unknown origin. Not going to go forward at this point until hearing back from owner...
I had a similar problem recently. I had replaced the fuel filter to a smaller filter. In doing so the fuel line was shortened. The fuel line was laying across the governor, not allowing the throttle to move. It was stuck wide open. Put a longer fuel line on and now running great again
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