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PK., Small Engine Technician
Category: Small Engine
Satisfied Customers: 1052
Experience:  Retired Owner of a full service shop and national parts sales website.
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I have a honda gx 340 and is backfiring out the carb when I

Customer Question

I have a honda gx 340 and is backfiring out the carb when I try to start it. To me it would seem like timing but diagrams I look at indicate that the crank and cam are gear driven. I took off the flywheel and cleaned out all the dirt back there. I have spark and gas. How would you time one of these engines?
Submitted: 7 years ago.
Category: Small Engine
Expert:  PK. replied 7 years ago.

It's probably an overly lean fuel mix. You can adjust ignition timing somewhat by moving the coil and changing flywheel keys. Cam timing is pretty much non-adjustable except for performance applications and then you can index the cam, but this requires some modifications.


Backfiring through the carb is indicative of a lean mix probably 90% of the time with a valve sticking or out of adjustment 10%. If yours has an adjustable carburetor, I'd try resetting the jets first followed by rebuilding the carburetor and adjust the valves last. I think the lash is .006" for intake and .008" for exhaust although the guys who race the Hondas will tighten that up a good bit to get a faster opening valve.



Customer: replied 7 years ago.
I took the carb off and clean it out, blew all the passages out with a blower and still had the backfiring. The bowl seem to have enough gas in it and the float seems to be non adjustable but looks right. I have also tried starting fluid twice and that didn't work. once I pulled on it and a flame about three inches flew out! The muffler isn't plugged, I did pull off the little valve cover and looked at the rocker arms and watched them work under a real light pull of the cord and they seem to indicate good cam lobe and pretty close to being right on adjustment. If it was a lean mixture wouldn't starting fluid at least make it run for a second or two? The gas flow is good too.
Expert:  PK. replied 7 years ago.

I'm sorry, I thought you meant it was backfiring while running just after started.


I would check my flywheel key first. If the key is bad and the ignition too advanced, it can do this.


You'll need to check the valve lash with a feeler gauge. With the piston at Top Dead Center the lash should be about .006" for intake and .008" for exhaust. You may very well have an intake valve that has a seating problem. If the valve lash is correct and the key in place, then I'd probably go ahead and pull the head and inspect the valve faces and seats.

Customer: replied 7 years ago.
Yeah I wish I could get it running! I did pull off the flywheel and inspected the key and that is all good . I will adjust the valves in the morning and one more question, if a valve isn't seating right then compression would be down    right   so do you know about what the compression is supposed to be off the top of your head? Thanks
Expert:  PK. replied 7 years ago.

Honda lists something like 110-180lbs with the compression release not functioning. I would want to see probably 75-85 minimum but even that may be high depending on how fast the engine is spinning. I've had engines with 50psi on the release that ran fine. A compression test is more useful on a twin, or larger, because then you have something to compare it to. A more useful test is a leakdown test, but in your case I don't think I'd do it, I think I'd just move on to pulling the head.


So a compression test alone might not be able to show you a valve that's not seating. I had a Kohler or a Briggs, I can't remember which, last summer that had a valve seat that had come loose and didn't show up on a compression test or a leakdown test at TDC. I was so sure that the valve was leaking that I pulled the head and there was a valve seat, on the intake side I think, that would just barely slip down when I compressed the spring. This engine would start and pop and buck, but the basic tests just wouldn't reveal it. Sometimes you just have to start removing parts, although that is my last course of action.


You're on the right course. I think if you inspect the valves and seats, head, head gasket and engine deck you'll find your problem. It could be nothing more than carbon built up or it could be a seat loose. I think close inspection would be the diagnostic tool to turn to with your symptoms. Thanks, PK.




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Expert:  PK. replied 7 years ago.
Thanks for the Accept and the bonus. Just come back to this same question if you have any troubles with the engine so you don't have to go through the deposit process again. PK

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