# A Bit of Backfire



## Dannoman (Jan 16, 2018)

I'm getting a bit of backfire when I run the 10HP Tecumseh MTD, I got a new sparkplug and will install it tomorrow, but that might not be it. I did notice that my gas cap is screwed up because the little metal disk is gone (probably in the tank) and the rubber dome that sits underneath it disappeared when the cap fell off the machine last time I took it out. So maybe it's the cap which isn't venting that's causing this. I can see sparking and an orange glow in the muffler when it acts up. Not good.


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## nwcove (Mar 2, 2015)

sounds like a lean condition or a valve issue. try running with the choke in the first notch to see if that helps.


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## RedOctobyr (Mar 2, 2014)

I doubt the gas cap would cause something like this. A non-venting gas cap would eventually starve the engine for fuel, and it would die shortly thereafter. 

A valve that's not closing fully, due to insufficient valve clearance, can cause this. Does it show up right away, or primarily after the engine is hot, maybe under a load? Checking the valve clearances would be a good idea. If it's a flathead engine, then *changing* valve clearances is more difficult. But checking them is still pretty straightforward. I had a Tecumseh flathead which eventually had a valve clearance that was too small. I had to remove the valve and grind it by a few thousandths of an inch, to fix it. If a valve isn't fully closing, that's bad for the valve and engine, it's not a great idea to keep running an engine in that condition. 

I believe some carb conditions can also contribute, but I don't want to mis-quote what aspect would do this. A lean-running carb will make the engine run too hot, and could cause a glowing muffler. But backfiring on shutdown can, I think, be from running too-rich?


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## Dannoman (Jan 16, 2018)

nwcove said:


> sounds like a lean condition or a valve issue. try running with the choke in the first notch to see if that helps.



Thanks for this. 

I might have the same problem this guy had...






Good old Donyboy has a solution. Clean the air mixture screw. I have a non-adjustable carb just like this so I'll try it tomorrow.


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