# Yard machine backfiring



## Rtreptow (Feb 8, 2019)

I inherited a snowblower from a relative. It says yard machine by mtd and has a 10 hp tecumseh. It has the carb with the fixed nut on bottom of bowl along with the spring loaded drain valve. I’ve had it for 8 years. Not sure how old it is.15 years? It’s always run well. This winter I started getting the occasional backfire and it has been getting worse. Seems to run good for 5 minutes or so then starts backfiring. When it’s under load and throwing snow it runs good. If it’s not under load then it starts backfiring. Fresh 91 gas. Always use 91 fresh gas. New plug. Cleaned the valve in carb bowl. When I look into the muffler I can see pretty steady blue flashes/flame. Not sure if it’s always been like that or not? I doubt it is supposed to.


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## tadawson (Jan 3, 2018)

Check valve clearance.


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## groomerz (Feb 7, 2015)

Does it idle? You say it runs good under load . To me I’d say it isn’t valve adj then . Is rpm going up and down when not under load when it’s backfiring. Might a be clogged idle and or low speed passage in carb. When it not under load low speed circuit is used to maintain rpm and if clogged it would be lean and motor would stumble and backfire then go full throttle and repeat. Let us know about the idle. 


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## Rtreptow (Feb 8, 2019)

It does idle pretty well. It doesn’t do any surging up and down. When idling it does still backfire but not as much. This may not be relevant, but in the past every time I would shut the engine down it would rev all the way down and just when I thought it was off, it would let out one backfire then turn off. It does not do this any longer though. Maybe just coincidence but thought I’d mention it. The thing runs really well when it’s chewing through deep snow, but as soon as I get to light snow or no snow it’s starts popping and spitting.


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## groomerz (Feb 7, 2015)

Have you checked the spark plug to see if it’s dirty or loose


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## tabora (Mar 1, 2017)

I think lean carb settings and/or dirty/sticking/mis-adjusted valves are the most common causes.

Other thoughts from the B&S web site:

*What causes a small engine backfire & how do I fix it?
*Backfire typically occurs when the engine is decelerated rapidly.
*Common Causes of Backfire:*


Lowering engine speed too fast
Gasoline, which contains higher blends of alcohol
Carburetor adjustment set too lean
Muffler construction can induce backfire
Higher than normal engine temperatures
Some carburetors can induce backfire due to the sensitivity of internal transitional passages
(This condition cannot be corrected).
 *Possible Fixes for Backfiring:*


Lower engine speed slowly
Follow small engine fuel recommendations and/or switch to brands with low or no alcohol
Adjust carburetor for optimum performance
Inquire with equipment manufacturer about increasing air volume to decrease engine temperature
 *What causes a small engine afterfire & how do I fix it?
*Afterfire occurs after the engine has been shut off.
*Common Causes of Afterfire:*


Shutting off the engine at high RPM, causing fuel to pump through the engine for ignition
Gasoline that contains alcohol has a tendency to ignite easier, which can cause afterfire
Small engine muffler type and manufacture
Carburetor adjustment may not be properly set for correct engine performance
Anti-afterfire solenoid may not be working properly
 *Possible Fixes for Afterfire:*


Allow the engine to cool by idling the engine down with proper speed (15-30 seconds)
Change to a different non-alcohol or alcohol brand fuel
Ensure proper carburetor adjustment for optimal engine performance
Contact the equipment manufacturer for updated designs in air control baffling, mufflers, etc.
Check the anti-afterfire solenoid for proper operation
*Note*: If equipped with an anti-afterfire solenoid, shut engine off at full throttle


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## Rtreptow (Feb 8, 2019)

As mentioned earlier, it does have a new spake plug. Ran the snowblower this weekend again. Just confirmed that backfiring is definitely worse after 10 minutes or so and definitely worse when not under load. I took the muffler off this weekend. It has a very poor seal with the engine. It appears to be worn. No gasket there. Is there supposed to be one? also, one of the bolts sheered off inside the engine while I was removing it. The muffler itself is "broke". The baffle inside is in 3 pieces. I saw on the internet that there was possibly a replacement muffler that screwed into the engine bypassing the need to use the bolts. I also noticed that the starter motor was hanging on by just one bolt and very loose. There are 4 little ears that come off the starter and bolt it tight to the engine. All but one have worn off somehow. Maybe they were loose and then wore off by vibration? But only one left. I never use the electric start, and certainly wouldn't try now. Can I just remove this and run the snowblower without the starter motor attached? It would seem to leave a pretty big gap going into the flywheel area there.


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## Rtreptow (Feb 8, 2019)

Did some work on the machine. Replaced the carb completely. Got a new starter motor. New Muffler. I pulled the head and adjusted the valve clearance. There was 0 clearance on both intake and exhaust valves. I did the exhaust valve first and ended up taking a little too much material off the valve. Got the Intake valve filed down to spec. I ran the machine and it ran better than it has for a long time. What kind of problems could I potentially have by having too much clearance on the exhaust valve? It ran great with no backfiring. Idled fine. couldn't appreciate any immediate problems from having too much clearance with the exhaust valve?


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## RedOctobyr (Mar 2, 2014)

I'd rather have too much clearance, than too little. Too much is safe, at least. 

With too little clearance, you can have a valve that doesn't close fully, giving up a lot of power, and potentially damaging the valve seat. 

With too much, you give up a little flow, so maybe a slight power loss, but probably not meaningful. The compression release on the exhaust valve will be less effective (it only nudges the valve open a small amount), so it might be harder to pull.


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## Rtreptow (Feb 8, 2019)

RedOctobyr said:


> I'd rather have too much clearance, than too little. Too much is safe, at least.
> 
> With too little clearance, you can have a valve that doesn't close fully, giving up a lot of power, and potentially damaging the valve seat.
> 
> With too much, you give up a little flow, so maybe a slight power loss, but probably not meaningful. The compression release on the exhaust valve will be less effective (it only nudges the valve open a small amount), so it might be harder to pull.


That's kind of what I'm finding. It's a little harder to pull. Starts a little harder. Sounds different when starting it...can hear air flow through the muffler maybe??? Is it constricted more and just making a different sound, like a whistling sound while pulling? Seems to run well. Haven't had a big snow to test the power. Now I just have to figure out how to get that broken off bolt from the muffler out. Have muffler attached just by the one long bolt into the exhaust valve and the smaller bolt on top. Don't think that's a good idea to run it for any length of time like that!


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## Fat City (Feb 11, 2017)

Many engine exhausts are tapped 3/4 NPT so you can get a screw in muffler, and not have to monkey with broken bolt .


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## orangputeh (Nov 24, 2016)

This is an interesting thread because have had similar backfiring issues with a couple Hondas. The OP did a lot of work so not sure which part of the work did the job. was it a new plug? A new muffler? a new carb? or valve adjustment? no way of telling unless he did one part at a time.

In my case I just decellerate slowly as suggested and let it idle before shutting down. But constant backfiring on high throttle would be a real concern. Could it be a clogged muffler? or valve adjustment?

why would it even out under load?


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## 140278 (Aug 27, 2020)

fat city
did you notice the last post was almost 2 years ago? the op last seen Feb 21, 2019 the date of the last post


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## Auger1 (Dec 4, 2020)

I have had 2 blowers in the past do this, running poor and backfiring after a few minutes and both turned out to be the gas cap was not venting well. Found it put by accident on the first one, I would take the gas cap off and the machine would run great again, put the cap on and after 5 minutes backfiring and running poorly. The second blower was a single stage 2 cycle and would just die after 10 minutes or so and would not restart well, I though what the heck, drilled a new hole in the cap and it's run perfect ever since. Not saying this is "your" problem but when it starts to act up take off the cap and see how it runs, cheap fix if it works.


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