# Yardworks backfire bang bang pow pow



## bbrian017 (Mar 3, 2016)

I notice my yardworks backfires often, I also notice my neighbors does as well but another neighbors doesn't.

In your experiences do these machines backfire often or will I be needing a tune up?


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## bad69cat (Nov 30, 2015)

More often than not that indicates it's running too lean. If the carb is adjustable you want to richen up the mixture just a touch. If it's one of the newer non-adjustable carbs you have to get a bit more creative _


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## skutflut (Oct 16, 2015)

bbrian017 said:


> I notice my yardworks backfires often, I also notice my neighbors does as well but another neighbors doesn't.
> 
> In your experiences do these machines backfire often or will I be needing a tune up?



What engine model do you have on your machine? 

Is the engine hard to start? Does it ever kick back when you pull it over and attempt to remove your shoulder from its socket?

You should ask your brother about the reason for changing the shear bolts to shear pins. Ask if he hit something that broke the pins and stalled the engine at the same time. 

If it's not just a lean mixture on the carb, it's possible that the woodruff key in the flywheel may have partially sheared, throwing the spark timing off a degree or two. It could also be valve adjustment, and/or excessive carbon buildup in the cylinder head and piston. 

Try the easy stuff first.


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## brickcity (Jan 23, 2014)

You should ask your brother about the reason for changing the shear bolts to shear pins. Ask if he hit something that broke the pins and stalled the engine at the same time

I must be missing something


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## cranman (Jan 23, 2016)

I'm thinking maybe a valve stuck open with either carbon or needs a grinding on the base of the valve? I think the timing would have to be way off for the backfire to come through the carb ...


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## skutflut (Oct 16, 2015)

brickcity said:


> You should ask your brother about the reason for changing the shear bolts to shear pins. Ask if he hit something that broke the pins and stalled the engine at the same time
> 
> I must be missing something


OP posted another thread mentioning shear bolts had been replaced, wondered if the motor stopped fast damaging keyway


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## DennisP (Jan 10, 2016)

If it is an older Tecumseh engine, check for exhaust valve lifter clearance. On my 1999 vintage 9hp HMSK90 I had constant backfiring and thought it was carb related. Upon teardown I measured almost no exhaust valve lifter clearance since the exhaust valve stretched.

Turns out the fixed orifice emissions carbs cause these engines to run way to hot (all for emissions) and the exhaust valves end up stretching as a result.

If there is not proper clearance on the exhaust valve, pull the valve, grind the lifter end to increase clearance, reassemble, check the clearance and repeat until it is right.

Upon restarting you will notice no more backfires and the exhaust won't be as red-hot as it was before.


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## Homkoguy (Dec 7, 2016)

i would start easy. change spark plug and clean and adjust carb. if that dosent work its for sure a stuck or carboned up valve which is very fixable.


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