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Hey everyone! I wanted to see if anyone else has experienced this issue before. It's minor but it's driving me nuts, because I can't figure out what's happening. I have a ton of small engines, mostly Briggs powered, and this season I noticed that several of my L-Head Briggs with the Float Style Carburetors (190CC usually) have been making a strange cough/backfire when running unloaded. It isn't loud enough to be backfire/afterfire, but it comes from the muffler and I have no idea what it is. You can physically feel it when holding the equipment. The carburetors are in good shape for each of the engines, I cleaned them to be sure, and the gas is new. The spark plugs should be fine. Timing is good on all of them. What gives? I recorded a few videos of different engines doing it. I just wondered if anybody else is experiencing this or knows why.



Gotta listen carefully, but it's especially noticeable on the second one
You also might notice the pressure washer has a slight rod knock. It was abused by its previous owner and I saved it just in time lol. I don't think that would affect this problem though.

Could be spark plug misfire, or even though the gas is new could still a fuel issue.

I have a similar type mower with the Briggs that also does this. I've always assumed it was pre-ignition. Maybe from some carbon build up or a sharp edge or parting line in the casting. Mine is not near as bad (vid.2) but same type sound. Nice quiet powerful easy starting mower otherwise, I just run it.

Didn't think of that. I'll try scrubbing carbon out of them and put in higher octane gas to see if it changes. I wouldn't think the compression ratio would be high enough but it won't hurt to test. Maybe try some ethanol free too.

I'll also test a different plug, but it happens in 3 different engines with different plugs...

It also seems to be happening on my lawn tractor. I suspect gas now. I'll try some higher octane later.

Sounds very much like you local fuel supplier has a bad batch of fuel.
Get some from a different branded place & see if it makes a difference .
Also check your fuel drum some times a little off fuel can make 5 gal go bad in no time flat .

bertsmobile1 said:

Sounds very much like you local fuel supplier has a bad batch of fuel.
Get some from a different branded place & see if it makes a difference .
Also check your fuel drum some times a little off fuel can make 5 gal go bad in no time flat .

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I'm gonna go nuts lol. I went to a different brand gas station today (I was using Shell, I went to a Quick Trip today). I got fresh 87 octane, 93 octane, and ethanol free 89 octane. I drained the carburetor and tank, cleaned the carb, and put in each fuel and ran the engine. After each run I would drain the bowl and tank and put in new fuel. I tested each fuel. STILL HAPPENS. Fuel isn't the problem.

So I figured carbon buildup may be the culprit. I took the head off and found normal buildup. I scraped off what I could (probably 75%) and no change. Maybe tomorrow I'll go get brand new spark plugs for all of them, but I don't think that's it. I'm gonna solve this though, if its the last thing I do.

Curious to know if you fixed it? I have a Craftsman YT3000 and it runs good, but has a sputter/miss as it runs. Still cuts the grass, but is such a pain not to run smooth

Coach Gates said:

Curious to know if you fixed it? I have a Craftsman YT3000 and it runs good, but has a sputter/miss as it runs. Still cuts the grass, but is such a pain not to run smooth

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Sadly, no, it's still happening. It bothers me too. I de-carboned the head of the motor, checked the timing and spark gap, cleaned the carburetor, etc. and nothing. Perhaps I should do a total rebuild, valve re-gapping, new gaskets, etc. to see what happens. Or try a different carburetor. I still want to know!

so if all that is fine then you are left with the exoteric stuff like a worn cam or loose valve guide or loose valve seat .
This is provided you have set the valve lash correctly.
Get a new rocker cover gasket & grease it with bentonite axle grease so it strips off cleanly.
Then check the valve lash and record it
Do the same every time you use the mower before you start it so the valves are stone cold
If you get results that are all over the place then you have a loose guide / seal
That being the case the head has to come off & get some staking done to hold them in
Tinker has a nice method that he shares readily
A loose guide or seat will some times show up in a low pressure leak down test

bertsmobile1 said:

so if all that is fine then you are left with the exoteric stuff like a worn cam or loose valve guide or loose valve seat .
This is provided you have set the valve lash correctly.
Get a new rocker cover gasket & grease it with bentonite axle grease so it strips off cleanly.
Then check the valve lash and record it
Do the same every time you use the mower before you start it so the valves are stone cold
If you get results that are all over the place then you have a loose guide / seal
That being the case the head has to come off & get some staking done to hold them in
Tinker has a nice method that he shares readily
A loose guide or seat will some times show up in a low pressure leak down test

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I was thinking the only possibility at this point had to do with valves. These are L head engines just BTW, so they have no rockers but there is the little port where the springs connect from the tappets to the valve stems. I'll check the valve lash at some point I suppose, but the only way to fix these is by grinding the valves or lapping the valves, and I don't feel like doing that lol.

L heads tend to burn the exhaust seat so after time the exhaust valve no longe seals tight.
Even then they will run like that fr decades till there is just not enough compression to start.

bertsmobile1 said:

L heads tend to burn the exhaust seat so after time the exhaust valve no longe seals tight.
Even then they will run like that fr decades till there is just not enough compression to start.

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I just fixed a leaky sump gasket on a Briggs Quantum that was on one of those craftsman 4-in-1 leaf sucker things. It was a bear to get to, the entire impeller assembly had to come off. Anyway, I noticed the exhaust had about .001 thousands! But it still ran and started fine. Guy didn't want to have me fix it so I just left it. Strangely though this engine did not cough and sputter even though it had such a good reason too.

TylerFrankel1 said:

I just fixed a leaky sump gasket on a Briggs Quantum that was on one of those craftsman 4-in-1 leaf sucker things. It was a bear to get to, the entire impeller assembly had to come off. Anyway, I noticed the exhaust had about .001 thousands! But it still ran and started fine. Guy didn't want to have me fix it so I just left it. Strangely though this engine did not cough and sputter even though it had such a good reason too.

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Side valve engines are very forgiving and will run in conditions that an OHV will not even fire with let alone run.

bertsmobile1 said:

Side valve engines are very forgiving and will run in conditions that an OHV will not even fire with let alone run.

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I’ve noticed this and I prefer the L head because they’re more durable even if less efficient. I seriously do not understand how they work with such abuse though, do you have any thoughts on how it’s possible? My OHV Briggs won’t even crank with the valves a few thou off because of compression release, but a L head can run with a blown head gasket, 1/2 oil, a dirt packed air filter, scored cylinder walls, debris in the float bowl, A partially sheared flywheel key, and valves with 0 clearance. I say that because I saw it once! All of those at the same time and it would still sputter to life.

Engines need a charge with a combustable ratio od air & fuel
What they do not need is a totally full cylinder
That is why turbo charging was invented so the engine gets at least a full capacity charge or even more every stroke.
Most side valves will happily run if they can get 20% of the capacity into the cylinder so a 500cc engine only needs 100 cc of fuel & air to run.
Most side valves are momentum engines so they store the energy in heavy flywheels & crankshafts.
OHV's are generally a lot lighter and a lot higher compression so need more energy to overcome their own compression so very few will run at anything less than 50% cpacity.
The amount of air/fuel mix that gets in is dependent upon the time the valves are open and how far they open so if the old side banger can stay open long enough for enough charge to get in and most of the exhaust to get out then they will chuff chuff down the road
Side valves can run down as low as 3:1 compression
Very few OHV's will run under 6.5:1

bertsmobile1 said:

Engines need a charge with a combustable ratio od air & fuel
What they do not need is a totally full cylinder
That is why turbo charging was invented so the engine gets at least a full capacity charge or even more every stroke.
Most side valves will happily run if they can get 20% of the capacity into the cylinder so a 500cc engine only needs 100 cc of fuel & air to run.
Most side valves are momentum engines so they store the energy in heavy flywheels & crankshafts.
OHV's are generally a lot lighter and a lot higher compression so need more energy to overcome their own compression so very few will run at anything less than 50% cpacity.
The amount of air/fuel mix that gets in is dependent upon the time the valves are open and how far they open so if the old side banger can stay open long enough for enough charge to get in and most of the exhaust to get out then they will chuff chuff down the road
Side valves can run down as low as 3:1 compression
Very few OHV's will run under 6.5:1

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thanks for that! Amazing explaination, I had no idea! Turbo charging makes more sense to me now, and so does the fact that I can run blown head gaskets in the L heads but not OHV. L head/side valve must have heavier flywheels. It also explains why they’re willing to run with plugged air filters. I guess the Side Valve engine is similar to an old timey hit and miss engine with the huge flywheels. With what you’ve told me I like the side valves even more, I think they’re the best option for residential mowing where the owners Beat and neglect their equipment.
Also I think I understand something about running mowers without blades now. I have some Briggs side valve engines and a couple overhead valve Honda’s. The Honda’s will run without the blade on, albeit rough, which I guess is because they aren’t reliant on the flywheel the same way side valves are. The side valve Briggs won’t run, they kick back too hard. very interesting!