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So here’s the deal. 2019 Briggs 33R777 0012 G1 fires right up every time. Runs rough and pops at high rpm (although it won’t get nearly as high as it should) but runs smooth at low RPM. I’ve both cleaned the OEM carb and replaced the carb, same issue. Valve clearance is good on intake & exhaust. Fuel is fresh. Flywheel key is in place. Magneto was swapped. In the video you can see a disconnected plug but that doesn’t change anything. Oil is new and at the right level. Valves bounce on each rotation (good cam). Choke moves freely (video shows this). Linkage is not restricted. All checks/adjustments made zero change to how it runs. Video link below. Any ideas?

Are both valve opening the same distance. I have seen one cam lobe wear down causing similar issues with rpm and running.

ILENGINE said:

Are both valve opening the same distance. I have seen one cam lobe wear down causing similar issues with rpm and running.

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I’ll check that out and report back. Thanks for the reply!

Does it have the correct spark plug in it? I've seen the "Torch" brand cheap plugs do this. I only use the correct Champion plugs in these engines.

Auto Doc's said:

Does it have the correct spark plug in it? I've seen the "Torch" brand cheap plugs do this. I only use the correct Champion plugs in these engines.

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Good call, but yes, it has a champion that looked good, I swapped it anyways with a new one and no change.

How did you check the valve clearances? "Looks good" is subjective. This thing is popping like an intake valve has poor clearance or the intake valve is leaking.

Check out Teryl Fixes All on You Tube. He has some good valve adjustment information on several different models.

Auto Doc's said:

How did you check the valve clearances? "Looks good" is subjective. This thing is popping like an intake valve has poor clearance or the intake valve is leaking.

Check out Teryl Fixes All on You Tube. He has some good valve adjustment information on several different models.

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Yes sorry, I realize my “looks good” answer was probably mistaken as an eyeball guess haha. But both gaps do measure within spec. I actually went back in and checked them again thinking I didn’t have enough coffee prior, but they were spot on.

Mowerflipperguy said:

I’ll check that out and report back. Thanks for the reply!

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Auto Doc's said:

How did you check the valve clearances? "Looks good" is subjective. This thing is popping like an intake valve has poor clearance or the intake valve is leaking.

Check out Teryl Fixes All on You Tube. He has some good valve adjustment information on several different models.

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Both valves open right at 1/8”

I'm not sure what you are referring to when saying 1/8". If they are barely opening, then someone as already tried adjusting them and the cam lobes are very worn down. These engines were bad about eating cam lobes. I'm not sure if B&S ever got their supply caught up, but aftermarket cams are junk right out of the box for these engines.

Auto Doc's said:

I'm not sure what you are referring to when saying 1/8". If they are barely opening, then someone as already tried adjusting them and the cam lobes are very worn down. These engines were bad about eating cam lobes. I'm not sure if B&S ever got their supply caught up, but aftermarket cams are junk right out of the box for these engines.

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1/8" valve opening is about half of what they should open. And I currectly have 2 OEM cams on backorder with an ETA of October.

Hello ILENGINE,

I've watched the initial video a couple of times and that popping back into the intake is indicative of badly worn cam lobes or a valve adjustment that was not done correctly. If this original cam design has press on lobes, there may be a chance that a lobe slipped on the shaft.

Might be partial blockage of fuel. Blow some air through line backwards to tank.

Auto Doc's said:

Hello ILENGINE,

I've watched the initial video a couple of times and that popping back into the intake is indicative of badly worn cam lobes or a valve adjustment that was not done correctly. If this original cam design has press on lobes, there may be a chance that a lobe slipped on the shaft.

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Briggs specs on your engine should be .002-.004 on your intake valve and .005 - .007 on your exhaust. Most Briggs this is standard on small engines. 1/8 inch would be way too much gap. Compare thousands of an inch to a piece of paper, for instance paper is .003 thick.

wekjo said:

Might be partial blockage of fuel. Blow some air through line backwards to tank.

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Take fuel cap off first, leave it off and test drive. Cap has one way vent in it. If vent stops working becomes a fuel supply issue. Costs nothing to test these issues before you start wrenching and open your wallet.

Is there supposed to be a sleeve spring from the governor to the carb linkage? That carb flange seems to wiggle a lot, like that spring tension would modulate that.

I think you you have a head gasket starting to blow thru.

How aboit a possible slipped key on the flywheel?

Zedo said:

Briggs specs on your engine should be .002-.004 on your intake valve and .005 - .007 on your exhaust. Most Briggs this is standard on small engines. 1/8 inch would be way too much gap. Compare thousands of an inch to a piece of paper, for instance paper is .003 thick.

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I think you misunderstand what was being discussed. The 1/8 inch was not the valve clearance gap but how far the cam lobe lifted the valve when it opened.

Sounds like it's running lean. Pull the choke out just a tad snd see if it runs better. If so, try fuel injector cleaner or check the carb for sludge or trash.

Did you do a compression test? Popping at high rpm is because intake valve is not seating properly, or valve seat and guide are not where they should be. Even with good compression you will still get popping.

From what I read you have a intake valve seat loose in the head, a bent or warped intake valve, or guide sliding out of the head.

Pull the valve cover and watch the valves move. Probably a push rod laying down on the job or a lean condition under high load. Lack of fuel delivery...... Do a leakdown test. Rocker arm loose.