Want to help diagnose my B&S V-twin misfiring issue?

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I bought a used mower primarily for the transmission and deck, but it does have an engine that starts and runs. Unfortunately it randomly sputters and loses power for 3-5 seconds. As it heats up it becomes more frequent and actually died a couple of times when I tried to mow a section of the lawn. Someone suggested it might be a fuel flow issue so I replaced the fuel line, pump, vacuum line and even swapped the carb for one taken off an engine with a thrown rod. Still does the same thing.

Any suggestions on where to start next? One odd thing I noticed after running it this last time was that there was a sound like water boiling coming from the front of the engine immediately after shutting it off. I thought it was the carb float bowl but I can’t be sure it wasn’t coming from the valve cover. The engine is a B&S 20hp V-twin OHV from around 2005 or so.
 
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Check for debris under the shrouding, grass clogging the engine cooling fins will cause several symptoms such as vapor lock. Also the gas cap that doesn`t vent will cause the symptoms described. When the engine stalls loosen the cap temporally.
 
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Just had the coil assembly finally go bad on my old B&S twin. I knew it was failing as it would start misfiring when hot and never could get it to start when hot unless I allowed an extended cool down.
 
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Make sure the gas cap is vented. My JD was hard starting. Saw the fuel tank was sucked in and opened the gas cap. Huge vacuum. Left cap off, mower started right up. The inside of the cap was all swollen (ethanol???) So I drilled the 3 holes and problem solved.
 

Elkins45

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Make sure the gas cap is vented. My JD was hard starting. Saw the fuel tank was sucked in and opened the gas cap. Huge vacuum. Left cap off, mower started right up. The inside of the cap was all swollen (ethanol???) So I drilled the 3 holes and problem solved.
I’ve checked that. Opening the cap doesn’t help at all.
 

Elkins45

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Well, it wasn’t plugs or coils. Valve adjustment or head gaskets are the next thing to investigate I guess. It was popping a lot when I ran it this time, which is new. Is there any chance this could be something weird like the carb solenoid intermittently disengaging or arcing on a switch somewhere?

This thing really isn’t worth the effort.
 

JTK

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Well, it wasn’t plugs or coils. Valve adjustment or head gaskets are the next thing to investigate I guess. It was popping a lot when I ran it this time, which is new. Is there any chance this could be something weird like the carb solenoid intermittently disengaging or arcing on a switch somewhere?

This thing really isn’t worth the effort.

Was the carb you swapped over to a known good one? Just putting that out there because the s/d solenoid is attached to the carb. I've seen the blade connectors get pretty loose on them, but that being the case, you'd think it would act up all the time.

Will it run better if you feather the choke on just a bit? This would point still to a carb issue IMO.

Looks like you've got everything else addressed. Not sure how to prove out a HG problem other than look for smoke, excess crank case pressure or external leakage.
 

Elkins45

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Was the carb you swapped over to a known good one? Just putting that out there because the s/d solenoid is attached to the carb. I've seen the blade connectors get pretty loose on them, but that being the case, you'd think it would act up all the time.

Will it run better if you feather the choke on just a bit? This would point still to a carb issue IMO.

Looks like you've got everything else addressed. Not sure how to prove out a HG problem other than look for smoke, excess crank case pressure or external leakage.
As far as I know the carb is good. I took it off an engine that was running right up until it threw a rod through the side of the crankcase. Choking makes it worse.

Someone suggested checking the valve clearances, so that’s next. At least that’s free lol. I also might pull the fuel cutoff solenoid and replace it with a screw just to eliminate that possibility.
 

Elkins45

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My 22HP Briggs was doing the exact same thing as yours. I replaced the plastic intake manifold with an aluminum manifold and cured the problem.
I can’t imagine those are easy to find anymore. Only 1 of my engines has an aluminum manifold. I might just have to give it a big squirt of RTV and see what that does.

I‘m not going to throw much more money at this thing unless I absolutely have to.
 
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