Briggs L head motor starts well but sputters after warming up

Patr said:

Hi cpurvis. To set the valve clearance I had the piston essentially at top dead center. I then moved it slightly to find where there was maximum clearance between the end of the valve and the cam follower. This is the position where I set the clearance to be 0.006" on the intake valve. On the exhaust I was careful to avoid the effect of the compression release. As far as the actual grinding of the valve stem, I held it perpendicular to the side of my grinding wheel and rotated it while grinding. I would take off a little at a time, put it back in the motor to recheck the clearance and repeat until I obtained 0.006"

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That's the part I was interested in. Setting clearance by grinding valve stems is usually beyond the ability, or willingness, of most people. You're the exception!

Do a compression test after the two minute run, because I would also be looking a valve problem.

Hi EngineMan. That's a good suggestion. I'm located in IN and it's cold here so the mower is put up until Spring. I am looking for ideas in case this problem exhibits itself again at the start of the mowing season. If so, I will check the compression when it starts to exhibit the problem.

Thanks to everyone who have responded to my problem with helpful suggestions. I especially want to thank Boudreaux who was first to respond and kind enough to speak with me by phone. I hadn't intended to work on this mower this Winter but was looking for ideas in case the problem returned in the Spring or got worse during the mowing season. Because of this forum and related suggestions my curiosity caused me to retrieve the mower and investigate further. I attempted to measure compression, even though the motor has a compression release on the exhaust valve, by driving the flywheel nut with a drill and socket. It indicated the compression was very low. This caused me to decide between repairing the motor or replacing the short block. I purchased a short block because one was available from Small Engine Warehouse for only $75. After incorporating the new short block, I tore down the old block to determine the cause of its problems. Before I get to that, I had indicated earlier that the flywheel key was OK. Before I removed the flywheel I looked at the alignment of the flywheel to the crank using a jewelers loop and found they were misaligned by .015". The key had been slightly compromised. Comparing this misalignment to the circumference of the shaft shows the timing was advanced by 2.8 deg. Back to the old block, the rings were worn significantly. Compression ring end gap measured .060" to .062" along the bore. This compares to a replacement spec of .035" let alone what a new ring dimension would be. In addition, there was significant scoring of the bore in a 1" wide section ranging from 1/4" down to 3/4" down from the top of the cylinder. This area was located roughly between the 2 valves. Some of the areas were deep enough to see light between the ring and cylinder wall by back lighting from inside the block and looking with the jewelers loop. There was significant carbon buildup on the cylinder head and top of the piston. The exhaust valve also had quite a lot of carbon and some of it was on the face of the valve and the face of the seat. This occurred even though I had removed the carbon a year ago and replaced the pitted exhaust valve and lapped it. I don't have a way to measure valve guide clearance but I thought the valve was a little loose in its guide. The exhaust valve plastic cam lobe was 1/16" wide at the top and pitted compared with the intake valve lobe which was smooth and rounded on top. The lift of both lobes appeared to be OK. All other internal parts were in good shape. It turns out that spinning the old motor at 425 RPM wasn't fast enough to eliminate the effect of the compression release because the new motor indicated low compression also. So, it appears I replaced the short block based upon an incorrect measurement but it got me to do what was needed and at an attractive price. I want to add that most of these posts, and my experience also, don't deal with motors that are worn out. Most are carburetor problems probably followed by ignition problems. Worn out motors are harder to deal with because compression is difficult to check with compression release involved and while carbon can be removed it returns fairly soon with worn rings or valve guides.