Briggs 20 hp intek model 31P977 Type D635 E1. This engine is on a Craftsman 46" rider. Engine barely turns over. Take out the plug and spins fast. Good battery. I jumped straight to starter and still slow. Adjusted valves, same thing. Here's the funny part. With the engine cover off and the carb exposed, I can cover the carb with my hand and it will spin and start and stay running. But if you don't cover the carb, it's the same slow slow turning and no start. Does this engine have the compression release and could that be it? Help me somebody.
#2
Scrubcadet10
Same here with this thread, https://www.lawnmowerforum.com/threads/compression-lock-b-s-21hp-540cc-platium-with-xrd.62914/
that engine uses the Briggs 793880 camshaft, which is known for the compression relief mechanism to fail, causing this issue they upgraded it to part number 84005207, to help diagnose the cam problem, i usually remove the valve cover, and spark plug, and turn the engine over by hand, i believe on this one watch the exhaust INTAKE valve, on the compression stroke, the rocker arm should bump the valve open. if you don't see the bump most likely the compression relief failed.
#3
NorthBama
here is a video at the 5:20 min mark this guy shows the bump on the cam to check compression release
I checked for the bump the last time I had it down and I never saw it. Guess that's what it is. Wish the compression release was like the old Harleys where you just twist the clutch side hand grip, kick it to a start, then twist it back closed. I watched my brother get kicked over the handle bars more than once until he learned to not forget.
Thanks so much for the help. Now to decide if I want to tackle a cam replacement job. Never got that far into one.
#5
Scrubcadet10
This video should help you out some, different issue with the Cam though.
Looks like I'm going to have to. Just tried everything again. No movement on the intake valve so I'm sure it's the ACR. Forgot my feeler gauges and had to drive back home to get them. The mower is my sons. Going to check valve lash one more time to be certain then start pulling engine either today or later this week. I'll probably bring it back to my shop.
Thanks to everyone for suggestions and videos. I watched two and I'm certain I can do it. I'll let everyone know when I'm finished.
Got the mower to my house today and got the engine off. Tore it down and sure nuff, pieces of the ACR was in the bottom. Got it cleaned up and old gaskets cleaned off. Waiting for the parts to put back together.
Finally was able to get to the shop and finish the job. Parts came in and everything fell back in place like a charm. Got engine back on mower and it fired right up. All that's left now is deck belt put on, motor shroud and hood. Thanks to all for your advice and encouragment.
