Briggs & Stratton 12 HP Flathead - No Compression

jgad60

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I am trying to resurrect an old MTD riding mower with a 12 HP Briggs & Stratton single cylinder flat head. It had been sitting unused for several years. I got it running briefly but it started backfiring including flames coming out of the carburetor. Then it wouldn't run at all and would not even fire (burn any compressed gas/air). I checked for compression and found none. I am getting good spark at the plug. I took off the head expecting to find a burned or stuck valve. The valves, and seats, look great and move freely. When rotating the crankshaft I observed that while the valves opened and closed at the appropriate corresponding times in the pistons travel the intake valve did not fully close until just prior to the piston reaching top dead center on the firing stroke. It opens fully when the piston is traveling down and appears to close as the piston begins its compression stroke but upon close inspection it does not fully seat, probably 95 to 98 percent, until just before the piston reaches TDC. It then fully closes. I spoke with a mower repairman who suggested that might be a compression release mechanism. I checked online and found that some motors do have some kind of compression release system which retract centrifugally. I put it back together and put on a new coil hoping that maybe it needed a stronger spark. That didn't help. At the intake manifold it definitely sucks in but then blows back out. Spraying in starter fluid does not even evoke a single "pop" and visually confirms the blowing out, with some force, of the expansion chamber contents.

Does anyone have any input or knowledge about this problem and what could be causing it? Is there a compression release aspect to these motors and if so could it be damaged or stuck internally and causing the problems I describe?

B&S model 289707 type 015401 code 93042641

Thanks
 

Tinkerer200

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Your flat head B&S single does not have a mechanical compression release mechanism. It does have a cam grind which does hold the Intake Valve slightly open until late into the compression stroke thus blowing back thru the carb. Due to "lag time" of incoming air, this has minimum affect at operating speeds. Sounds to me like you need to make sure the valve clearance is OK but probably lap the valves first. Might also want to check that the flywheel key is OK. You can remove the flywheel nut, washer, screen, and look down the key way and make sure fly wheel and crankshaft key ways line up.

Contact me at address below, put in proper format, remind me what you want and I will send you a Service Manual covering you engine.

Walt Conner
wconner5 at frontier dot com
 

jgad60

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Thanks for the advice. I will check those things and get back to you.
 
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