I have a briggs and stratton 12.5HP MN: 286707-4520-E1 cranks fasts,pops wont turn over. I've replaced the carb,spark plug theres spark found out hard way gapped to .03. Battery checks out 12v with multi meter, fuel pumps pumping gas. Engine just spins and spins wont start up.(also) all safety are off seat is bypassed tied together.
I'm unsure at this point im going to buy a compression tool after work here. i'm unsure what range if should be in for good or bad though. and what to fix if it's bad
Sorry i'm a newby here. I thank you all for your input i'll do your suggested compression test tonight and get back to you guys with the number later on.
I’ve had them old flatheads running with only 60lb compression. Hard to get a true reading with decompression mechanisms on the small motors. On your original post saying it spins fast on cranking has me wondering if there is compression loss. Could be a head gasket as stated earlier or a valve not sealing correctly. A leak down test is actually the best to pin point a leak. Most auto stores with loaner tools have these and normally is free.
I'll have to look into a leak down test then. i wont even pretend to be any-kind of expert on this subject. leak down test is a new phrase for me. ill see what i can find on that.
It’s a very simple test but you do need a air compressor for the test. You basically put the piston at top dead center on compression stroke...this will have valves closed. Remove spark plug and insert the leak down test adapter in its place. Compressed air from the compressor fills combustion’s chamber and you listen for leaks. Noise from carb is intake valve leaking, noise from muffler is exhaust valve leaking, noise from oil fill tube is blow by or a head gasket blown. You normally hear a little noise in the filler tube but you don’t want much. I know this sounds confusing but to get an idea of how it works go to YouTube and search “Briggs leak down test” and there are examples. BTW you don’t need a big air compressor for this...I little tiny one works fine. There are ways to properly test using the gauges on the tester....but for small engines I just listen for air.
Thank you for that description. ill have to see if there is a tester at one of my local autostore. ill still find out the compression and get back to you tonight.
So I bought a compression tester and tried if. Results were 0 the needle didn't move during test. I checked the key under the flywheel bolt looks to be in good shape. Not sure what's next here. Attached is photo of key
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#13
Scrubcadet10
This isn't a bulletproof test, but put the spark plug in, and take off the oil filler cap, rotate the engine over and listen for air "hissssing" a little bit is normal for an old engine, but if it seems excess it may be the rings...
I would try replacing the head gasket as bert suggested, or at least look for blown spots.
Problem found the ring under the intake valve popped out of place. I was able to seat it back in and and have gotten alot of compression back. I have a exhaust gasket and head gasket on order.
Attached is vid of issue
Once I have the gaskets I'll let you guys know what happens!
That is the valve seat and it will continue to do this
IT popped out because the head got too hot
Some people have managed to get by staking the seat in position with the valve in place.
I just replace the head after working out why the head is overheating in the first place
#19
Fish
Sounds like a stuck valve,
or something like that....
