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Briggs & Stratton 8.75 190cc on a self propelled push mower, 1 yr old

5 years ago

last modified: 5 years ago

This mower just won’t start.. backfires loudly. This is what I have done and checked so far. Changed spark plug, drained and replaced gas, drained carb. Probed main jet with tip cleaner, checked spark from coil, and from plug. Pulled flywheel and found keyway partially sheared, and replaced with new.

i have also checked valve clearances and reset to specs. Checked piston and valve travel with plug out and all looks good. Coil is spaced properly between flywheel .. this has me stumped.. After reassembling.... Problem still exists . Could this be a valve not seating ?

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Comments (5)

Steve J
5 years ago
Is the plug wet, as in, is it getting gas? You mentioned you probed the main jet, how about the pilot jet that's responsible for starting? If you pour some gas into the cylinder, does it start even for just a short time?
redbaron129 thanked Steve J
Jake The Wonderdog
5 years ago
Ok, so if it backfires then let's think this through:

That is caused by un-burned fuel getting to the exhaust and igniting there.

That can happen if it's flooded, or if the exhaust valve isn't closing or if the valve timing is off a fair amount.

Does that engine have a compression release?
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I took the carburetor completely off and apart. I cleaned every orifice I could find. I didn't have a pipe cleaner or wire, but I had a very small paper clip which I used to gently push through the holes in the main jet. I also picked up a copy of the B&S manual from Northern Tool. I cleaned the carb with carb cleaner last night. I was careful to remove all the washers and o-rings that could have gotten solvent on them. When I got home today I put everything back together. I put sea foam in my gas. I had put Stabil in my gas canister before I refilled the tank today. I tried to start it. It wouldn't start. I removed the plug again. This time, I noticed what looked like kind of a wet oily black substance on the tip of the plug which I wiped off. I replaced the plug and pulled. Nothing. Tried it several more times. Then I re-attached the coil cable to the plug :) and pulled again. Still nothing. I sprayed some carb cleaner in the carb intake. Two pulled and it started. Ran for about 2 seconds and died. Pulled. Would not start again until I sprayed the intake. Rinse and repeat. It just won't stay running. I thought it was a fuel problem, but I've exhausted everything that I thought would be related to fuel.
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redbaron129
Original Author
5 years ago
1st for Steve, engine will not run either by squinting gas in the cylinder or carb.. just backfires once. Today I did a compression Check on it and found 0psi compression.

did not have a lot of time to do further today, but obviously I either have a real bad head gasket, a valve not closing or broken rings. Will post more after disassembly
redbaron129
Original Author
5 years ago
Jake: don’t believe it has a compression release, but will know more when I pull it down to check head gasket, valves and cylinder . Found engine has no compression today. As previously mentioned, I had found flywheel key half sheared. Post more after disasembly. Below is a pic of this mower
redbaron129
Original Author
5 years ago
OK. Had a major breakthrough today.. tore it down and found 1 headbolt slightly loose and sure enough head gasket was blown out in that region. Surprises me that an engine that size, 190cc 8.75 lbs of torque and only has 4 head bolts. Replaced with new, reassembled and ”Walla”. Started right up. Thanks to you guys I got to lean on for this issue.

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