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I've been repairing small engines, as needed around the house, for about 35 years, and this time I'm about stumped.

The mower I'm working on is an MTD Yard Machines, Model 126-238M062 90174.

It has a Briggs & Stratton Engine, Model 128802-0829-01.

I got it working last week after cleaning the carburetor and replacing the seals and gaskets. It worked fine and I mowed the whole lawn.

Then, this week, after I mowed a couple of rows, I grazed a brick and the engine stopped. At first, the engine wouldn't turn, but after it cooled down I put the mower on its side (muffler side down), and worked the blade back and forth until it moved freely. Now the engine turns fine, but it won't start.

Here's what I've tried so far:

The flywheel is in proper position, the key is not sheared. I've removed the flywheel and reinstalled just to be sure.

The spark seems fine. I tried with the original plug and a new one. For test purposes, I bent the electrode out on the old spark plug so the gap is about 1/4" and grounded it to the engine block. The spark jumps the 1/4" just fine. I reset the gap from the coil to the flywheel using the business card method. I did notice that the resistance from the frame to the kill switch terminal on the underside of the coil is 0.7 ohms, which seems troublesome, but there's a spark in the spark plug, even with the gap set very wide. The kill switch wire is not grounded to the block through any other contact, and I've tried starting it with the kill switch wire disconnected without any success.

I opened up the engine from below and checked the valve timing. The timing marks are aligned. I also pulled the head and the valves appear to be working fine and synced with the piston. Nothing looks wrong in the drive shaft, connecting rod, piston, valves, or cam.

The last diagnostic experiment I did was to buy a can of old-fashioned starter fluid spray. I figured if the spark and intake were working properly, then the engine would at least sputter a little. No success at all. I think this means something important, but I'm not sure what.

My guess is that the engine is not pulling any air/fuel mixture in at all. Otherwise it surely would have popped and sputtered when I sprayed the starter fluid and pulled the starter rope. Since I hit the brick it hasn't seemed to fire at all, not even sputter, not even once, not even a little bit.

Could the exhaust port be blocked? Anything else to check? I've never disassembled/reassembled/disassembled/reassembled a small engine as much as this one [so, there is also the definite possibility of messing up one thing while fixing another thing].

At this point, it may make sense to take it into a shop, but now it's become a puzzle and a challenge that I'd like to solve.

Any suggestions for meaningful diagnostics? I'm about stumped...

--Carl

I forgot to mention that I already put in fresh gas.

Doesn't anybody have an idea to share?

Have you checked to see if you have compression, without it you won't get it to fire. I would have replaced that flywheel key when I had it apart, they only need to be a little cut to create problems. Don't like the idea that you had to work it back and forth like something was stuck. That would be where I would have started, why would it turn freely.

I agree with rivets about the flywheel key, replace it. Very cheap to replace. Seeing you hit something it sounds like the timing might be off. Could you have bent the crankshaft?

+1 to the above