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I have an old (2006) Cub Cadet CC500 push mower that gradually started dying shortly after starting and then began tearing the starter cord out of my hand and, of course, now won't start. The engine is a Briggs Model 1P70FUA, 173ccs. I have: 1) made sure the flywheel key is not sheared, 2) repeatedly adjusted the valves (cold; 0.004" intake, 0.006" exhaust), 3) reset the magneto gap with a business card (0.012" and I read somewhere that the gap spec is 0.006" - 0.010"). Blade is properly indexed but I intend to put a new one on just in case. Compression is 120 psi. Has good spark and clean carburetor. 10W-30 oil changed seasonally. The parts guy at the place where I bought it says the engine diagram doesn't show a compression release on the camshaft like the larger engines have. I only use fresh premium gasoline in it.

Does anyone know for certain whether this engine has a compression release mechanism on the crankshaft (that can't readily be seen from the exploded engine diagram? Is it possible that the flywheel key or the key slot in the flywheel are bent or damaged and it isn't obvious? Or am I overlooking something else?

Thank you in advance!

I would say with 120 psi compression that the compression release is broken on the camshaft of your MTD engine not Briggs. So the clearance on MTD engines for ignition module is .008-.016. So .012 would be normal clearance. May have to double check the flywheel key to make sure it hasn't partially sheared.

pull the spark plug and see if it pukes gasoline out the plug hole. You said it started dying before the pull cord problem began. I wonder if it was getting too much fuel while sitting and now the cylinder is filling up even more.

Blade is properly indexed?

I agree to re-check the flywheel key by REMOVING the flywheel. They can be partially sheared & you can't see that without removing the flywheel.

With the rocker cover check the ACR bump on the exhaust rocker as the engine approaches TDC compression stroke. IF not present then the ACR has failed.

With OHV engines I always set the clearance at the minimum spec as the valve train does wear over time and opens up these clearances. I do this to extend the time between valve clearance adjustments.

Thanks. I guess it being an MTD engine makes a lot of difference; wondered why the model numbers wouldn't correspond. I didn't realize that it had a compression release lobe on the cam (probably because I was under the impression that it was a Briggs). Got one coming from ebay and so will see if that's it.

The blade has been properly indexed all along (except for the times I hit something and had to adjust it). No gas comes out of the spark plug hole when the rope is pulled.

It just gradually became harder to start and finally ran for something like 30 seconds at a time before quitting.

I've had the flywheel off twice to look at the shaft key. No evident shearing or visual distortion. But the fact that it's a straight rectangular key and not a Woodruff key has me wondering whether it had sheared sometime ago and I replaced it with a piece of straight keystock? Memory is foggy on that.

Thanks for all the replies.

So I bought a used camshaft assembly and see the spring-operated compression release valve. Is there anything tricky about replacing the camshaft? Any tiny parts that tend to fly out or get lost in the engine to watch out for? Thanks!