Briggs 9P702 hot starting issues - Husqvarna 5521P

[engineeringpunk]

New to the forum. I like to rebuild and fix lawn mowers for fun. I have kept my 1973 Lawn Boy running for years.

I recently acquired a Husqvarna 5521P mower for free because it didn't run. It needed some obvious stuff, which I did:
new muffler (and auto-choke mechanism) - old one was frozen up
cleaned the carb out really well
new spark plug
new control cable

The mower has a Briggs 9P702-0010-F1 motor on it, which is the 550EX, I believe. Starting from cold, this thing fires right up and runs strong. If you shut it down long enough to empty the bag or drink half a beer, it won't start back up. If you let it hang out for maybe 15 or 20 minutes, it'll start again.

I'm confident the choke mechanism is working properly, since it's new. I read somewhere to check the valve lash, which I did. It was not adjusted properly, so I adjusted it to the factory spec. I'm currently sitting here waiting for the RTV to cure up on the valve cover before I give it another whirl.

What else could cause issues with hot starting?

 

[ILENGINE]

When it doesn't restart do you still have spark at the plug. Could be a heat sink issue with the ignition module. Other than that you could have a problem with the auto choke system. There have been cases where the engine won't restart after being run for a short period of time because the engine isn't warm enough to restart without choke, but the exhaust choke heat sensor is holding the choke open.

 

[engineeringpunk]

So adjusting the valve lash did not help the hot starting issue, but it was out of spec so I'm glad I fixed it.

My next thought was the coil as well. I measured coil resistance cold at 5.8K Ohms. I think this is a little high, but I could be wrong. While the mower is running, the fan atop the flywheel keeps the coil pretty cool, but if you shut it off when hot the radiant heat from the jug heats that coil up something serious. I ran for 20 minutes and then stopped for 10 mins to let the heat soak into the coil and re-checked coil resistance at nearly 9K Ohms. I didn't even bother pulling out the park tester. That's too much resistance to get any sort of good spark.

29 dollars to B&S for a new coil and the problem is gone. Same test repeated and it starts right up. Looks like they updated the coil design too. The new one fit right in, but was not identical to the one taken out.