Briggs 6.5 Quantum won't start when hot

[bluedakotasky]

I have a Briggs Quantum 6.5 hp pushmower that will not restart when hot. It's for a customer. Prime the mower (cold) 2-3 times, and it starts on the first pull. Mow for 30 minutes, shut down to empty the bag. Empty the bag, put it back on (5 minutes, max) and it will not start. Leave it for 15 minutes, and it will start right up.

Checked: kill switch - disengages properly, engages properly.
Kill switch wire - no bare spots, routed correctly.
Air filter - clean.
Fuel system clean - this mower is garage kept. Very clean.

Replaced: carburetor and ignition coil.

Customer took the mower home. He mowed 30 minutes, repeat problem.

Help??

[dutch1]

Check compression when hot and cold. If there is a lack of compression when hot you could have valve problems--valve clearances not sufficient when hot.

[piston slapper]

Ditto Dutch.....might also want to make sure the head bolts are tight.
Make sure the plastic intake tube isn't broken or cracked.
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[dutch1]

Good point on the crossover tube, Slapper. I've seen a few of those over the years along with a defective crossover tube intake gasket where it is retained to the block.

[cgaengineer]

Valves not seating...
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[bluedakotasky]

Thanks everyone, I'll look at that stuff today. I'll just pull the head off and check everything. One other thing I haven't done is change the spark plug. I haven't seen a bad spark plug probably in 20 years. But you never know.

[piston slapper]

Originally Posted by bluedakotasky

Thanks everyone, I'll look at that stuff today. I'll just pull the head off and check everything. One other thing I haven't done is change the spark plug. I haven't seen a bad spark plug probably in 20 years. But you never know.

Hmmmm.....Where have you been for 20 years??????
Sparkplug would have been at the top of the list....

Here we go....Its springtime again.......

[bluedakotasky]

Sorry,
I was working on General Electric turbines for American Airlines, lol.... : )
We called them "ignitors" there.

[piston slapper]

Originally Posted by bluedakotasky

Sorry,
I was working on General Electric turbines for American Airlines, lol.... : )
We called them "ignitors" there.

Nothing to be sorry about....They dont call me " Slapper " for nothing.
I've been doin this stuff since Reagan was in office and I still aint got it all figgered out.

Nothin personal......Slapper

[bluedakotasky]

No problem! I appreciate you guys immensely! I certainly don't know it all, and I don't trust someone who claims that they do : )


I pulled the head and valves today. There was so much carbon build-up, that I'm not sure the intake valve was closing very well. It was really caked in there. That's crazy to me. This is a very new engine. Very low time. I don't know if it's from fuel leaking in after shut-down, or if it's oil leaking in after shutdown or while it's running. It's a mystery. There is just slight movement of the piston up and down in the cylinder (cold) but there has to be some clearance there. I will media-blast the cylinder head and valves on Monday. I won't have the head gasket until probably Tuesday. We'll try this and see if cleaning all the carbon out fixes the problem. I will put in a new spark plug, too. Although I know that when troubleshooting, we should only change one thing at a time : )