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I have a newer sears 21.0 briggs (mrs) motor on my sears lawn tractor!After 20 or so minutes it runs terrible spits and cuts out ?Any ideas?Less then 75 hours on tractor!

Remove the gas cap, clean the vent hole thoroughly, cover the cap with aluminum foil and you'll be on your way to happy mowing forever after.

Wow, that is a bold claim!!!!!

I was just going to suggest checking your valve gaps!!!

But wrapping stuff in foil is always a good plan.....

Re: 21.0 briggs sputtering stalls after 20 minutes

Just ran it with gas cap loose still same results any other ideas???

The engine needs 3 things to run. Fuel, air and spark.

Is it getting a spark when it dies? If not several causes possible Loose plug or plug wire. Seat switch, clutch switch, blade engagement switch. Ignition switch. Check connections. Maybe bad coil.

Fuel problem, check fuel solenoid (under carb) check for loose wire or remove plunger completely, venting gas cap (you checked that). Kinked fuel line. Check valve clearance.

Air, carb bolted down to engine? If loose it will suck air. You have a air filter on engine?

Will it start up after dieing or do you have to let it sit for awile

Check the battery voltage just after it dies. I realize it cranked with that battery but the solenoid valve at the carburetor bowl needs 12 V to stay open.

IT starts right up but runs terrible