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I have a vertical shaft 17hp ohv briggs that will only run at idle for maybe a minute then backfires and dies. Compression is good, flywheel key is good, compression is good, valve lash is good, and spark is great. As far as i can tell timing is good. Tia.

Carb

Almost another has all required but won't run post.

Take off the rocker cover again, and watch the rocker arms moving while rotating the engine by hand, see if they both open/close the same, that the valves are opening about the same distance, etc..

Thanks for the replies. It has a carb off a running mower. The valves are opening and closing as they should and the decompression is working.

Flush out the fuel tank and install new lines from tank to carb.

Zav_5669 said:

Thanks for the replies. It has a carb off a running mower. The valves are opening and closing as they should and the decompression is working.

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So you watched the rocker arm travel?

OK Zav we will do it this way.
Get a can of carb cleaner and start the engine
As soon as it starts give the carb short shots of carb cleaner ( air filter removed ) and keep on doing this for for a minute or more ( it will run rough & sound aweful ).
If you can do this then you have a fuel problem
If you can not do this then you have an electrical problem
Let us know what the results are .

bertsmobile1 said:

OK Zav we will do it this way.
Get a can of carb cleaner and start the engine
As soon as it starts give the carb short shots of carb cleaner ( air filter removed ) and keep on doing this for for a minute or more ( it will run rough & sound aweful ).
If you can do this then you have a fuel problem
If you can not do this then you have an electrical problem
Let us know what the results are .

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If i ket it sit for a couple days it will run perfect for a few minutes then it acts like i turned the key off. No sputtering. Im stumped

Zav_5669 said:

If i ket it sit for a couple days it will run perfect for a few minutes then it acts like i turned the key off. No sputtering. Im stumped

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That is not what you were asked to do
If you do not do what your are asked to do then post the results I can not help you because the mower is there in front of you, not in my workshop .

FWIW, I'll give you an example that goes to what Slomo suggested. I had a customer with a Craftsman tractor with the gas tank just under the front shroud. The space between the fuel cap and the dashboard was relatively tight and as a result collected grass clippings and more importantly dead leaves. My customer wasn't too careful about clearing around the cap when he filled up. He got a number of big chunks of dead leaves in the tank. They'd slosh around the tank and eventually they'd cover the output of the tank and it would choke out. Time varied, and degree of choke out varied. Good repeat customer though even after I told him how to prevent it, LOL! Definitely do Bert's test as well!

Good luck!