Outdoor Power Equipment and Small Engines - Briggs quattro has me at wits end ?

I have a 4hp briggs quattro 10A902, Problem is it will start and run for about 30 seconds than will loose power and die. It will not start again until it cools down in about 5 min.Than the cycle repeats. Have cleaned carb,installed new diaphram,changed gas,new plug,checked plastic intake for cracks etc. same problem persists, have tried to keep it running by using the primmer buttom but it doesn"t seem to be a fuel problem.Throttle seems to be functioning properly.
Where do i go from here????????? lawnmower is about 3 years old with maybe 30 hours on it..............

Thanks Kent

Try it with a loose gas cap.... just in case the gas cap isn't venting......
30 seconds sounds too short of a time for a bad ignition coil...., but check for spark immediatly after it cuts out.

Not too likely, but, check that the flywheel brake cable is getting a full pull on the brake/kill mechanism.

Could it be a sticking intake valve Puey?

Unlikely, since it does run, if only for a short period. A sticky valve situation generally lessens as the engine is run since the valve guide material expands at a greater rate than the valve material with heat.

Are you sure the primer is working? Might try adding a bit of fuel in the carb throat when it cuts out. If this keeps it going I would go back to the diaphram or carb.

Thanks Guys, Have tried everything,but engine will not keep running,Gas in carb wil not keep it going. It has spark. Plug is wet after it quits. Valves are functioning,but exhaust valve seems to have alot of play in the guide and the seat may be a bit worn. I was thinking if it was the valve it wouldn"t even start.
It runs for 20 seconds every time like its on a kill timer????
The search goes on, I may abandon the search and go buy a new one as these are basically a disposable unit after looking at the parts used in the motor. Thanks Kent

If the plug is wet when it dies then it sounds like it is loosing spark. The brake/kill assy as Puey mentioned should be checked. Just the cable stretching a small amount can cause the kill switch to make and ground out the coil.

Try disconnecting the kill wire to the coil and see if it wont shut off.

Thanks Az Should have mentioned that I have tried that.I had thought for sure that it was shorting out but to no avail.
Thanks kent.

Check for spark immediately after it dies. If no spark, you probably have a bad coil.