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Discussion Starter #1
My lawnmower starts fine when cold. But if I allow it to stop (like for emptying the grassbag), it will not successfully restart unless it cools down. If I prime the fuel line by a few pumps, it will run smoothly for a few seconds, but then stall out again. After it cools a little and restarts, it will run without a hitch until I stop it again.

Any thoughts?
 

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Not likely, but loosen the fuel cap when you try to restart to make sure the vent is not partially plugged and causing a vacuum in the tank. Usually this will cause the engine to die however. Worth a try. Another guess, maybe the coil is getting heat soaked. The fan on the flywheel blows air over the coil and around the cylinder while running. It may be getting superheated and providing a weak spark when you turn it off, possibly requiring a rich condition to fire like when you push the primer.
 

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Discussion Starter #3
Not likely, but loosen the fuel cap when you try to restart to make sure the vent is not partially plugged and causing a vacuum in the tank. Usually this will cause the engine to die however. Worth a try. Another guess, maybe the coil is getting heat soaked. The fan on the flywheel blows air over the coil and around the cylinder while running. It may be getting superheated and providing a weak spark when you turn it off, possibly requiring a rich condition to fire like when you push the primer.
I did try this earlier with no apparent effect. I don't know how to spot my coil, or how to alleviate the hot spot. What does the coil on a lawnmower engine look like?
 

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Post the model number for the mower and engine so we know what you have. Sounds like something is expanding when the engine warms up creating an air leak. Have a good one. Geo
 

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If you take the shroud off of the top of the engine you will see the black box with layered metal wings that hugs the flywheel. The sparkplug wire goes into it. That is your ignition coil. No way to fix a coil, if it shorts out when hot you replace it. What engine is this? Have you checked for a good spark when it won't start?
 

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Discussion Starter #6 (Edited)
The engine is a Briggs & Stratton Quantum 120000 series; model #123K02. I did manage to find the coil. There wasn't any debris that seems to have been capable of creating a hot spot.

Also, since the engine will run for a second or two after priming I'm taking that as a signal the spark is good.
 

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Have you checked for spark when this happens? Remove the spark plug, hook spark plug wire back to plug. Have someone touch the tip of the plug to unpainted metal, like the area of the plug hole. Pull on starter rope and look for spark at the tip of plug.
 

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When the engine is cold, start it, while running spray some carb/brakeparts cleaner behind the carb where the carb mates with the block, then when the engine is warm, as if it were time to empty the grass bag or whatever, again spray along the mating surfaces, if in either case the engine tempo changes you have an air leak, that carb uses a large "O" ring on the intake tube to seal the intake, the "O" ring fails or gets lost when the carb is removed and the "O" ring comes off stuck to the carb or is cracked in multi pieces and then falls off, because most people don't know anything is supposed to be there. Have a good one. Geo
 
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