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Small Engines (Lawn Mowers, etc.)/Snapper mower no start when warm


QUESTION: Hi Eric,My Snapper mower model#7800845 with Briggs Stratton engine128M02-0412-F1 starts cold easily and will run/cut fine.But when warm the mower will not start until left to cool down and then will restart.I checked the choke butterfly is working properly (closed when starting cold and air vane opens choke fully when trying to start hot.I have a new coil and spark plug on mower.My ? is what else changes or could affect starting hot.Thank you for any suggestions.Doug

ANSWER: Is the thermostat behind the muffler working?

FYI, this has been big problem at higher elevation...they will not start when warm as the air is thinner and generally cooler.  The less air does not push the vain and the thermostat won't open the choke.

I've been told the system works great lower elevations.

The reason the engine will not start when warm is the chock butterfly is still closed.  Bet if you use a screwdriver to prop the choke open it will start right up when warm.


---------- FOLLOW-UP ----------

QUESTION: Thank you, but thermostat is working properly and choke is open when hot.I am leaning toward the intake or exhaust valve problem.Could you tell me which of these valves are opened or closed(sealed)-when starting mower hot.In other words what do valves do when starting hot?Doug

In theory, both valves will stretch, a tiny bit, when the engine is warm/hot...metal expands when heated.

The valve check procedure states to check the valves when the engine is cold.  You can find the clearances by Googling "Briggs and Stratton Engine Check Chart".  You have a L-head 12 series engine.

I don't recall every have a similar problem due to valve issues.  Have you check the intake tube to make sure it is tight where it bolts to the engine and the o-ring behind the carburetor is not flat on one side?

If you remove the air filter and open the choke by hand when the engine is hot, will it start right up?


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Eric A. Jones


Lawnmower Repair . Certified Master Service Technician from B&S. Have 23 years experience on B&S, Lawn Chief, Weed Eater, Echo, Peerless, Wheel Horse, Snapper, Atlas, MTD, McCulloch, Homelite and many other numerous brands. Specialize in electrical repair.


Born and raised in the midwest. Started tinkering with engines when I was about 14 on my Suzuki RM-80. I began lawn mower repair at a small hardware store. I knew absolutely nothing. I read lots of repair manuals and met an older fellow who taught me many lessons. I continued working on small engines through high school and paid my way through college working on mowers at the same hardware store. Decided to get away from the midwest and mower repair so I joined the Air Force. I repaired air traffic control electronic equipment and ended up in Hawaii where I got a part time job at Small Engine Clinic. I gained a lot of experience from the Small Engine Clinic and had a blast repairing small engines. I then took the Briggs and Stratton Master Service Technician test and earned my MST. I then traveled to Wisconsin where I attended the factory update training seminar and received formal training. Continued working on mowers part time as I completed 20 years of military. Retired from the military on a Friday and continued in the lawn and garden industry the next Monday.

MAS Aerospace Operations BA Mathematics AAS Electronic Communications AAS Electronic Technology

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