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Small Engines (Lawn Mowers, etc.)/B&S Engine wont start, weak spark



I have a Snapper Hi-Vac push mower that I have had about 20 years, though its not used a lot usually, and sat for the last year unused. Never had a single issue in the past with it. Now it refuses to start. Seems to be good compression. I replaced the fuel line and filter today due to age. I also replaced the spark plug with a new one, and checked the coil as well. It read around 4600 Ohms on the voltmeter, which should be good, but, holding the plug wire from the plug about 1mm its a faint spark. Any farther and there is no spark. Just barely a tickle to while holding the wire. Also tried it with and without the kill switch wire connected. The engine is a B&S 124707-3114-01 (numbers above the head on metal shroud over top of engine), which I believe is a Quantum, though the engine only says, built by B&S for Snapper.

My question is, what do I need to do to get this engine running again, or better yet, what is keeping it from starting? I'm thinking either the coil is weak, or its time for a rebuild.

Second, (I doubt the engine is bad, but worse case) is there a drop in replacement from B&S or would it need a rebuild? Mower is in great shape, no rust, etc, always stored inside and washed/dried.

BTW, Ive grown up around and always had Snapper, and older Cub Cadet (pre-MTD) products, so I would like to stick with what I know.

The engine should have electronic igntion so weak spark is not an issue...usually.  Only found one weak coil on a Briggs in 20 plus years.

To get it running I'm betting the carb just needs a good cleaning along with some internal parts, inlet needle and seat plus a bowl o-ring gasket.

Did you try priming the engine to see if it will fire?  This is the first thing we do with all products that come in for repair.

As for a new engine, it should be a piece of cake.  I replace several a year on old Snappers...older Snappers are better than the newer ones they are currently selling.

What is the crankshaft output (blade side) diameter and length.  The diameter should be 25MM (1 inch) or 7/8 inch and the lenght should be  3 and 5/32 inch.  Very common crankshaft.

Let me know if the engine starts by priming.

Small Engines (Lawn Mowers, etc.)

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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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