Small Engines (Lawn Mowers, etc.)/B+S 31c707 problem.


Hi. My parents have a Toro mower with a B+S 31c707 17.5 hp engine. For the last year or so it has been very hard to start, even with new battery. Turns about 1 rev and then stops. I spin it past compression by hand then try again until it starts. The last time they used it they reported a fuel leak at the carb/line but could not find it and it all leaked out. Today I got to it, pulled off fuel line and reattached added fuel. No leak now. Hard starting as I said then when it did start it had much popping/backfireing for 5 seconds then real quiet or not fireing but still spinning, then popping, then quiet and so on all the while spewing lots of grey smoke and dripping black fluid from exhaust. Gas + carbon rather then oil I think. They did not have this issue when they last used it. I figure it may be blown but the smoke doesnt smell of oil. Could be bad carb?... but then why the hard start? What do you think? If I know what it is I can fix it, finding that out is my issue. They are not against replacement rather then repair depending on the cost/what it is. Thanks.

Does the oil smell like gasoline?  How old is the mower?  How old is the battery?  Have you disconnected the battery cables and cleaned the battery posts and cable ends with a wirebrush?  Fixed a lot of hard starting problems just by cleaning the connections.

Remove the spark plugs and see if the starter spins the engine quickly.

Have you had the battery load tested?

Let me know.


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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