I've got a Yard Machine mower with a 4hp B&S model 10A902. Always change oil, plug and filter in spring. It surges or revs, belching a little black smoke each time. Used to do this only under a very heavy load.

The manual says you need to remove carbon buildup every 3-4 years.

1. Do you think that would solve my surging? This is the 4th season I've had it.

2. I'm thinking this might be a good diy project for me and my teens. Although I haven't seen an engine torn down since I was 13 when a neighbor and I 'rebuilt' his go cart engine and he did most of the work. Any special tools needed?

TIA
Jim

My lawnmower was surging before the fuel problem and I cleaned the rust off the magneto and it no longer surges so I would try this first since it is an easy fix. Remove the plastic cover over the flywheel(my lawnmower just had two 1/4 inch bolts). Then remove the pull cord housing(3, 3/8" bolts) than remove the magneto(2, 1/4 inch bolts) and unplug it from the spark plug. Then I used some sand paper to clean the end of the magneto contacts. I also used sandpaper on the flywheel to remove a little rust on it. I then used a matchbook to provide the gap between the magneto and flywheel and bolted the magneto back in place, reconnect the spark plug and put the cover and flywheel cover back on.

If this doesn't resolve it, I would think it is fuel related but I think this will resolve your issue.

Walt

Black smoke is an indication of rich fuel or too much fuel, bet your spark plug is sooty,if you have the tank type carb with primer on the side,usually this carb is made of plastic with no adjustment for air or fuel, your lucky if you get three years out of it without replacing it. IF this is your type carb,replace whole carb for 19 to 23 bucks,run for another two or three years.
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Is the air filter clean?

Yep. I clean it every year, splurged on a new one this year. The carb has been off for a cleaning too, although there's not a whole lot to do to it. Think I'll try the magneto cleaning tomorrow.

Lucky to get 2-3 years out of those carbs? very easy, heck mines going on 6 years. You can clean the carb, and still not get it all the first time... what was your cleaning method exactly?....
I'd replace the diapragm and gasket atleast as well.

BTW, the carb or tank can warp as well

Twas the old magneto. Didn't look that dirty/rusty but I cleaned it anyway and she's good as new. Course there's a clean carb, new plug, new air filter, new oil also.

Thanks

i had that problem i rebuilt the carb and seen there was a oring missing where the carb attached to the motor