I've got a B & S 10A902-2072 lawn mower, recently pulled the magneto off ... yes, I know, not a good idea - I didn't think the gap would be too crucial.

But it won't start now, it started OK before dismantling.

Anyone know what the gap from the magneto armature to the rotor should be on these machines?

I've looked at the parts list and owners manual from www.briggsandstratton.com, nothing to help there.

tia, Peter

put the coil on, get the bolts just tight enough so you can still slip it closer to the flywheel (turning the flywheel so the magnets are away makes things easier) take a dollar bill, slip it between the coil and the flywheel, until it slides out with a little force, but doesn't tear, then spin the flywheel by hand, to make sure it doesn't rub until you get it right. then tighten the one bolt that doesn't hold the windvane on tight, the other one just tight enough to not make the vane stick.

The gap should be .006 to .010" at the magnets.
I set the magneto back and snug it up. Then rotate the flywheel so the magnets are directly below the coil, and put a .010 feeler gage between the magneto legs and the magnets. Loosen up the magneto bolts to allow the magnets to pull it down onto the flywheel with the feeler gage between them. Tighten up the magneto bolts, then pull the feeler gage out by rotating the flywheel.

thanks guys (and Tranny on lawnserviceforum.com), it worked a charm. it's amazing how much grief you can put these little engines through and they still keep chugging along. I just mowed the front lawn - about a metre high, mostly weeds - partly because the machine was laid up, partly because I was putting it off - and it went a charm.

Cut out midway and I had to let it cool down, my theory is that the air filter bolt is loose, where it threads into the carb is stripped, so too much air is getting in, too lean a mix, running to hot, having to take breaks occasionally.

The next job is to find a slightly oversized bolt for that air filter.

Thanks again for your advice, Peter

so the bolt going into the carb is stripped, that holds the air filter on? wouldn't run it long without fixing that first. dirt getting in isn't good. she'll still last you a quite bit more....... briggs can take punishment, and keep going..... use nothing but briggs