Small Engine Advice

Saturday was my first chance to mow the lawn in about 2 weeks. I do my whole yards with a conventional push mower (21" Craftsman with a 6.25hp Honda motor). The mower has never given me a lick of trouble before, been using it for about 5 years now...its always been a 1 pull start. Grass was pretty long and thanks to a good application of springtime fertilizer its also quite thick now, so I was going along pretty slowly. After about 6-7 passes the motor just starts sputtering and it dies on me. Restarted right away and it happened again after a few more feet. It wasn't bogging down due to hevay grass, it was the motor just dying on me. Pulled the air cleaner, clean, oil level was correct and clean. Let it sit for a few minutes and it started right up again, only to die again after a couple more passes. At this point I'm thinking either an ignition problem, bad gas or something more serious like a scored piston wall.

Pet the motor cool off enough to get the plug out...it was a bit on the gross side but the electrode cleaned up ok, checked the gap and reinstalled, also pulled the air cleaner and sprayed the carb with gumout to see if I had any varnish stopping up the butterly...all to no effect, still crapping out on me. So I finally pulled the carb off and used the gumout on the intake itself, reassembled and fired it up. Spewed a cloud of black smoke for a minute and ran strong after that for a few minutes. Died again but when I restarted it I got a bog (for a single cylinder motor) backfire and got a pop of black smoke out of the exhaust that looked for all the world like someone fired a gun and it ran great for the next couple of hours.

So, based on what I've laid out here, I cna certainly swap out the plug and make sure its got clean gas...other than that, anyone have any useful suggestions? Small engines with carbs are not my forte, so apart from the very basics I'm a bit over my head.

Start saving up for a replacement mower?

Replacing the plug is definitely a no brainer for me. It was not gunked up with oil, rather it was a dark grey and the crud that was on it came rihgt off with a quick pass of a stiff bristled nylon brush (on my gapping tool).

My guess is its probably running rich and has been from the day i brought it home as I've never done anythign except change the oil in this mower...its been remarkably reliable. Its never smoked except when I put the carb cleaner in there, so I'm reasonably confident that the internals are in good shape. I bet I'm giving up a bit of power and using more gas than I should be.

Anyone have a link to a handy "how to adjust your lawnmower cabeurator" link? I could use a good primer on how to do it properly...though for all I know its locked and has no provision for adjustments...there is no throttle switch...its on or off with the grab handle, though there is a butterfly handle that will rev the motor if you tinker with it...seems to be some sort of self adjusting mechanism as I can see it move back and forth on its own when I was spraying the carb out.

I'll check the gas cap too. Other than the backfiring it sounds very much like fuel starvation. No fuel filter to plug up...could be the fuel cutoff valve full of crap though. Have to check that too.

Went to go tinker last night and when I tried to start it, the safety switch cable snapped right in the middle of the liner. Realizing god was trying to tell me to spend my time playing with my daughter instead of mowing the rest of the lawn I took the hint and got out the wiffle ball and a couple of squirt guns. One of the very rare summer evenings we've had this year...we decided to enjoy ourselves and let the work be damned.