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You are right about the blowby. You can verify this with a compression tester; if the compression is improved by squirting a *small* amount of heavy oil into the cylinder, it's a bad ring or heavily worn cylinder. Since it started suddenly, it's a bad ring (and repairable).

R Really? I've had more than one that produced feeble spark. One that I couldn't even see spark, but I could feel a twitch from it when my wife pulled the ripcord. Might be enough to run when the motor is revved, but not enough to fire at cranking speed.

Air Filter.

Are you sure there's no water getting in? Tarps can get holes.

What kind of motor is it? It might need nothing more than a two-pole, two-throw switch with a key. Or, if it's a matter of kids driving it without permission, the same switch with no key and a leather "do not drive" belt.

I've seen coils that would produce a tangible spark but wouldn't fire starting fluid. I think starting fluid is not all created equal. Some of it seems better at wetting the plug than exploding.

I've had no problem at all with leaving the carb wet, as long as the gas has Stabil in it and the engine is run periodically to keep it wet. Let gas dry out in a two-stroke carb and you're hosed.

Yeah I don't know why The Instructions always say to empty the tank for storage. Always keep it full of stabilized gas over the winter, and this goes triple for two-stroke engines. I normally run every small engine every 2-3 weeks for at least a few minutes, too.

Bingo. The knock was at about 300/minute, so probably half engine speed. There was a little crumb of rust in the oil.

Best never to use ethanol gas in anything. Ethanol absorbs water, and the underground gas tanks at stations often have leaks. With straight gas, the water settles to the sump at the bottom and is unlikely to get pumped out. Ethanol diffuses the water all through the underground tank. I have...

Frozen will hell be before ever anything from walmart I buy.

Safety devices work by cutting the ignition (or sometimes starter current) so that ain't it. You can't reliably judge plug wire insulation visually (unless it has been plumb rotted off by oil). If you don't have a pacemaker you can try grabbing it while the motor cranks.

One thing you could try is putting the plug in the jaw of a jumper cable and clamping the other end of the cable to whatever metal you can find on the block. I dunno why SAE 30 is so hard to find now but it's a pain. I've been using 10w30.

Slomo is right about the plug wire insulation. If it's weak in some spot, it might be partially shorting out to adjacent metal but if you move the wire to test the removed plug, it might no longer be adjacent to metal there. It's also possible (but very unlikely unless it's old and abused)...

An inductive pickup won't trigger consistently on a really weak spark. You're right it doesn't actually prove the spark is strong enough, but it's usually right and easier to use than pulling and ground the plug. At least if you have a 12V battery handy to power it. ...and you can always have...

Mounting bolts are rock solid and not a trace of blade/deck contact. The engine had sat for a couple of years, is it possible that a bearing could knock and then stop? Perhaps a spun bearing insert?

This is my favorite kind of problem: cut two wires, crimp.

Usually when a flywheel key shears from hitting a rock or whatever the engine stops immediately and you know what you hit, but I once had one that had apparently sheared previously but the flywheel didn't move enough at the time to notice. It later shifted from nothing but heavy grass that...

With advice from Scrubcadet10 I got a 16 year old Briggs 3.5 running. It clattered very steadily and very loudly. I replaced some oil with motor honey, no change. I assumed the rod was knocking and figured I'd beat my money's worth out of it and started mowing. About an hour in, the clatter...

I don't think the picture successfully attached, and it's bad anyway - took it in the shed and had to shrink it in half. But zoomed in, even I can easily see that it's 9J902.

Thank you!!! The other mower may very well be 9J902, it's not exactly easy to read the numbers anymore.

I have two push mowers made in 1992 and 2005 that have the old B&S Classic 3.5 HP engine. My last ignition coil finally failed, and after a dozen hours of searching I can't find a part number. Not even B&S seems to know these engines existed, although millions of them were built. The engine...