My mower, a Craftsman model 917.378941, uses this spark plug. But the plug gets loose, and the mower backfires and blows the plug out. Now I can't get the plug to tighten on the mower. Help?

If your maintenance did not include changing oil each season that made the engine wear out sooner. Clean on the outside does not help the inside. You can get a price on rebuilding the engine but I have a feeling that you can replace the mower for not much more. Use it till it dies if you plan to replace it.

The most common reasons any motor backfires are timing being out of adjustment or dirty spark plugs, in this case a dirty plug. Carbon deposits cling to the electrode, effectively changing the gap of the plug, or glowing long enough after the actual firing to cause another firing in between the normal firing, (back fire). I'd lean towards a good spark plug cleaning or just a new plug.

Hi, check that you are getting a spark at the spark plug 1st. Also check that you my have an air lock in your fuel pipe.

I would check the big three first of all. Spark, compression and fuel. Sounds like you have spark and fuel. I would get a compression tester to check actual amount of compression. If all three of those things are good I would check valve lash. You will need make, model and serial number of engine. If you post these I can help find specs for you. Good luck.

I’m happy to assist further over the phone at https://www.6ya.com/expert/layne_0fccff498c319c32

James, it is at the front of the engine. It is next to the 4" by6" metal cover that says OHV. It is on the right hand side ( sitting on the tractor), There is a black wire the size of a pencil ( diameter ), hooked to it. It is set deep in the engines cylinder head. There is a rubber boot over the end of the spark plug. Remove the rubber boot, you will hace to grip it hard and pull it off. Then you will be able to remove it, using a deep 5/8" socket with a 4 to 6 inch extension.

Hope this helps, if you need more let me know.
Matt

If you need further help, I’m available over the phone at https://www.6ya.com/expert/matt_8dbc24bf722649ac

suggest to look at the carb adjustement unscrew

manual said use champion qc12yc spark plugs gap 40 hard to found i use b&s plugs fail out. do i need to use qc12yc plugs

Have you changed the spark plug and engine oil? The oil can become fuel contaminated and cause your symptoms. If this solves your problem you need to fit an inline fuel tap to stop it happening again. Regards Phil

First, take the air filter off and spray a shot of carb cleaner/ starting fluid in the carb. If it starts for a second and runs on the prime then its a fuel issue. Check fuel filter, fuel delivery, and carb.

Hi
Very possible that the valve is stuck open and piston could have hit valve causing it to bend at the head. This would cause valve not to close completely. Would have to remove head and remove valve and most likely replace intake valve.