I have a jd 240 mower [vintage 1989]. The mower started this season just fine! But started back firing and now it just won't start.

Okay after trying all of the about things I went on youtube and look for videos of others who had the same problem. (Not that I hadn't already done some of that.) I found 3 videos by Small Engine DIY
1.
2.
3.

The videos showed the diagnosis of the same symptoms i have been seeing. He changed a number components. The last thing he did was put in a used spark plug NGK BRR6ES. Mower started and ran great. The recommended plug was NGK BPR5ES.

After doing some of the same things as the videos, I remembered I put a new spark last summer and my problems started a little later. SO I changed the spark plug to the BPR6ES. EVERYTHING runs great. I haven't run it for very long yet because I stripped the locking nut on the valve adjustment. But we will see after that is fixed if it continue to be good. I'll get back and report later.

Thanks for all the suggestion.
 
The 5 is a hotter plug than the 5
If it needs a 6 to run that says the rings are on their way out and oil is depositing on the plug electrode.
 
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