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The mower started just fine at the start of this season. Then it started back firing on some restarts and now it will not start. I have looked at the fuel pump and it's fine. Looked at the carb bowl and it was clean although I did not clean the jets. Tried using start fluid to no avail. Didn't even try to start. Looked at the spark with a spark tester. It fires with no misses, bur the spark is yellowish. What do I do now?

Pull the flywheel and look at your Key - sometimes when backfiring it will shear the key and throw it off time

charles.harward said:

The mower started just fine at the start of this season. Then it started back firing on some restarts and now it will not start. I have looked at the fuel pump and it's fine. Looked at the carb bowl and it was clean although I did not clean the jets. Tried using start fluid to no avail. Didn't even try to start. Looked at the spark with a spark tester. It fires with no misses, bur the spark is yellowish. What do I do now?

Click to expand...

check your valve lash and possible bent push rod.

Thanks all, I will look into both things when I get a chance. I looked at the relative position of the flywheel magnet, the ignition coil and top dead center of the piston. Where TDC is, the magnet are almost past the coil. It would seem the spark happened just before TDC. I don’t really know how the alignment should be. I'm just a beginner at this "stuff".

Check to see if the oil level is high and the oil on the dipstick smells like gas.

Take the fastener off the flywheel and look at the keyways.
The 2 sides should align to make a perfect square.
If not the key has sheared
Your symptons are consistant with a sheared key as if not catastrophic the mower gets progressively harder to start and power output decreases till eventually the flywheel is so far out of position the mower will not start .
Usually this will have the 2 sections 1/2 apart ( like a Z )

Now there are other things that will cause the same symptoms .
But this one is easy to check .

Rivets said:

Check to see if the oil level is high and the oil on the dipstick smells like gas.

Click to expand...

Will check,

bertsmobile1 said:

Take the fastener off the flywheel and look at the keyways.
The 2 sides should align to make a perfect square.
If not the key has sheared
Your symptons are consistant with a sheared key as if not catastrophic the mower gets progressively harder to start and power output decreases till eventually the flywheel is so far out of position the mower will not start .
Usually this will have the 2 sections 1/2 apart ( like a Z )

Now there are other things that will cause the same symptoms .
But this one is easy to check .

Click to expand...

Will check tomorrow.

charles.harward said:

Will check tomorrow.

Click to expand...

I checked the fly wheel alignment and it is correct.

bertsmobile1 said:

Take the fastener off the flywheel and look at the keyways.
The 2 sides should align to make a perfect square.
If not the key has sheared
Your symptons are consistant with a sheared key as if not catastrophic the mower gets progressively harder to start and power output decreases till eventually the flywheel is so far out of position the mower will not start .
Usually this will have the 2 sections 1/2 apart ( like a Z )

Now there are other things that will cause the same symptoms .
But this one is easy to check .

Click to expand...

Looks good., thanks!

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.

bertsmobile1 said:

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.

Click to expand...

Thanks for info. The mower has started to smoke some. I will think about rebuilding the motor.