What was wrong with this mower?

[kjonxx]

valve keeper came off

 

[scoyote]

Solution was simple in the end---change out the plug or clean and re-gap---apparently the plug sparked in air but failed in the cylinder. Probably a simple step to add in our troubleshooting rather than tmaking assumptions? Usually when everything seems to check out correct but you're still not getting a result---you have missed something!

 

[doug9694]

When I first read the problem my 1st thought was try another plug. Maybe I am a little psychic or psycho??
Then thought of other reasons. Conclusion was excessive grass build up. That has the same effect too.
So. If another plug don't work it may be the cleaning that does.

 

[ErnieN85]

On small engines always try a new plug first

 

[scoyote]

You're correct on that Doug---If your blade is stopped up with packed grass engine likely not starting--but it would become noticeable fairly quicklythat the condition would be a problem?

 

[first pull]

I had an issue with a fix that seemed uninituitive enough to me that it might have some value here. Anyone interested in venturing a guess on what was wrong? Let me emphasize that I have fixed the issue, so the thread is more along the lines of entertainment/education vs. helping someone desperate to get something working.

I was cutting very high grass with essentially your basic 21-inch push mower with no self propel. It was powered by a 190 cc B&S L-head engine. While going through the grass, the mower hit a hidden piece of wood and stalled. After that it would not start. There was a bit of an attempt by the mower to run when the recoil was pulled, but it would not stay running no matter how hard I pulled. I took the spark plug out and saw a typical small-engine spark when I held the body of the plug against the head and pulled the recoil. I also did a compression test and measured a respectable 86 PSI reading. I will also mention that the flywheel key was undamaged. What was the problem?




I gave the true problem about a day after the initial post. It is towards the bottom of page 4. A few subsequent posts made it hard to find, so new readers are guessing and not finding any answer. Here it is again right at the beginning so nobody will miss it!

ANSWER

In the first post, I described two test, one for ignition, and one for compression. The ignition test needs to be looked at carefully.

Ignition test procedure: Remove the spark plug, hold the body of the plug against the cylinder head, and pull the recoil.
Ignition test result: There were sparks that were as expected for a small engine.
Ignition test conclusion: NOTHING

If no spark appeared, the test would have indicated that there was a problem in the ignition, so either the armature/coil, the spark plug, the air gap, or very unlikely the magnet on the flywheel was bad.

Seeing sparks only meant that the ignition might be OK. Why? Because the spark plug was firing in air rather than in the cylinder. The voltage needed to fire at atmospheric pressure is less than when the pressure is increased. I changed the spark plug, and the engine started right up and ran normally. Apparently the impact with the log dislodged a deposit in the spark plug which fouled it enough that it would fire in air but not in the cylinder. To confirm what had happened, I cleaned the original plug with a spark plug cleaner and reinstalled it on the engine. The engine again started right up and has been running with this plug for several hours now.

You don't need to do all of that to test spark, just go to harbor freight and get a spark tester for under $5. You don't have to remove the spark plug that way. If I had to guess you may have bent one or both of the push rods

 

[scoyote]

If you were following--the plug tested fine in air or as it would with a spark tester which tells you coil is fine but not the whole story---the plug was not firing in the hole!

 

[GrumpyCat]

Mid 1990s I had a Kawasaki engine on a John Deere 21" SP that did the same sort of thing. A new NGK spark plug would last 2 years, not 3. Sparked outside the engine but would not run. IIRC it would more likely start cold than restart hot. Once running it stayed running.

 

[SlopeMan2]

I had an issue with a fix that seemed uninituitive enough to me that it might have some value here. Anyone interested in venturing a guess on what was wrong? Let me emphasize that I have fixed the issue, so the thread is more along the lines of entertainment/education vs. helping someone desperate to get something working.

I was cutting very high grass with essentially your basic 21-inch push mower with no self propel. It was powered by a 190 cc B&S L-head engine. While going through the grass, the mower hit a hidden piece of wood and stalled. After that it would not start. There was a bit of an attempt by the mower to run when the recoil was pulled, but it would not stay running no matter how hard I pulled. I took the spark plug out and saw a typical small-engine spark when I held the body of the plug against the head and pulled the recoil. I also did a compression test and measured a respectable 86 PSI reading. I will also mention that the flywheel key was undamaged. What was the problem?




I gave the true problem about a day after the initial post. It is towards the bottom of page 4. A few subsequent posts made it hard to find, so new readers are guessing and not finding any answer. Here it is again right at the beginning so nobody will miss it!

ANSWER

In the first post, I described two test, one for ignition, and one for compression. The ignition test needs to be looked at carefully.

Ignition test procedure: Remove the spark plug, hold the body of the plug against the cylinder head, and pull the recoil.
Ignition test result: There were sparks that were as expected for a small engine.
Ignition test conclusion: NOTHING

If no spark appeared, the test would have indicated that there was a problem in the ignition, so either the armature/coil, the spark plug, the air gap, or very unlikely the magnet on the flywheel was bad.

Seeing sparks only meant that the ignition might be OK. Why? Because the spark plug was firing in air rather than in the cylinder. The voltage needed to fire at atmospheric pressure is less than when the pressure is increased. I changed the spark plug, and the engine started right up and ran normally. Apparently the impact with the log dislodged a deposit in the spark plug which fouled it enough that it would fire in air but not in the cylinder. To confirm what had happened, I cleaned the original plug with a spark plug cleaner and reinstalled it on the engine. The engine again started right up and has been running with this plug for several hours now.

enjoyed the read. I have been fooling with lawn mowers, starting with push type reel mowers for 80 years. With help, I built one of the first twisted belt rotary power mowers in our small town, , using a Model N B&S, a wooden deck, and wooden plow handles. While using many mowers over the years, I have never had a spark plug fail due to a hard jar like that. Interesting story, thank you

 

[TobyU]

I had an issue with a fix that seemed uninituitive enough to me that it might have some value here. Anyone interested in venturing a guess on what was wrong? Let me emphasize that I have fixed the issue, so the thread is more along the lines of entertainment/education vs. helping someone desperate to get something working.

I was cutting very high grass with essentially your basic 21-inch push mower with no self propel. It was powered by a 190 cc B&S L-head engine. While going through the grass, the mower hit a hidden piece of wood and stalled. After that it would not start. There was a bit of an attempt by the mower to run when the recoil was pulled, but it would not stay running no matter how hard I pulled. I took the spark plug out and saw a typical small-engine spark when I held the body of the plug against the head and pulled the recoil. I also did a compression test and measured a respectable 86 PSI reading. I will also mention that the flywheel key was undamaged. What was the problem?




I gave the true problem about a day after the initial post. It is towards the bottom of page 4. A few subsequent posts made it hard to find, so new readers are guessing and not finding any answer. Here it is again right at the beginning so nobody will miss it!

ANSWER

In the first post, I described two test, one for ignition, and one for compression. The ignition test needs to be looked at carefully.

Ignition test procedure: Remove the spark plug, hold the body of the plug against the cylinder head, and pull the recoil.
Ignition test result: There were sparks that were as expected for a small engine.
Ignition test conclusion: NOTHING

If no spark appeared, the test would have indicated that there was a problem in the ignition, so either the armature/coil, the spark plug, the air gap, or very unlikely the magnet on the flywheel was bad.

Seeing sparks only meant that the ignition might be OK. Why? Because the spark plug was firing in air rather than in the cylinder. The voltage needed to fire at atmospheric pressure is less than when the pressure is increased. I changed the spark plug, and the engine started right up and ran normally. Apparently the impact with the log dislodged a deposit in the spark plug which fouled it enough that it would fire in air but not in the cylinder. To confirm what had happened, I cleaned the original plug with a spark plug cleaner and reinstalled it on the engine. The engine again started right up and has been running with this plug for several hours now.

All true but the theory for a discussion like this is kind of flawed because I don't have the machine right in front of me.
I will tell you how I would have troubleshooted it though.
I would take that compression tester and put it back in the toolbox and leave it for good because they are a little to no use on small engines I'm not worth the aggravation of walking over to get them. Lol
I also would not have touched the spark plug or even bother checking for spark first.
You describe the mowers running fine and then it hit something and I would not restart but you could pull it normally.
You didn't mention anything about it jerking the rope out of your hand or popping her back firing or being abnormally different to pull other than like a mower that was out of gas so I wouldn't have immediately suspected the flywheel key but I would have kept it in mind.
The first thing I would have done was to go get my car spray because that is the first way I diagnose all mowers now because over 85% of the time it is a few related problem where they're not getting enough.
A quick shot into the carburetor and a few pulls in this case since you've told us the problem, would have ended up giving the same result of a no start so ONLY THEN when I remove the spark plug.

You are completely right about you can't trust 100% I plugged that fires like that but I'll go one step further... Some people don't even use the plug! Some people use one of those dumb little spot testers and put it in line or they check for spark from the spark plug wire to the block or to a screwdriver stuck into the block etc.
This only checks your ignition system on your mower for spot but leaves the barely likely possibility of a flood not working properly or at all!!

So I would have checked for part too but you get pretty good after doing thousands upon thousands of these knowing what a good spark looks like from a weak spark.

If I would have determined it was a good spark but still would not run when I screwed the plug back in my very next step would have been to swap it for a known good plug..

I keep good used plugs just for this reason because as you've proved, a plug can seem okay sitting beside the block but not run in the machine.
I do 100s of these a year and sometimes over 1000.
Even though the overall number of plugs being the problem is very low, I've had two this season that was exactly like you presented this..
Another plug and it was up and running.
The stats I find on bad plugs is about 1 in 450.

Smart, systematic troubleshooting will get you to the root of the problem much faster than what some people do and no need to check flywheel keys etc when you don't rule out something like plug based on bad actual info.