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.
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.
Last edited:
