Lawnmower Won't Start When Engine Hot

[jkon]

Well I had some time to look further into the problem. Seems to be pointing to the coil but still not 100% positive. Checked the condition of the spark when engine was stone cold. Spark looked weak. Also tested resistance of coil when cold 4.93 K ohms. Started it right up and used it for a good 1/2 hour. Shut it down and tried restarting. No deal. Checked the spark and it looked very weak. Also tested the resistance of the coil at it read 5.61 K ohms. Looked on line and B&S does not publish the resistance values of their coils. Found one guy on line who says B&S coils should read between 2.5 K ohms and 5.0 K ohms. He says that any higher than 5.0 K ohms is too much resistance and the coil is no good. The only thing I know for sure is that my test definitely indicates that when my engine is hot the resistance value of the coil is out of the range that he states. Any thoughts? Thanks all.

 

[HerrStig]

what's the plug look like after it has run, since it apparently will run when cold?

 

[Quest]

on B&S flathead with side-draft (low emissions type) carb type: check flywheel key distortion for ignition timing shift.

Q.

 

[jkon]

OK this is what I did over weekend....Pulled flywheel to check for sheared key. Key was OK. Checked ignition coil air gap which was OK. Removed ignition coil and cleaned mounting surfaces and posts. Re-installed ignition coil and set air gap. Pulled carburetor and disassembled. Purchased new carb rebuild kit. Carb was remarkably pretty clean but cleaned carb and all jets and ports. Inspected float and was OK. Not filled with fuel. Installed all new carb rebuild parts. Reassembled carb and re-installed carburetor. Engine has new spark plug and air cleaner filter. Re-assembled everything. Checked for spark before stating. Spark OK. Primed and it started on second pull. Ran and used it for 1/2 hour until good and warm. No luck on restarting after warm. Checked for spark. Spark OK. Coil is good. Now sure but seems to be pointing toward the valve clearances???

 

[EdwardC]

Just curious, you said that the coil may have been suspect due to weak spark and high resistance. Did you change the coil? The valves seem like a good thing to check.

 

[old1]

I would check the valve clearances. If within spec's then I would replace the coil. I have seen them seem to have ok spark, but just not enough spark to start hot. Just out of curiosity, how do you check the spark? I made a spark tester by using a new plug and breaking off the side ground electrode. Spark should jump the gap to ground if it is good.

 

[PhilsSmallEngine]

Folks, we're skipping a possible easy fix. Check the cable on the flywheel brake. If it has a kink in it or it feels really stiff when you pull the bar back, replace it. I've fixed many a machine simply replacing that cable when it won't restart hot and the coil isn't the answer. It starts cold just fine, but with the heat swelling, it just doesn't pull back quite far enough to allow an easy restart.

 

[jkon]

I re-checked for spark when cold and hot. Spark is present and good in both states so I did not change the coil. I checked the brake handle shutoff mechanism and that is working fine. Yow...one thing I haven't checked is the auto choke air vane mechanism and thermostat. No one has mentioned that. Wonder if that could be the problem???

 

[jkon]

It was the ignition coil after all. I replaced it this weekend. Mower now starts and runs like a top. Thanks all.

 

[bubbatime]

Originally Posted By: jkon
It was the ignition coil after all. I replaced it this weekend. Mower now starts and runs like a top. Thanks all.


Wish I saw this thread earlier - would have saved some headaches.

When you think your coil is going out, drop the plug gap down to .020 from .030. If this seems to "fix" the hot start issue, your coil is going bad, and needs replaced.