Won't Start When Hot

[jkon]

I have a push lawn mower with a B&S engine 123K02-0185-E1. It is 13 years old but I maintain it very well. Starts right up when cold. And it runs fine. However I have an awful time starting it again when it's hot. Sometimes takes 5 or 10 minutes before I can restart it. Has new plug, new gas, new air filter and gas cap replaced also. Like I say...starts right up on first pull when cold. Thought it was the cracked fuel supply line but I replaced that and it did not make a difference. Haven't checked for spark when it is hot. How about the valves? Doesn't look like they are easily adjustable. Any ideas please? Thanks.

 

[darrinster]

Try starting fluid when it wont start. If it doesn't start then it might be the coil. Also make sure the brake handle is pulling the piece near the engine all the way back.

 

[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 5.43 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.

 

[Rivets]

Using resistance reading on today's CDI coils is not an exact science and please don't make your decision on the readings you got or what someone says. I would try the following. Remove the coil and remove all corrosion from the legs where they contact the block. We want a good clean surface. Reinstall the coil and set the air gap at .010". I use a brass feeler gauge, but you can get by with a good quality business card. Reassemble and give it a workout, at least 30 minutes. Shut off wait five minutes and try restarting. If it restarts problem solved, no start check for spark. Weak or no spark, replace coil.

 

[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???

 

[bertsmobile1]

Firstly what you were told previously about coil testing is right.
You need a lot of expensive kit to test them properly .
Secondly heat increases electrical resistance so a higher value when hot is expected.

There are a few things that have to be happening right to start the engine.
There is the O ring on the manifold.
The long manifold is prone to cracking, generally on the underside.
The manifold is prone to leaking where it bolts into the engine, replace the gasket and don't over tighten the tiny bolts.
Then we get to the valves which loose clearence when hot, which is why there is a clearence there in the first place.
If the inlet is too small you get too much decompression when hot .
Get a new inspection cover gasket because it will break when you remove the cover.
If you have to adjust the valve clearence then the head has to come off and the valves need to come out.
Usually this is the end of one of these engines as a valve adjustment is chargable at 1 hour + parts.

 

[darrinster]

Make sure the engine brake handle is pulling the lever on the engine far enough. After running the mower for a while it could of made enough slack for it not to pull it far enough to start again.

Have someone hold the brake handle down or put a zip tie on it and see if you can push the lever further.

 

[mechanic mark]

http://bsintek.basco.com/BriggsDocumentDisplay/iorpzOMV.HvjhJvcv4.pdf valve adjustment specs. are on page 9.

https://www.briggsandstratton.com/n...nginemanuals.html?searchrequested=123K02-0185 operators & parts manuals.

 

[jkon]

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???

 

[darrinster]

Still could be the coil. I had a Lawnboy that wouldn't restart after it was hot, it was the coil going bad. If you spray starting fluid into the carburetor it should at least try and start if not run for a few seconds.

Go to your local mower repair shop and ask if you can take a coil off a non running mower. The local shop here just scraps them anyways.