Back firing and after firing at the same time.

[StarTech]

Here is one for you guys to figure out.

Kawasaki FX751-AS04

After operating for 30+ minutes, the engine will after fire on shut downs and will back fire thru the air cleaner on start ups. Luckily I already repair one that was backfiring out the air cleaner a month ago so I had inking of where to look for the problem. Problem was confirmed so the parts are on order; although, the distributor said they are on back order for nearly a month.

It is bad as I had already had to replace the starter due a hot restart problem. The starter flat refuse to operate when it was hot. Not good when the client uses the mower commercially. And it also took me nearly 5 weeks to get to mower due to severe backlog due health issues here.

 

[ILENGINE]

First thought was faulty ignition modules. Spark advance not working correctly causing it to fire too soon or too late. Or valves out of adjustment

 

[StarTech]

My initial thought was that because they were aftermarket coils but I had already swapped them out using OEM coils. Valves rockers were in spec when checked but it is a valve related problem.

 

[txmowman]

Check the valve lift. Might be a worn lobe on the camshaft.

 

[StarTech]

Cam lobe fine.

 

[Tiger Small Engine]

Cam lobe fine.

How many hours on Kawasaki FX engine? Have you pulled engine shroud to check for debris buildup on cooling fins, etc.?

 

[StarTech]

3000+ hrs. And the engine shrould has been removed twice already and I have remove yet again to effect the final repair but those parts are on B/O currently. No debris found and the both after market coils have been replaced with OEM coils. But after what I found I know those after market coils are fine.

 

[gearz]

Probably not the issue but a flywheel out of time? Maybe the key way is sheared or worn. Tight valves will also cause the issue but you said they were good to go. I had Iridium spark plugs installed at one time and they caused a cold back fire like the spark was to strong so I installed the old ones and it never happened again.

 

[hlw49]

It may not be an engine issue at all. Had one with a similiar problem and it was due to wet grass packed around the engine control module wiring harness. He had taken the grass chute off the deck and it was blowing grass all over the mower and it got packed around the control module and was shorting it out.

 

[Tiger Small Engine]

It may not be an engine issue at all. Had one with a similiar problem and it was due to wet grass packed around the engine control module wiring harness. He had taken the grass chute off the deck and it was blowing grass all over the mower and it got packed around the control module and was shorting it out.

3000 plus hours is a lot of hours (pretty much maximum for a Kawasaki FX series engine). Has this engine been rebuilt, to your knowledge? Even if it runs, it will be way down on power.
Perform a compression and leak down test and go from there.