Toro Backfire on Cold Startup

[jshep1102]

Hi guys,
I'm working on a Toro zero turn (Model 75755) with a Toro branded engine # 139-0602. The complaint is that it intermittently backfires on a cold startup. It runs fine once started. I have only been able to replicate the issue twice and both were on a stone-cold engine startup. It backfired immediately upon cranking the engine (not after an extended crank condition). I figured it was either a partially sheared flywheel key (nope) or an overly tight exhaust valve lash keeping the valve open during the compression stroke (nope). It doesn't backfire when you shut it down after the engine gets to operating temp. Any thoughts on what would cause a backfire on a cold start?
Thanks
JShep

 

[bertsmobile1]

Backfire or after fire ?
Most likely either a bad plug or bad safety switch
Pull the kill wires off and see if you can eplicate the backfire
Yes = bad plugs
No = bad switch

 

[jshep1102]

Thanks for the quick response. Not sure what "afterfire" is. My understanding is that "backfire" is fuel being ignited in a hot muffler. Since the engine is cold, it can't be that. The 2 times I was able to replicate the issue, it backfired immediately upon starting the cranking. I have new plugs, air filter and fuel filter showing up on Tuesday for a general tuneup task (i.e. not firing a parts cannon but it sounds like maybe plugs may be a possible fix).

As for the possible bad safety switch, are you thinking it may be inadvertently killing spark with raw fuel still in the cylinder that gets ignited by the muffler?

 

[bertsmobile1]

Engine spins X number of times, fuel is sucked into the engine then blow ot of the exhaust unburned because the air"fuel ratio was wrong or the spark plug is bad or the spart is at the wrong time
Some of this is blown out of the exhaust while some stays in there
Then the engine finally ignited the mix in the cylinder and the still burning exhaust enters the muffler igniting the fuel that is in there .
IF you remove the exhaust pipe you will see about 1 to 4 " of flames coming out the head.

 

[jshep1102]

Ah! I get it now. I was only considering the hot muffler as the ignition source and not the HOT exhaust from the first ignition. I'll get the new plugs installed and see if I can replicate the issue. As you know better than anyone, troubleshooting an intermittent problem can be difficult. However, sounds like you've made my efforts much easier. Thanks! I'll post an update once I get the new plugs installed.

 

[bertsmobile1]

I like to run a customers mower for at least an hour before I hand them back
Even with this field testing a couple always come back

 

[jshep1102]

My neighbor loves me because I do the same and usually run out of grass so I cut his.

 

[jshep1102]

New plugs installed and no more backfiring. Thanks to Bertsmobile1 for all the help.