John Deere Lawn Tractor deonation

[FrankP]

Non-ethanol gas is getting harder to find locally in 87 and 89 octane so I use 91 octane in my Deere twin-v lawn tractor. The thing is it developed detonation regularly shortly after shutting off the tractor. The unit is only 2 years old and a addition of gasoline cleaner did not help. Lately I started adding 2-cycle oil to the gas at a concentration of 1:500 and whola no more detonation!
Anyone have some thoughts about this?

 

[tom slick]

Just to clarify, the engine has "run on" or "diesels" when you shut it off?

Oil lowers octane, it's odd that it solved your problem. If you were to run straight gas now would the problem come back?

 

[JTK]

Frank, I've found this to be a product of today's OPE carbs set to EPA standards. The Kohler Command 20hp V-twin in my tractor will diesel on forever unless I kill the ignition at more than half throttle. If I s/d at idle it will afterfire... *Ka-POW*!!! out the exhaust, slightly more than idle and it will diesel.

 

[FrankP]

After I turn it off after 10-15 seconds there is a"pop" sound. Well was a pop sound.
Afterfire is perhaps a better term.

 

[JTK]

All I can figure is the exhaust temps run so hot on them, the last bit of fuel running through the engine after the ignition is cut, ignites in the muffler. If you 'lean out' this mixture by opening the throttle, it wont pop. All the engines today have the fuel cutoff needle on the carb bowl, but there's always the last blip of fuel that squirts through on s/d.

Joel

 

[Brett Miller]

I let mine idle a few minutes while I blow off the tractor and I've never had this happen.

 

[doubleclutch]

I'm with Brett on this one. I let my equipment idle for a few minutes to let it cool down. If you don't, you will soon be replacing your muffler.......

 

[JTK]

Hehee.. Neither of you fellas owns a late model Cub Cadet GT with a Kohler Command then. You can let it idle until the cows come home and it will still *BANG* on an idled s/d. It's very common with this particular machine, I'm assuming because of the design. It's even in the owners manual that you need to s/d while throttled up. Tons of posts on it on other OPE forums. OTOH, I've used other brand equipment with the exact same Kohler twins that will not backfire on idled s/d.

Joel

 

[FZ1]

Try Shell V Power in 93 octane.

 

[RWEST]

Fairly common with Kohler v-twins, unless you let 'em idle down for a few minutes. Scared the heck out of my puppy the first time it did that, from then on, every time he saw the tractor coming - it was look-out, hiding place here I come!!

 

[FrankP]

Guys after the addition of 1 oz of 2 cycle oil per 4 gallons fuel it no longer backfires nor detonates.
Has to be the 2 cycle oil. Nothing else ahs changed.

 

[RedOak]

I was having the same issue with my kohler twin cylinder also. The kohler air cooled engines tend to run very rich anyway. Anyone ever when you start them they tend to smoke heavily for 30 seconds or so even when you back off the choke? I to let the motor idle for a few minutes and then shut it down and has stopped the back fire. Trouble is air cooled engines are more efficient at near or full throttle because that is where the cooling comes from the air fins on top of the motor.