Lawn Mower Backfiring after shut down

[Gus]

2007 Cub Cadet 1000 series with a 20HP Kohler. 33 hours on motor.

Recently it has been backfiring when I shut it down. The engine will wind down and 3-5 seconds later a big ole pow! I haven't done anything to the motor besides clean the pre-filter, inspect the air filter, and change the oil after last fall and this spring. Performance does not seem to have changed at all.

My operating methods are to typically idle the motor down slowly as I'm heading to the garage after cutting and pull it into the garage on the slowest setting and turn it off. I have also cut the key at fast throttle once and it also backfired.... Dog took off to meet a jogger and I jumped off the tractor rather quick to get him. Jogger turned to a Runner after the tractor backfired!

Any clues for me guys? I'm assuming I'm getting some extra fuel that is combusting somehow.

The tractor has been doing this for probably the last 3 hours of use on the motor.

*no joggers were hurt in the making of this post*

[fishing tech]

Sounds like it is running rich. Mine has been doing that for as long as I can remember. The kids think it is cool but I don't think it will hurt anything. Do you use it in the winter? If so let it run rich.

[Down2Earth]

This is a problem, well not so much a problem just backwards from what normal people are used to doing. If you read in your manual it should say to shut down at half to full throttle because of the fuel shut off solenoid on carb. it shuts off fuel to the main passages but not the idle circuit. thus you draw in just enough fuel to cause this... So put it on 3/4 throttle for a few seconds before turning the key off. By the way back fire can cause big problems in these motor. you can sheer off pins and i've even seen it blow the muffler in half. Remember you can buy a Kohler 20horse motor from a few hundred dollars to into the thousands. you definately get what you pay for.

[paul pachowicz]

take a look at your manual. Some of the Briggs say to bring the throttle to full idle and let er run about 30 seconds before turning it off. That is one of our SOP's for all the types of mowers we have on our golf course. The new JD utility wagons we have run for a few seconds after the key is turned off. You might wanna give your dealer a call about this.

[Gus]

Well I finally had a chance to dig into this problem.

First things first, I took the plug out and noticed that it was very fouled up and covered with carbon. I removed the air cleaner and there was a noticeable amount of extra fuel around the carb intake. It wasn't pooled but it was wet to the point of probably just short of pooling. The precleaner was somewhat dirty however the air cleaner itself was spotless. I planned on replacing both anyway as part of yearly maintenance so it was changed and a new prefilter was oiled and installed as well. New spark plug installed.

I read over the manual and much to my surprise, just like DownToEarth and Paul mentioned, the manual states to shut the beast down while at or near full throttle. As most things are today, the high speed jet is fixed and the only adjustment that can be made is on the low speed mixture and low speed idle, two separate screws.

Since I haven't been operating this according to the manual (Shutting down on low speed) I felt that I should leave the carb settings alone for now and see how things run after changing just the filters and spark plug and operating it correctly. I also put a good dose of seafoam in the gas to hopefully help clean out anything that may have accumulated in the carb, although I feel that is somewhat unlikely as it has seen regular use since new.

I ran it a dozen or so times yesterday for mowing as well as some yard work and it never backfired at all.

I will continue to run it as is unless the backfires return then I think I'll dig into the carb settings. I also plan to check the spark plug after a few hours of operation and see how it looks.