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: Backfires When Starting...Engine Warm

12-02-2007, 03:17 PM
Hi All,

I've been using my tractor for leave sweeping and noticed that it backfires out the exhaust when I jump back on the and start it...Sounds like a shotgun. I adjusted the carb as per the manual and I thought I fixed the problem but it's back again.....I'm not sure but I think it only does it now if my throttle is low and I start it....

Tractor is a craftsman II 18 hp kohler..1993 model.


12-02-2007, 04:11 PM
fuel entering the cylinder after shutdown (stuck needle? incorrect float level? I dunno)? If so, wont take too many more of those before something goes snap inside. If thats not it I am sure someone will be along with the exact fix for you shortly!

12-02-2007, 04:16 PM
Also, the only time it does this is when the engine is warm...When I start it cold it starts normal...

Could it be the way I start it? My throttle is low when I start it and the engine is warm...


12-02-2007, 04:28 PM
Also, the only time it does this is when the engine is warm...When I start it cold it starts normal...

Could it be the way I start it? My throttle is low when I start it and the engine is warm...

I doubt the throttle set low has any effect since the butterfly valve (throttle plate) in the carb will be wide open at cranking speeds. If you are killing it at wide open throttle you might consider lowering the speed to low idle before the ignition is shut off. Maybar

12-02-2007, 04:50 PM
I always shut it off at low throttle....

tjw in kans
12-02-2007, 08:05 PM
dont know if this applies to your year/model, but my 2003 kohler has an electric fuel shut off in the carb probably an air polution thing, has a visible wire coming out of the carb, might ck yours, dont know what year they started this, it must be shut down at 3/4 to full throttle for this to operate which doesnt sound correct, but it is, i know from experience and not completely reading my operators manual. if yours isnt equipped this way, i would go with a needle and seat sticking in the carb and letting fuel drip in after shutting off, but you would think the oil level would be over full after sitting for a week.

12-02-2007, 10:13 PM
I've been looking on the internet and found article about a john deere owner who had a similar problem and his turned out to be an ignition coil....Sounds to me like it's a timing issue..If it trys to start at the wrong time it will backfire....

Also, if fuel leaks into the cylinder wouldn't I have a hard time starting it such as engine flooding as opposed to a backfire?

Just trying to put 2 & 2 together..I could be wrong...


tjw in kans
12-02-2007, 11:32 PM
i guess you have looked and yours doesnt have a wire coming off the carb, just trying to start out with the simplest/most obvious before spending a bunch of money. if i remember correctly from reading posts, you purchased used in late spring? get an operators manual with it? have you tried your shut down and restarting proceedures yet? yours may be designed entirely different than mine. just trying to save a fellow member a buck. as far as a coil problem, it is either on or off momentarily, controlled by an electronic device or a set of points, usually if a problem occurs, you would have runnability problems. i wouldnt think timing problems should occur if it normally starts and runs correctly, unless something is triggering a coil at the wrong time, which is pretty unlikely. i dont know everything, but i would start with the simplest, due to my older age, been there, tried that, keep us posted, and try to keep the wife off your butt for purchasing something this old, [to me, not that old]. good luck.

12-02-2007, 11:51 PM
I just read posts about starting and stopping the engine as well as the manual. My manual says to start the engine (warm) put the throttle half way and turn the key. To shut down, move throttle to low, then turn key off. Read different posts, some throttle to low and wait a minute to shut down.

Since my issue is starting while warm I'll just shut it off when I'm done and not try to start it so many times...

And yes my wife did say not to buy an old tractor but I didn't have much money and I couldn't pass up the deal....she's not rubbing it in my face or anything since I bought an old tractor. Although I was expecting some kind of remark from her.


Small Fry
12-03-2007, 01:57 AM
Hello New here... reading your post. Hope you don't mind me sticking my nose in here ?
Throttle down and wait a minute is the right way for your tractor. That is so it don't build up gas in the manfold and back fire shutting down.
I would think repeated start up and shut down, in a short time could also cause this, even more so if your not shutting it down and/or starting the right way. Throttle down like you do but wait a 15 to 30 sec before turning the key off, and do the half throttle start up. Once your doing the way they want you to you might not have any problems.
You have a good cast iron kolher engine, the good old days......
Thanks for letting put in my 2 cents.

12-03-2007, 09:51 AM
Thanks for all your input...I'm going to take all your suggestions and see what happens...I'm hoping it's not the ignition coil. I'll need to take lots of parts off to replace that..

The other thing I was thinking about is that during the summer it never did this. This started with 1 backfire, then 1 here and there and now more often. It does this 1 out of 3 times.....

Also, this weekend while using it I was driving at 1/2 throttle which may or may not be causing this. I know the manual say to operate at full throttle but to me it's strong enough at a lower throttle....

I'll try my testing this weekend.


12-03-2007, 11:49 AM
A dirty air filter can cause a semi-choke rich fuel condition. Not that you don't have good maintenance practices but sometimes those things just sneeeeek up on us. Just a cheeeeeep thought. Maybar

12-03-2007, 11:55 AM
Hello New here... reading your post. Hope you don't mind me sticking my nose in here ?

No apologies needed Small Fry...That's what it's all about here....members helping one another out by brain storming a problem.

Welcome to the forum. :Welcome1:

12-03-2007, 07:32 PM
patch, it may have alot of carbon build up in the combustion chamber; cylinder head, piston, and valves. May have to decarbon with Sea Foam, or take the heads off and clean with a wire brush and install new head gaskets.
I say this because you say its fine starting cold... just a thought, Tony

12-03-2007, 07:39 PM
:Welcome1: , Small Fry, :ditto: on what Argee said! Enjoy the forum,:trink39: Tony

12-03-2007, 10:50 PM
What is and how do you use sea foam?

12-04-2007, 12:30 AM
Welcome to the forum, Small Fry! :Welcome1: Glad to have you with us!

12-04-2007, 04:38 AM
:Welcome1: Small Fry!

I hope you get your problem sorted out Patch. I have no idea, but it looks like you have plenty of leads to chase down.

12-04-2007, 07:56 AM
Just a shot in the dark pun intended, have you checked the valve adjustments lately? It sound as the valves maybe tight and causing a back fire if the intake valve is too tight.

12-04-2007, 12:15 PM
What is and how do you use sea foam?
try this for information on sea foam


12-04-2007, 07:31 PM
My first thought was the timing, too. When did you last have a full tune up?
I'd check all the basic stuff before buying coils and such.
When I decarbon, I use Permatex Gasket Remover. Spray on, leave it set and then wipe it of with a clean rag. I'm not carzy about using a wire brush on aluminum.

Small Fry
12-04-2007, 07:33 PM
One more thing.....we did change a coil on the same engine you have, this past summer. Tractor would start up cold, start mowing, once it got hot, it quit, no spark. Once it sat a while and was cold again it would start right up and do the same thing again once it was hot. So a coil will act up when hot if its going bad.
You could pull the plugs and check the spark when it hot, if it is delaid a second maybe gas is building up and bango when it does spark.
Try all the real easy stuff frist like has been said...good luck and be carfull !!

Thanks for the Welcome Guy's.

Bill Kapaun
12-05-2007, 12:47 AM
I always shut it off at low throttle....

Are you having back fire (through the carb) or after fire (through the muffler)?

If afterfire, I think that's your problem.
If the carb has the fuel cutoff solenoid, the engine is supposed to be shut off at high speed to purge fuel vapors out the exhaust system.
Try it!

12-05-2007, 09:58 PM
Thanks all your your help!!! Since it's cold the only time I will be using it is for leave sweeping and snow removal....and it gets dark during the week so I wait till the weekend.

I'll definitely do the easy stuff. And try shutting it off after idling down and waiting 30 sec.

I haven't done a tuneup since I got it 6 months ago and I don't know when it was last done. I have some tuneup parts and I plan to do one next year....

As far as pulling decarbonizing, is it easy pulling the heads off? I'm used to working on chevy motors and pulling the heads involves taking the rocker arms off, pushrods, etc., I guess with this motor the valves are facing you like a 5hp briggs engine.....

I'll keep everyone posted the next time I use it...