127 Backfires
 

My 127 runs pretty well and has done so for so for nearly 40 years. However, if I don't let it idle for 30 seconds or so after cutting it will backfire thru the exhaust when I turn the key off. What is up with that.

Same problem with mine. I think that is normal.

I don't think that would be normal, particularly as it did not do that for the 1st 38 years or so.

Several things to check: carb --perhaps your air or fuel intake settings need adjustment or maybe the carb needs rebuilt, spark plug, timing, or are you shutting it off and then while the engine is stopping are you pulling the throttle down to minimum?

Cub Cadet 123

Sounds rich to me, my mower (1015) does that. I'm not changing it though because I like how it starts. I may also be completely wrong.

I always let mine idle,it helps cool the motor. Also I run 89 which will also help solve your issue, not to say there isn't another cause but if it runs good otherwise I wouldn't mess with it

you could have a carbon buildup on the head

I have not done this but one way to clean it is to mist the carb while running with water so it sucks the water in and will help break up the carbon

My 149 has done this like clock work ever since I've known it (the late 80s when Grandpa bought it) and Dad's 127 has done it for my entire life. My 1650 will do it on occasion but its fairly rare. I'm in the camp of not fixing what doesn't appear broken. If it runs good and you're not having other problems I wouldn't worry about it. Letting mine idle for 30 seconds is about 90% effective. Sometimes I get a slight pop. For me, 30 seconds seems to be magic, 25 and it will backfire nicely and more than 30 just seems to waste gas.

I had high hopes for carbon build up theory when I replaced the head on my 149 a month or two back. It made absolutely no difference and there was 0 carbon in there after I cleaned it up. I had a shop teacher mention the water "trick" once but that guy wasn't the most credible source. Pulling the head and cleaning it out manually isn't that hard. I think I read in the service manual that you're supposed to do that at some regular interval anyhow.

I have my own wild theory (it is just that) about the deteriorating muffler having a hot spot combined with trapping fuel rich gasses remaining from throttling it down. That would explain why some tractors do it worse than others, not at all, or at different ages. I may be grasping at straws.

An exhaust valve job has always cured the backfire on shutdown for me.
About three times on various 125/127's that I have owned over the years.

You just lap a new valve in or pull the entire motor down for the full monty?

Dad's 125 did that for years. We got to where we would just shut the key off, and instantly clap our hands over our ears before the BANG would arrive.

i'm also a "if it's not broke..." guy, but lapping the existing exhaust valve wouldn't hurt.

the spraying water theory works good at decarbing the combustion chamber. seen it do a real good job on a 350 olds motor once the head cracked from a leaking water pump. i use to do it occasionally to my old T/A, but i can't say it actually helped it run better.

You sure about that? Seems like reducing the air flow from the flywheel wouldn't do much to cool it.

If you work on it soon after it starts backfiring, you can just lap in a new valve.
I have even lapped in the existing valve to refresh the contacting surfaces.

Rich fuel mixture + hot exhaust = BANG!
BANG can = split muffler, ....sometimes.

Guys, this is not hard. Too rich, too hot. Or just too hot. Let your engines idle to cool down for at least a couple min. Lets them cool, much better on an engine. Do you like to run as fast and as hard as you can, then just stop and lay down on the ground panting? Neither does an engine.

The flywheel ALWAYS is moving air. It does not have to be running full throttle to do it's job. It does not hurt these motors to idle. I am 110% sure.

I always idle down my engine before shutting it off too.

some of you guys sound like whinny little school girls. buy a new box store tractor with the anti bang fuel shut off on the carb then you won't get scared or just take 30 seconds out of your life and idle it down.
common sense should tell if the velocity of the incoming air is to hi it keeps drawing fuel in when shut down. just because you throw it down to idle does not mean the air velocity has slowed down. a strong enough vacuum signal keeps pulling fuel. take the spark away and you get a bunch of gas and air mix in a hot muffler :Boohoo: need some cheese with that whine?

yes, unless my temp gun lies to me.

No temp gun necessary to measure that. Just put your hand in front of the tractor where the air comes out. You can feel it. :biggrin2:

Your the best! Almost pissed my pants I was laughing so hard. :beerchug:

better put a diaper on before you read my weenie of the week post in the suggestion area

I owe you a cold one! :beerchug:

An idling k series engine is enough to send the cottonwood in my garage flying in the front of my 147. As long as you got all of your tin work in place it moves plenty of enough air IMO.

I phrased that wrong. Thanks for the straight answer, I really didn't know.

:beerchug:

Well that was a lot of fun. Glad I asked. I do idle it down, unless I forget, and the backfiring stops. Just wondering if I had a defect. Perhaps an exhaust valve could be causing it, but that would be beyond me. My old 127 with power lift and a hydrostatic is a most amazing unit. It was 10 years old when I bought it from my next door neighbor and other than a couple filter changes and new oil ever 15 hours or so it still runs like new. I don't let rust get to it and while faded in places it looks good. I can't believe the condition of some of the Cubs I see. Some looks like they have been salvaged from a Spanish Galleon. Kind of sad really.

I agree, I've seen some really rusty cub.