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?