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.
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