Having grown up with a Dad that was a mechanic and been one myself since I was old enough to start helping out in the shop, I can say for sure the stories you hear are not an old wives tale. I don't know what it is about the 'new' gas but left setting it will gum up a carb just as bad as the old stuff did. Actually it often seems to do it worse because it seems to evaporate faster. In other words leaving it with the carb bowl full for very long seems to be a thing of the past unless it's a gravity fed carb. I've seen carbs done both ways and ultimately the end result is always the same.
Case in point I just spent several hours getting both a Miller and a Lincoln welder back in operational condition after setting about 6 months unused, with the bowls left full. Since the fuel has to be pumped up to the carb the bowls on both were dry and the needles were gummed up in the stuck position due to the fuel evaporating over time. Basically the fuel in the bowl kept the float up and the needle shut then gummed it up. When the fuel eventually evaporated the needle was actually stuck to the degree that it kept the float from dropping when the fuel was gon. After a minor cleaning and unsticking I finally got them both started by using my hand as a choke. For ny of you that worked on the old stuff and have seen the new stuff, you know the spring loaded pieces of crap they call chokes nowdays won't cause a high enough vacuum in the intake to pull the gas through even a partially clogged port. Once running I fed them both some fresh gas with a dose of Seafoam. I had to begin their run time with the choke partially on to keep them going but after letting them run for about 8 hours apiece, varying between WOT and idle each time I walked past them, before they eventually got back to where they'd start and run like they were supposed to.
In the past couple of years I've gone through the carbs on two different 4 wheelers that had both set with gas in them. One for about two years, the other for 6 months to year. On them the tank is above the carb and it's all gravity fed. By the time I got them there was still gas in the bowl and tanks it had gone bad. The insides of the carbs each had a nice layer of varnish all over as well as a nice layer of thick goop in the bottom of the bowl. The carb on the machine setting the shortest amount of time I was able to salvage by using a piece of stranded copper wire to clean out every port in it and replacing the needle. The one setting the longest I had to replace. Never seen it happen before but when I pulled the bowl it looked fine but I didn't get time that evening to start cleaning so it set out overnight. The next evening when I got back to it the tower housing where the float pins as well as the center tower where the jets are, for want of a better term, had both disentegrated. Again never seen it happen before in my life but it did with this carb.
Now growing up I've worked on and seen dad work on many, many carbs off of the old B/S, Kohler, Onan, etc, etc engines. I've seen them left with gas in them and seen them run dry. Typically on those the ones with gas left in them tended to gum up and need some TLC before they'd run if left too long. On the other hand the ones that had been run dry were usually not that much trouble to put back into service with some fresh fuel and a few minutes run time with the choke left partially on.
What's different nowdays when it comes to the gas and the material the carbs are being made out of I can't say, I'm not the engineer designing them. What I can say is that the gas had somehow changed and it's not for the better, and I know the material the carbs are being made of has somehow changed also. With the combination of the two you'd better not leave any engine setting very long with or without gas if you think your going to need it again anytime soon. Your best bet, if you are going to leave one sit for any length of time, is to put some Seafoam, Sta-Bil, etc in the tank. Then run it enough to get the chemical mix into the carb and you'll be a little better off come time to get it going again than if you left straight gas in it....not alot better if it sets too long, but even a little better is better than the alternative....
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Today's Featured Article - Identifying Tractor Noises - by Curtis Von Fange. Listening To Your Tractor : Part 3 - In this series we are continuing to learn the fine art of listening to our tractor in hopes of keeping it running longer. One particularly important facet is to hear and identify the particular noises that our
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