Which gas do you all you

[BC_coops]

Pure Amoco White gas, labeled "Ultimate" 93 octane.  I've heard reports that it has the lowest additives of all American blends (in fact one tank farm operator told me that Amoco Ultimate is shipped from the refinery in a specially monitored slug in the pipeline, and that it is reportedly pure distilled gas, without any additives.)

I think it has the lowest vapor pressure, e.g. your gas can doesn't "spurt" at you when you open it.  (I bought some Shell 93, and it had a totally unacceptable vapor pressure.  You could watch it, e.g. the additives, evaporate the blend down to 3/4 of its original volume before your very eyes (actually say 1 hour, if left open on a dish!)  I think this effects the operation of hte pump in the carb, and the operation of the pop-off pressure of hte metering valve and jets in the carb -- lessening the likelyhood of leaning out.  If metering valve and pop-off pressure of your carb was set up for regular (lower octane) vapor pressures, then going to a high vapor pressure fuel (non-Amoco 93 octane) would theoretically tend to run the saw leaner.  It woudl be the same thing as increasing the pressure of the metering spring.