i had a small piece of sand in my carburetor one time and it got lodged where the fuel flows past the mixture screws. it threw everything out of whack! the truck ran funny, i got bad mileage, and it wouldnt hold an idle. i took the entire carb apart and the 2nd mixture screw (mines a weber 38) was the very last thing i checked and sure enough there was a piece of sand in it. that was the only particle of dirt in the whole thing and it made it not want to run. carbs are real sensitive
