so i've been having trouble with the idle on my '79 columbia commuter. it idles fine when on the kickstand, but stalls if the back tire stops spinning (like at a complete stop). of course, i've tried adjusting the idle screw, but no setting works. the other day i took off the air filter and tried stopping at it idled fine and didn't stall. what does this mean? i think i need a new air filter anyway, but does this point to anything else wrong with the air flow in the engine? are there performance air filters i should try?