could be were they are running. could be their gearing. could be a malfunction in the motor. you have to have it idle the same on basically every motor. all engines idle the same. one might idle higher then another because of the truck its on or the clutch its using, but they all idle the same temp and speed basically. its all in the gearing and runnign conditions. or else a malfunction in the motor. something in the exhaust line maybe. too lean. way too lean. when you get your rusty, remember this. it doesnt kill to have your engine running to rich. but it kills your engine(literally) to have it running too lean. always make sure your truck has blue smoke coming from the pipe, idles fine, and has all around good performance. it needs to be able to grab hard off the line and still go wide open without cogging and sputtering.