Analyzing Needle Clearance NEEDLE JET CLEARANCE The needle jet is the brass tube the carb needle slides inside at the bottom of the venturi. If the area between it and the needle at closed throttle isn't right then that negatively affects the idle jetting and jetting up to 3/8 throttle. One way to tell if it's off is to adjust the slide opening for a steady idle speed and then measure the distance of slide opening from the bottom of the venturi. If it's more than 11% of carb size (multiply size by .11) then the needle clearance is too much. If less than 11% then the clearance is too little. [read more] It's fairly common that engines with noticeable vibration eventually cause the needle jet to be wallowed out by the needle vibrating in it. The bigger the engine, the more vibration even though the crank is balanced. Due to inaccurate carb jetting charts we have been under the impression that no gas comes up past the needle at idle. But that just isn't true. If you don't believe me then stick a toothpick in the main jet and then assemble everything and try to start it. If you can even get it started then it won't idle. SPREADSHEET Needle clearance can be off even when the carb is new. There is a free Excel spreadsheet that can help you know what the "new" needle clearance is. Just download it and enter the required data into the light blue cells and it will tell you the % clearance compared to what is ideal. HOME