<VV> RE: Base Vent

grant young gyoungwolf at earthlink.net
Sun Jul 3 22:26:22 EDT 2005


I read a few quips on the HV base vent and thought I would set the record straight :-). The vent is simply a hole in the mounting base (after 1963) that results in two new air passages. Since the hole is drilled all the way into the throat, it connects the inside of the carb to the outside air at idle when the vent is open (if adjusted properly). The hole in the throat is still above the closed throttle plate. Since it is drilled into the mounting base, it also hooks up with the internal vent tube that runs up into a void in the top of the carb. This is the small tube that sticks up out of the top of the bowl in all carbs after 1963. If the vent is allowed to stay open more than a crack it will reek havoc with your adjustment efforts. If it is missing, your carb just isn't going work right - you can probably guess how I know this.... :-). While I have never encountered a cracked one, I find many that are hardened, stuck shut, or bent open. Occasionally, over-adjusting them will cause the tip of the spring to wear or break off, rendering it non-functional. (If your vent plug is hardened/shrunk, you can improve its condition with a short soak in some carb cleaner). The "book" says to adjust the actuating tang on the throttle lever so that it is just starting to touch the spring arm with the lever on the highest step of the fast idle cam - but, this is only good if the spring is properly bent to 90-deg. at the arm. A better adjustment technique is to set it while you are looking at it work in order to determine how much "flex" is in the arm of the spring. If it is weak, it might not open at all, and if it bent less than 90-deg, it will probably open too far. Most importantly, this setting has to be made after adjusting the fast idle setting with the speed screw and throttle lever, or it will be for naught..
The Carbmeister


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