so today, I had to start and move my 68 Dodge truck, w/ a Grooved Edelbrock Performer 625 CFM carb. The truck mostly sits, only used occasionally. I Grooved this Edelbrock carb several years ago, I'm pretty sure I crammed a Large bit Groove under the throttle blades.
Ron says the Medium bit size is for most carb's, I think I know why now. The large size Groove removes enough material width-wise. That in order to fit a large Groove into the available bore wall left under the closed throttle plates on THIS carb...there is very little idle RPM adjustment possible. I don't remember now, but I might have backed off the idle speed screw some, closing the throttle plates, when I cut the Grooves.
So now, though the engine runs well, the idle RPM has a mind of it's own. Today, after it was warmed up enough, I had to back the idle speed screw off some to get idle RPM's below 1000.
So I think I made a mistake when I Grooved this carb.
My bottom line: Though the rule of thumb when Grooving anything is go as large size as possible--think twice on carb's!
IF Idle RPM is already set well on a carb, before Grooving it-- then only use the bit size that will fit under the throttle plates without further closing 'em!
For those interested in more detailed discussion re: Edelbrock Performer and Carter AFB's, and the Groove, try this thread:
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Gadgetman Reno, NV