How Much Idle Bypass Air?

[von]

Chevy 454 with 9.0-1 CR and cam with a fair amount of overlap. 10" vacuum at idle. Using a '68 Q jet #7028210 that had no idle bypass air, rebuilt to recipe #2 specs. Drilled holes in the primary throttle blades for idle bypass air. What size holes would be correct? Started with .088 but doesn't seem to be enough.

[Cliff Ruggles]

You will also need to add idle fuel by enlarging the idle tubes and idle down channel restrictions.  Adding bypass air alone will most likely not be enough for that low of a vacuum reading at idle.....Cliff

[77cruiser]

I ended up @ .140 on my 385 that has about 11" vac. but it whistles a bit @ idle.

[von]

Yes as I stated I rebuilt to recipe #2 specs. I added idle fuel by enlarging idle tubes to .038 and idle down channel restrictions .055. Idle mixture screw openings are .093.

[von]

Also, would idle bypass air openings in the throttle plates be the same size as openings in the base plate as far as effectiveness? In other words, to equal .135 of factory idle air bypass in the carb body, would .135 holes be drilled in the throttle plates?

[73 Z28]

With idle bypass air, too much will create a fast idle that cannot be reduced other than by plugging the holes and starting over. To get it dialed in, increase the size of the bypass air by about .010 at a time until you get satisfactory results. It involves more work as you have to remove the carb more times, but it will be worth it. How far out did you adjust the mixture screws?

I'm using .095 idle mixture screw holes set at 4.5 turns out and .099 bypass air. This works well with my cam which has low vacuum at idle.  Now my engine idles strong in gear at approx 700rpm.

Paul

[77cruiser]

I'd try to add the idle air bypass rather than holes in the plates.

[von]

My first choice also was to add idle air bypass to the carb body and baseplate, BUT the base plate was mismatched to the body and I couldn't see a way to make it work with the baseplate gasket in the kit. The carb was a friend's carb (now many miles away), numbers matching '68 big block Chevy carb, previously beautifully restored, replated, etc., by others, but ran terrible. The air horn was a mismatch too and there weren't any upper idle air bleeds at all (I drilled some). The by far easiest way to get idle air was to driil the throttle plates. I went conservative at .088 but my friend tells me the engine wants to die when put in gear from a 900 rpm idle. I know there could be distributor issues that could cause it too or a too tight converter for the cam, but I'm thinking it needs more idle air. we're just exploring this possibility among others.

[Cliff Ruggles]

Working with miss-matched parts can be difficult.  I've had quite a few carbs come in here with no upper idle airbleeds, and plenty with two pairs of upper bleeds, when the wrong tops were installed on them......Cliff