Are all quadrajet idle tubes generally the same size?

[fussfeld]

i'm really wondering if quadrajet idle tubes are basically the same size, in general?

 

[75gmck25]

Simple answer - no, they may be different for certain carburetors, but many are the common sizes of about .035-.037. AFAIK the outside collar and body are always the same size so that you can press them in, but the small end can be drilled out to whatever size you need.

Quadrajet idle passages and adjustments are a complicated mix that I don't fully understand. However, they work together with the jets, rods and step-up spring during the transition from idle to part throttle power. If you made quite a few mods to your motor, you may need to also modify the idle circuit so that it works well with the part-throttle adjustments so that you get a smooth power transition.

Bruce

 

[fussfeld]

How do you measure these tubes?

i have two good q-jet books, but can't find them at the moment.

 

[75gmck25]

I had to use a set of bits for a pin vise (small hand drill). I used a micrometer to measure the small bits, and then trial and error to see which one fit in the small end of the tube. My measurement was .038 for the new tubes I got from Cliff Ruggles.

In Ruggles tuning info on pages 94-95 of his book, idle air tubes start out at .033 for a very mild engine, and graduate up to .036, .037, .038, and finally .039 for a highly cammed engine.

He then recommends .035 as a good choice for a nearly stock engine. However, for that engine he also opens up the idle down channel to .046 (from .040), and the mixture screw holes to .090 (from .070). I only changed the idle tubes and opened up the mixture screw holes to about .80. I'm not really sure how it all works together to affect the idle.

When I was tuning and setting up my carburetor I had problems just identifying some of these other passages in the carburetor so I could measure them, since the Quadrajets use a little different design from year to year.

Bruce