Cliff's Quadrajet Parts and Rebuild Kits

General Category => Quadrajet Carb Talk and Tips => Topic started by: 78anniversary on February 06, 2012, 04:41:41 AM

Title: Idle tube size with new fuels
Post by: 78anniversary on February 06, 2012, 04:41:41 AM

I have a stock 78 corvette with the original q-jet.  It seems I have heard talk somewhere of drilling the idle tubes out to a bigger size due to the new fuel formulations for better idle quality.  I am not sure of the idle tube size in my carb now, besides just knowing that they are original. 

Has anyone heard if this is true?  IF so do the down restrictors need to be drilled also?

Thanks

Title: Re: Idle tube size with new fuels
Post by: Schurkey on February 06, 2012, 09:06:44 AM

E-10 has about three percent less energy content than non-ethanol-fortified gasoline.

First Guess:  Three percent isn't enough to make a difference in terms of passage sizing.

Might want to do a proper job of re-adjusting the idle mixture screws, though.

Title: Re: Idle tube size with new fuels
Post by: 78anniversary on February 11, 2012, 02:32:52 PM

Any more thoughts?

Title: Re: Idle tube size with new fuels
Post by: 73 Z28 on February 11, 2012, 06:19:34 PM

If idle quality is poor there are several things to look at. First I would check the condition of the vacuum lines going to and from the carb to make sure they are not dry rotted. Next I'd remove the carb and check for excessive wear where the throttle shaft goes through the baseplate. If those are ok I would check the choke operation as it will have a big impact on idle quality.

On the other hand if you are just looking to increase the amount of fuel flow to the engine at idle,
start with opening up the idle mixture screw holes to about .090 then drill out the idle tubes to about .038 and the down channel restrictions to about .052 These mods will increase the fuel to the engine at idle speeds. After drilling out the mixture screws, I set them at 4 turns out from fully seated as a starting point and adjust them with the engine warmed up and using a vacuum gauge.

Hope this helps
Paul

Title: Re: Idle tube size with new fuels
Post by: Cliff Ruggles on February 12, 2012, 05:16:55 AM

The only time you would need to open up the idle tubes, is when you don't have enough fuel at the mixtures screws to control the idle A/F ratio from rich to lean.  If it idles fine and you can slow the engine some by turning them in, then smooth it out by backing them out slightly from that point, no additional idle fuel is necessary.....Cliff