Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

Can't get my rebuilt '78 400 to idle; runs great, but surges about 100 rpm

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Shark Racer:
The restriction in the idle tube is at the very bottom of the idle tube - I doubt it's .046. That's HUGE. You need to be measuring at the necked down section which is ~2" below the surface.



Pic above is from the parts Cliff offers: http://www.cliffshighperformance.dreamhosters.com/parts.html#a

corellian corvette:
How long are those tubes? The smallest bit I have is .040, but they are super short so I want to make sure I'm getting them all the way down (note - I have not removed the tubes)

Given that I need to make the idle RICHER - what would be the best bet? Drill out the IFR (Idle down channel)?

What about blocking the Air bleeds? I notice my system has 2 - one at the top (.070), and a smaller one at the ventrui (I believe this is called the main air bleed or lower air bleed) which is very small (.055).

Could it be as simple as just blocking the upper idle air bleed which would enrichen the fuel mixture?

Shark Racer:
You really need to size them as close as possible to determine where to go. Like I said in the last post, the tubes are about 2" long.

Personally, I'd open the IFR and tube up as matched pairs.

Cliff sells screw in idle tubes, which would probably help with speeding the process up. Prying the tubes out is kind of a pain.

You'll need a 61-80 drill bit set, and pulling the tubes will require a drywall screw, .090 punch and a pair of pliers. Probably some heat too.



I took that photo that shows all the tools I used to enlarge the idle tube restriction... :)

corellian corvette:
OK thanks for that note - I measured again and the idle tubes are exactly .042. I have a .043 and it doesn't fit.

I also realized that I was measuring the MAIN idle bleed, not the LOWER idle bleed. Since I don't have the baseplate off, I can't measure it but I'm assuming it's the larger size.

These would be my new measurements:

Idle Tube: .042
Idle Down Channel:  .046 fit, but not .052 (my next size up)
Upper Idle Air Bleed: .070
Lower Idle Air Bleed: .070 (assuming)
Idle Bypass Air: None.

Given that I need to make the idle circuit RICHER, it seems like I only have a few options.

1. Trying going up a size or 2 on the Idle Down Channel.
2. Try blocking the upper idle air bleed. Doesn't this mix with the fuel going down the IDC to lean it out?

It already looks like my idle tube is plenty large based on some of the "recipes" in Cliff's book - it appears it's the down-channel that severely undersized.

Make sense?

Shark Racer:
I'm still really surprised your idle tubes are that large. I would go up, VERY slowly on the down channel restriction until you solve your problem. I might consider pulling the idle tubes anyways and necking them down to 036.

Do you know the static compression ratio of your engine?

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