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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corellian corvette:
Thanks Shark Racer!

I'll do that then, first. It's hard because on the PY string people think opening up the idle bypass is also a good idea. So hard to parse the advice.

I'm getting more freedom with the idle screw then I had before. I can get an easy 1000 + RPM on the idle screw without venturi drip (before Max I could get was ~ 850).

But idle screws are still out quite a bit, with a very light lean condition as I stated.

So you think going to .089 on mixture screw, and a very small opening of the down-channel would be better?

The PY forum recommended going to .055 on the Down-Channel, but I'm going to look at my drill-bit options and was thinking of shooting for something more in the .049/.050 range...

RE: XE274. I probably could have made it run, but it was MUCH worse than what I was dealing with on the XE262. I was also told by Lars Grimsrud and other Pontiac guys that the 274 was not a good match for my engine - sacrificing low-end torque for high-end HP, but my stock iron heads and low compression would die out before I was in the sweet spot of the cam. I already can tell this cam is better matched to my engine as stoplight performance is much better. I really had to get into it before to feel any power...

Shark Racer:
It's much easier to go bigger than smaller, so that's why I'm recommending stepping it up a little at a time.

I went from 55 to 59 on my DCR and that took me from a little too lean to a little too rich. If I did it over again, I'd go with a 1.45mm bit which is about 57. 2 years away until my next smog test, I'll leave it alone for a bit. :)

As your idle tubes are already quite large, I'd be very cautious going up on the DCR.

corellian corvette:
I think we're in good shape.

The next size drill bit I had was .052, so I opened up the DCR to .052.

To compensate, I opened the Bypass Air from .082 to .086.

Finally, I went to .089 on the mixture screws.

I think we've nailed it. Idle mixture screws are perfect at 4 turns out. I did need to crank the idle screw in just a bit to bring it up to ~ 900 rpm in Neutral, but I'm still below transfer slot so no venturi drip.

Idle holds steady in drive at 700 RPM, Vaccum is pretty steady at 14hg.

So I think I'm done at this point. I'm going to drive it around and let the engine break in, and I have a rebuild kit coming from Cliff that I'll put on when it arrives.

Thanks for your help!

Shark Racer:
No problem at all... glad to be of help. Now come take care of mine. :)

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