Just finished swapping 2 for 4 barrel carb and manifold. Runs fine at higher rpm's but starts popping and backfiring when trying to idle it down. The small butterfly is not closing all the way. Is it supposed to be closed at idle? If the distributor is off, could that cause the same symptom? Just trying to figure this out, any help appreciated! Thanks.

Sounds like the timing is off to me. it's easy to get it off when pulling dist. all the way out to change intakes. You could have it 180* out (rotor pointing to #6 on #1 compression stroke) when it should be pointing at #1

"Could" be timing.

Since you say "small" butterflies, I assume you swapped to a Qjet? Most post 1973 Qjets are a bear to get right. What are the numbers on the side? They are located up and down above the secondary linkage. It will be a number starting in either 70 or 170. 170 is post 1976 and those are the really lean versions, although all Qjets are lean to a point.

Well first off the timing can't be 180 out or it wouldn't run. Secondly the primaries (front butterflys) are supposed to be open. The only way they can close all the way is to back off the idle screw all the way out. There is a good chance that your timing is too far in advance. Do you have a light you can put onto it? If its a stock set up you should be looking at around 8-10 BTDC.

another thing to check along with the timing is the mixture of the carb. sometimes if it is too lean on a deceleration it will pop or even back fire out the exhasut

Thanks for the input so far guys. It's tough to set the timing because the idle is so high. The throttle linkage isn't even touching the idle stop screw so adjusting that won't do anything at this point. I can only get to stay running close to idle by holding the primaries closed by hand. What does that mean? Gonna check plugs again next. Maybe I fouled some. It just doesn't seem to be running on all cylinders.

you do have a throttle return spring on it right??

Idle screw in correct position. spring is on. What else would make it idle so high? If choke housing adjuster ( the black dial with the hash mark) is set wrong could that be causing this?

What brand of carb did you put on it? If its a Holley with electric choke, then the choke will come on for the first 2-3 minutes no matter how warm it is outside.

its a rochester quadrajet.

I would check for a vaccuum leak at the carb somewhere. Another thing that could hold the primarys open is the fast idle adjustment screw below the choke housing. Was it a known good carb before you swapped it over?

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If all else fails...Tap lightly with a 5lb hammer!

There is a fast idle cam on the choke side. Just under the choke assembly and to the front of the carb there is an adjustment for the fast idle cam. That may be misadjusted and set too high. Try backing off a few turns. If that doesn't work, there may be a vacuum problem with the carb. RH

<font face="Arial,Verdana" size="2">Originally posted by chrs6788:
What else would make it idle so high? If choke housing adjuster ( the black dial with the hash mark) is set wrong could that be causing this?</font>

Yes! The fast idle could be staying on.