[HO455]

I am suspicious that the carb may have issue in the main body or possibly a mismatch with the base plate.
What kind of shape was the carburetor in when you received it?
I have seen lots of frankenjets over the years. Different year carburetor sections bolted together or a Chevrolet main body bolted to a Buick base plate. In almost every case this causes problems for the vehicle operation. Sometimes it is just some one trying to solve a problem other times it is an unscrupulous seller looking to sell an incomplete carburetor for more money.
I do wonder too about your quote from QP: From Mark @QP
Quote:

Carb can idle with them closed. It has bypass air built in and we richened the mixture based on your engine descriptions.

This essentially means they physically modified the idle circuit in the main body of the carb which is may not be problem but they did it before the carburetor was on the vehicle and driven. If I am not mistaken you have a basically stock engine with the correct carburetor on it. If that statement is correct then such a modification should not be necessary. I have modified the idle circuits on my Burban's Qjet but it is off of a 1978 400 cu engine. The idle circuits were designed for emissions and as a result too lean for my engine. By modifying the circuits we were able to get the idle mixture screws to work properly. Scroll to post 455 in the link below for more info on what was done.
A common problem these days comes from corrosion. Most people seem to think that letting a carburetor sit out in the weather is not a problem. This is far from the truth. The metering circuits in the carburetor are machined to very exact specifications that are matched to the specific engine it is going to be installed on, whether or not it has to meet California emissions, and if it was going to be shipped to a high altitude dealership. When one of these precision machined passages become corroded they become larger and now will no longer meter correctly resulting in driveablity issues that are difficult if not impossible to solve. Folks who drag carburetors from swap meet to swap meet and let them sit in the rain time after time deserve a special place in he11. Photo below is a good example. Not a Qjet but a reasonable looking carburetor on the exterior but corroded badly on the inside to the point that rebuilding it may not be possible. Not something a person who spent $600 for a correct carburetor wants to hear.
One other problem that comes to mind which is rare ( We've only seen it twice that we know of over the years) is you may have a crack in the main body that only causes problems when the carburetor is hot. This is a hard to troubleshoot problem that many people have never run across.
Good luck and keep us posted.

https://67-72chevytrucks.com/vboard/...698377&page=19