71 Pontiac 400 left idle screw no response and off idle flat spot

[SP Steve]

I have a 71 Formula 400 with a TH400 and a 7041263 QJet.  The engine is very cold blooded and I have to pulse the throttle to keep it running until warmed. The drivers side idle adjustment screw is almost completely unresponsive and there is a flat spot when accelerating a few hundred RPM past idle.  Once the engine is out of the idle circuit the carb performs fine.  I'm a long time car guy and pretty knowledgeable about many aspects but I've never been a carb guy.  Do I have a clogged passageway that is causing the drivers side idle screw to have no affect and if so can this be unplugged by simple means such as a can of brake cleaner?  Would this fix the flat spot off idle?  Would this fix my cold start problems?  I am interested in purchasing an HP kit for this carb if I have some confidence my problems would be solved.  Keeping this carb would be my first choice but in the past I've fixed QJet issues by installing a Holley.

I would appreciate any feedback and diagnosis from QJet experts.

[Cliff Ruggles]

Right to start with those carburetors (single booster models) are extremely lean in the areas where you are seeing problems.  On this new fuel they will be even worse before the idle tubes or any other passages get restricted or plugged up.

When you get time call the shop, I'll spec out a custom rebuild kit with tuning parts to get the carb up to par for this fuel and working much better everyplace.

Vacuum test the choke pull-off, or just replace it unless it's pretty new, the rubber diaphragm will go bad and cause choke issues and problems going quickly to full throttle.

We're here 8am-4pm, M-F EST...tks....Cliff