6 VOLTS/POS. GRD. NW INDIANA

?  The stripped threads are in the float bowl, not the cover. A replacement cover would be appreciated, since mine is damaged, but the rebuilder would likely need another bowl also, though I suspect many used ones have bad threads too, from over-tightening the AC wing-nut. Cover gasket is new, BTW.
An air horn seal failure is a possibility, since trouble started when I reestablished the cover’s vacuum seal. I’ll check the manifold bolts though.

6 VOLTS/POS. GRD. NW INDIANA

Another possibility is a leaking intake manifold gasket.  Have you tried just retightening the intake manifold bolts?

How does the top fuel cover gasket on the carburetor look?  Your symptoms points to too much air coming into the air bleeds and a worn or broken gasket can do that.  The choke is likely compensating for this when the engine is cold assuming that the choke is functional.  If you can give me the stamped number off of the top of the fuel bowl cover, I might have an extra cover here which would at the very least, take care of the boogered up air cleaner stud hole.  That enlarged A/C hole could be the source of your issues.

Lorena, Texas (South of Waco)

It is possible that it is not a carb issue.  A lot of time when things go bad after warming the engine up  up it can be an electrical or mechanical issue.

54 Victoria 312;  48 Ford Conv 302, 56 Bird 312
Forever Ford
Midland Park, NJ

6 VOLTS/POS. GRD. NW INDIANA

Maybe give this guy a call and describe the situation. Tell him the carb has been modified by PONY and that you want to retain the calibrations as he finds them but need a complete rebuild to make it serviceable.

If it needs donor parts and he doesn't have them, I am sure we can find them.

- http://www.thecarbshop.com/home.html

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'Diluted oil an unlikely cause though.'

It is not a cause but a result. 

'Why don’t you try to explain why it still runs great initially when cold, with a carb that has multiple problems?'

If it runs good at cold start up and then worsens as the car warms, it is indicating a rich condition with the choke on and as the choke opens it goes lean, possibly a vacuum leak.   

https://www.mustangtek.com/Library5/PDF/Holley4000.pdf

https://duckduckgo.com/?q=HOLLEY+H4000+4V+REBUILD&t=newext&atb=v420-1&ia=videos&iax=videos&iai=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DqtCR6uwcnGg

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Site won’t post my quote text.  

6 VOLTS/POS. GRD. NW INDIANA

6 VOLTS/POS. GRD. NW INDIANA

Opening post was only due to a sudden problem, which would seem obvious.  Diluted oil an unlikely cause though. Why don’t you try to explain why it still runs great initially when cold, with a carb that has multiple problems?

6 VOLTS/POS. GRD. NW INDIANA