Newly Rebuilt 400 backfires and stumbles

[Gold Rush]

I am still trying to solve the back fire on my newly rebuilt numbers matching 1974 400 4 barrel (non HP) engine. Of all the engines I have built over the years, this is the first that has given one ounce of problem. The symptoms are: The engine starts and idles smooth. Once warmed up I can gradually increase rpm and feel just a little stumbling; then around 2100 rpm it gets real bad. From idle if I stab the accelerator the engine backfires through the carburetor. If I give a couple short shots of gas then stab the accelerator it will come to life and wake up the neighbors!

So far I have had the guy that did my machine work and installed my cam (very close to OEM) come and check things out without any discoveries. We played with timing and found lots of advance made it better but not gone. He suggested I verify the harmonic balancer had not slipped throwing off the timing mark. That is dead on so not the problem. I have also checked the compression and all cylinders are very close.  The accelerator pump nozzles seem to be giving a good squirt so I'm guessing that is also good. I understand backfire through the carburetor indicates a lean condition so I have removed and disassembled the Summit Racing replacement "Thermoquad" bought several months ago. I did not find any obvious issues except a missing "Bowl Vent Seal." The lever is there – just no seal.

I compared the Metering Rods and Primary Metering Jets. The original carburetor had 2005 rods and 4098 jets. The replacement carburetor has 1950 rods and 4096 jets. What difference will that make?

I know there are some real "Thermoquad" GURUs out there so hopefully someone can give me some guidance. Thanks so much in advance..

[RollinThunder]

You have confirmed this is a Thermoquad carburetor.  And the motor likes lots of timing.
I have a few questions regarding your build:
- Single plane or dual plane intake manifold? 
- Stock exhaust manifolds with functional heat riser? And opened and functional exhaust crossover passage in the intk mnfld?
- Stock cam shaft?  What is the overlap duration?
- On the carburetor, how many turns out are the idle mixture screws (from lightly seated)?  Does lightly seating the screws stall the engine?
- Does the coolant temp increase at the 2100 rpm point?

[Gold Rush]

You have confirmed this is a Thermoquad carburetor.  And the motor likes lots of timing.
I have a few questions regarding your build:

- Single plane or dual plane intake manifold?  - Stock exhaust manifolds with functional heat riser? And opened and functional exhaust crossover passage in the intk mnfld?  To be honest I do not know the difference.  It is the OEM manifold and still had the exhaust crossover.

- Stock cam shaft?  What is the overlap duration?  The OEM cam was trash but my machinist got an ES327 (replacement for OEM 3577181) cam.  I believe the overlap is around 113

- On the carburetor, how many turns out are the idle mixture screws (from lightly seated)?  Does lightly seating the screws stall the engine?  I did not count the turns but set the air/fuel mixture for max rpm then lean one turn.

- Does the coolant temp increase at the 2100 rpm point?  I did not let the engine run at that rpm very long.  Never had any overheat issues.

Hope those answers are enough for you to gather some insight.   Thanks !!!!

[Nacho-RT74]

what's the deal on iddle mixture screws if problem is not at iddle ?

BTW, In my experience they usually are nice between 1.5 and 2 turns... sometimes 2.5. Everything depends on the screw needle shape... some are longer than others changing the cone shape.

Whats the carb conditions? rebuilt ? old used ?. I'm not a carb expert but I have deal with TQs since ever ( I want to keep that carb on my car ). The weakest point of them are the bowl cracks and the leaking gas well passage, as well the O ring on them. They are also a bit hard to tune up, maybe to me since I barelly know how to tune a car.

[RollinThunder]

Gold Rush,
I think you are saying the intake manifold is stock, with the exhaust crossover passage opened.  But you did not say if you are running the stock exhaust manifolds with functional heat riser. 
You did not report if lightly seating the idle mixture screws will stall the engine.  You did not report how many turns out from lightly seated the mixture screws are at for best idle (highest idle rpm).  Also, if you leaned them in one turn from highest idle rpm, that is a lot and should have not been something the motor would not have liked.
By the way, 2100 rpm in Park (or very light throttle in drive cruising) will very much be controlled by the idle circuit.
Good luck.

[c00nhunterjoe]

The fact that it idles great but a slow steady increase in rpm causes it to run poor the higher the rpm tells me you need to verify fuel delivery, verify good hot spark 1st.