Engine Backfiring and Stumbles during acceleration HELP !!

[goldrush]

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 cylinder compression and all 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..

[DynoDave]

Oh boy. NOT a TQ GURU. But just have to ask a few questions.

What I know about TQ's you can fit in a thimble. But I do know that when setting one up from new, you have to go through the settings, and do them in the right order.

Here's a factory manual on them...

Imperial Club TQ Book

...and here's a great 20 minute "film strip" (remember those?) that covers the carb. as well. This IS from 1972, so there may be a few small differences, but nothing to crazy I hope. See if this helps.

Imperial Club TQ Film

I do know from experience that with the mechanical secondaries, if you push into them too far too fast, you will get a bog. If you were to do this and the vac. control for the secondary air door were not adjusted or working, and allowed that door to pop open too soon (cold), then you would likely get a lean backfire. The vac. prevents door operation when cold (no secondary operation when cold). Then the spring tension controls how easily that air valve opens. If too soft/easy, it could give you this.

While that secondary air valve adjustment can be made with a couple of screwdrivers (tricky, as it wants to unwind on you), there is a proper tool being repro'd for that, and it's probably worth having.

The Cheap Version of the Tool That I Have

An Expensive But Nice Version of the Tool

[goldrush]

Thank You Dave!!!  Some of this I have but not all of it.  I appreciate the help!  This is getting frustrating.    **   Had to use it!!!!!  

[odzking]

I tried every TQ book with instructions I know including the Mopar one and service manual. All were worthless. You need to find a fellow that knows how to adjust TQ's, it is an art. Not like and AFB, they are quite picky.