I have a 1970 440 6-pack set up. Center carb was leaking so I took off and had rebuilt. No leak now but backfires at low speed. The place that rebuilt it does put it on a engine and fine tune it. One thing I did notice when I got it back was the nipple which was plugged when I got it back was the lower one. When I took it off before having rebuilt the upper nipple was plugged. I believe the black hose connected to it is a vacuum line see picture

Anything else I should check. Car was not backfiring before rebuild by highly recommended shop.

 

Not a good clear shot . And I'm not to clear on what the question is?
I would hook the hose back were it was, and plug the other.
Once you have all the roughting done. Then you can check for the problem.

When you say it Back fires at slow speeds. you mean it is a constant pop,pop,pop,pop,pop, as your driving slow?

IF its just a single pop. Then Re-check timing for one. And also check to make sure the accelerator pump discharge has a good squirt to it when you crack the throttle.

 

Read your shop manual. You can download a '70 over at MyMopar. The pages are not numbered with the proper "dash" system, so you'll have to play with the page numbers

I would check out the accelerator pump and idle mixture settings

Are you certain the stumble wasn't there before the carb work? What prompted you to have that done?

 

Yes it seems like a constant popping at midrange. Car was running ok before carb rebuild, was just dripping from bottom of center carb. Was hard to start after sitting for a couple days.

 

MidRange as at about 3000 rpm's?????
IS it popping out the exhaust or back up threw the intake?

 

Sounds like its coming out the exhaust. I'll look at it Thursday I'm off.

 

Not to be funny, but I've seen this issue before where it was the rear license plate had been loose and was banging on the body.

Just thought I would mention it.

 

Lets start here by checking this first:
Tighten mixture screws, decrease squirter size,
lower float levels, reset throttle plates, decrease size of
main jets, check ignition timing.
If some of those where changed get the corrected....

Thin In short.. From what i'm hearing, this is what I get out of it.....

Assuming the carb is hooked up correct and, good to go?
Id look at Plug Wires and or Timing.

Not sure if you had the plug wire off? but hears a thought.

Possibly the plug wires got disturbed and are now mixing up between a couple of cylinders causing the pop out the exhaust? However most of the time this has a related miss that goes with it.
Now this part is only if you didnt move your timing or cross a plug wire at some point!!!!

Now... All mechanical being in good order... (Valve springs) per say....
I will say that really late ignition timing will cause a rich fuel mixture to pop threw the exhaust....

And if it's a Dual point Dist...... You can throw what is said out the window.... It just need new points....

Just My 2 cents
Good Luck on the hunt.... Keep us posted.....

 

So originally they vacuum hose was on the lower nipple and the upper was capped. When I got back from the shop they had capped the lower. I hooked it up as original and it didn't run 100% Then I hooked to top nipple and capped lower now it's running like a beast again. Don't know why but it's fixed now. I do have a little squeal now when I punch it. Seems like a belt. I will check tension next. Looks like this hose is called a distributer vacuum advance tube.

 

Lower one should be for the Choke pull off... The upper one should be for the outer carb vac. diaphragms. I thought there was one more port up there but I'll be dammed if I can remember what it was for? Maybe Dist Vacuum if needed.