Howdy, this is my first time posting, my Dad and I have looked on here to answer our questions but this time we're perplexed.

So i've got a 71 Demon, recently got dual exhaust and replaced the exhaust manifold,(old one had a big crack) now I'm noticing unburned gas coming out all ends, my car is white but the back end is caked in black soot from the exhaust, it also looks like exhaust is coming from the tube that goes from the bottom of the exhaust manifold back into the air filter and clogging the outside of it. (I know not very technical but I don't know how else to describe it) Almost to the point it couldn't breathe.

We've rebuilt the carb twice now and it runs fine otherwise but my gas milage has got to be around 10.

Anyone else have a problem like this?

Hello and welcome! My fiurst question is what kind of dual exhaust exhaust manifolds did you get? Headers or Dutra Duals?

My second question is could you possibly post a picture of this tube you are describing?
I can't think of anything that matches that description.

Have you checked to make sure the choke is functioning correctly? What kind of carburetor do you have? Have you made ay other modifications to the motor?

Is it the original engine to the car?

The only things I can think of that match that description are:

Thermostatic air cleaner exhaust heat duct.

Emissions piping from rear of head to underside of air cleaner.

Number 2 there is only on the much later cars (approx early 80s and up) so unless you have a newer head/engine, that pipe should not exist. A picture would be most useful, as Reed mentioned.

Glad to have ya aboard!

~THOR~

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working on pictures, far as i know original motor, the guy before me was kind of a tweak. holley one brl carb. choke works. no other mods

ya know the little aluminum tube that brings warm air back into the air cleaner when its cold. the inside is caked in soot.

the exhaust manifold is stock, off a 70s dodge van, then the new exhaust starts after it leaves the engine splits into two flowmaster mufflers.

Welcome on the board! Right away, as soon as you can, go get these three books, which will quickly and thoroughly get you up to speed on what you need to know to understand, diagnose, and repair your old slant-6 car.

The flex pipe you describe as sooty inside is the "carburetor preheater" hose. (or "stove pipe"). If it's sooty inside, then there has been exhaust in there. If removing or installing this flex pipe changes how the engine runs, then there's still a manifold exhaust leak that needs fixing. In any event, it's probably time for a new stove pipe. Correct diameter is 2". Where I am, I can buy them in 18" or 36" lengths; an 18" item is more than long enough (I cut it to fit).

It sounds like there are other issues remaining to fix. It's easy to create (or not seal up) an exhaust leak where the intake and exhaust manifolds bolt to each other, especially if you use a parts store gasket kit that contains the stamped-metal gasket for this junction. It is very much worth your while to get the good gaskets for the manifolds-to-head and intake-to-exhaust junctions.

"Back end caked in soot" and rotten gas mileage means your car is running way, way too rich. Your spark plugs are probably black, too. Eventually those Holley 1920s simply "end" and cannot be made to run correctly no matter how hard you try or how many rebuild kits you put in, because the metering block inside gets internally corroded and/or begins losing check balls. Before condemning the carburetor, though, have you installed a new float? That's important on a Holley 1920 rebuild. Don't buy a "rebuilt" or "remanufactured" carburetor from a parts store; they're junk.

Tune-up parts and technique suggestions in this thread. Remember, this engine has solid lifters and requires periodic valve adjustment.

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