Any idea what causes this?

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put a rag on top of the carb slowly .does it pick up speed or lower speed /if it goes up its running lean (needs more fuel the idle circuit is restricted or there is a vacuum leak somewhere ) and most of the holleys rebuilds are junk they dont take the metering plate apart to clean it they just soak/blast them .the tiny fuel passages are never open up.I have been dealing with one like yours for 3 months. also there is 6 different accelerator pumps for these models they are very close but you may need to adjust the accelerator pump linkage . the one i bought was just bead blasted and new parts/wrong parts/wrong adjustments and sold . good luck
 
gas starvation. your tank is clogged up (the sock is probably rotted, dead leaves in the tank, junk, sludge, etc maybe even a dead mouse in it). or the gas line is clogged up, or both.

my gas tank when we got it off the duster, you could push your foot through the side...

you can test with a gas can/fuel line straight to the fuel pump, but i dont think you can test that going down the road too easily.

sometimes you can just use compressed air and blow the gas line out. but your tank ... you have to remove them to clean them out, then you can clean it up and paint the outside etc...but by the time you do all that a $150 brand new tank dont sound all that bad. the only problem a lot of people run into is the sending units tend to not work right on some of them. keep your old sending unit, gaskets etc from the old tank.
 
I hate to beat a dead horse, but you should still make 100% sure its not a vacuum leak. I dont know how comfortable you are with this kind of stuff, but if you're looking for an easy way, a can of carb cleaner will find a vacuum leak faster than you can imagine. It's not very high tech, and it can be bad for engine paint, but it will do the trick. The other thing that comes to mind is an improperly adjusted float level in the carb.
 
I did take carb cleaner and spray all around. Only time it idled up was when I accidentally got over spray in the top of the carb. I turned the fuel WAY down and now it is doing a lot better but still spits and sputters when I punch the throttle. I realized I screw up that I did and am going to fix tomorrow. I set the timing without pulling the vacuum from the distributor and then adjusted the valves after doing that. Yesterday I re did the timing after I unhooked the vacuum and now I have to redo the valves. I was wondering if that may cause my problem? It only does it when I mash the gas to the floor, does great any other time I am driving it.
 
This question comes up a lot, and post #27 is spot on. My 69 225 idled like that for decades, thru ~3 rebuilt carburetors. I finally got a 4th Holley 1920 that made it idle extremely smooth and responsive. It was like a different car. Of course, a vacuum leak would give the same "lean idle" condition, as mentioned.
 
Does it die or want to die when you mash the gas to the floor from idle and stopped, or mash it to the floor from 50 MPH cruise conditions, uphill or on level ground, etc.? Is this when the car is fully warmed up? Deos it actually die, or does it recover and smooth out? It is important to identifty the conditions under which this happens and exactly what happens; it can mean different things are wrong.
 
nm9stheham said:
Does it die or want to die when you mash the gas to the floor from idle and stopped, or mash it to the floor from 50 MPH cruise conditions, uphill or on level ground, etc.? Is this when the car is fully warmed up? Deos it actually die, or does it recover and smooth out? It is important to identifty the conditions under which this happens and exactly what happens; it can mean different things are wrong.
Click to expand...

If I mash the gas to the floor it will spit and sputter till it dies if I dont back out whether I am stopped or going 50. Only time it "recovers" is when I pump the gas and then it will pick up like its supposed to
 
First thing, check for vacuum leaks. Don't forget to check the lines inside the car if yours originally had factory A/C.

I had a '73 /6 that acted a lot like yours does. The problem was no accelerator pump. At the time, I couldn't find a carb overhaul kit that included accelerator pump parts. I sprung for a Holley reman that I got through Auto Zone. While it fixed the pump issue, fuel mileage went in the crapper. I opened it up and found it had a huge jet in it. Changed the jet, and it ran flawlessly until I sold it three years later. Part of the point of this is that even the folks who should know better don't always get it right.

To check the accelerator pump, take off the air cleaner, look down the throat of the carb, and quickly manipulate the throttle linkage to wide open. You should be able to see fuel spray in the carb. Repeat this several times and you should be able to smell fuel. Normally, I'd say you can do this with the engine running or not. In the case of this car, IMO it would be better if the carb bowl was full of fuel but the engine not running. I've experienced a backfire to the face...so you don't have to. lol.
 
Grab the carburetor and twist it from side to side. Does it move? For some reason on the slants, the gaskets in the carburetors shrink really bad and cause the bolts to loosen up and that causes major vacuum leaks. Probably cause the carburetor sits basically right on top of the exhaust manifold.
 
I hate to add my suggestions to a thread that already has TONS of good info but I feel it is worth mentioning.

I'd highly recommend getting a vacuum gauge. That way you can set the idle to the highest vacuum reading. The vac gauge can tell you things you wouldn't know otherwise.

Also, spray carb cleaner (with the red straw inserted into the can) next to or ON the throttle shaft where it sticks out of the carburetor body (by the base of the carb, and even around the gasket if you're so inclined like Rusty Rat Rod has pointed out above). If your motor stumbles, you have a leaky throttle shaft (or gasket). The only fix is to bush the shaft or replace the carb with one that has a known good shaft.

Sure we all want our slants to do a tire-incinerating burnout like the Valiant from the film Duel, but you have to walk before you can run.
 
I have a similar problem but I think it's choke related now (it's cold here and the Super 6 lemon is my DD) I'm not sure if it opens to slow or fast but if I drop it to drive before good and warm it dies. I had a loose carb gasket I didn't notice til I had the air cleaner leaning on the carb and it moved. Also when I bought the car with a holley 1920 it was exactly like yours. I got a carter 1bbl off a friend. I found that the guy that had the car put the carb gasket on backward. There is one hole on the manifold, one on the carb so wouldn't you think the hole in the gasket went between the others. I got a new one and put it on right...car ran much better. The 71 swinger i used to have had a manifold gasket leak and ran like yours.
 
Have you replaced your PCV valve. If they are stuck open will make an internal vacuum leak. And if you have power brakes could be a leaking booster
 
Food for thought. Do NOT use anything flammable to check for vacuum leaks. Water ina squirt bottle will make them show up just as easily and not be dangerous.
 
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