I have a 1964 Ford F-100 that I bought a few years ago. When I bought it it had a Chevy 350 vette engine in it and had the front sub-frame of a 1972 Nova. I supercharged it and raced it for a bit until it spun the bearings in the bottom end.

Having only mopars I knew it would get a mopar mill and I happen to have a fresh 1989 factory roller cam engine with a 4-bbl. on it. I added the exhaust manifolds from one of the 1993+ 5.9L Magnum engines I have laying around (headers will not fit). The engine is just a factory stock rebuild and the truck has a true dual 2.5" exhaust with no H or X pipe yet. The truck runs a stock mopar electronic ignition like the mid 70's cars have and it now has a T/F-727.

My question: The carburetor I used was a brand new Holley 670cfm vacuum secondary unit with an electronic choke. Ever since I put it on it has been trouble some. It always starts and runs good, however the truck gets poor gas mileage. I realize this truck is heavy (~4,000 lbs.) however I think it should be descent. Anyways my big gripe is the hesitation right off idle. I called and talked with a couple techs at Holley and they all say it needs larger squirters. I find that hard to believe being this engine is stock, but it maybe the case I guess.
It smells rich and I thought maybe it is putting too much fuel in the chambers causing the poor fuel economy.

What do you guys think?

My 670 Street Avenger did that off idle and from googling it, it seems that nearly all of them do. Some people have fixed it with a bigger or smaller accelerator pump cam. Mine is on a stockish 73 400.

No biggee..
First of all make sure you have the accelerator pump adjusted properly..
Heres how to do it.

Engine OFF, push the throttle linkage all the way.
Adjust the accelerator pump arm to have a slight clearance such 0.050s..
Now try it.
If it still bogs slightly try another position on the plastic accelerator cam. Recheck the accelerator pump arm clearance clearance again.
Now try it.
If it still bogs, bump up the squirters 1 step.

Holleys are super EZ to tune, be patient and take your time it will run fine..

Just my

Thanks I'll try that. I have never really messed with carbs myself much so I am learning. Just do not really know where to start.

First thing, make sure everything is right...no vacuum leaks, etc as that can cause poor performance.

Another thing you should do is calibrate the carb...now, you really shouldn't HAVE too, since the street avenger's come fairly well calibrated, but you give it away when you mention it smells rich....if it smells it, it could well be!

So first check the fuel bowl float levels...with the clear sight plugs on the avenger, this should be easy to visually check. If too high or low, adjust accordingly.

Then, where I think you'll get the most help is calibrating the idle mixture and getting the idle speed correct. You'll need to make sure you've got enough initial timing advance (~ 14 BTDC should be good), and then set the idle speed as low as it can go while still idling steady.

While watching a vacuum gauge (hooked up to the full manifold vacuum port under the carb), turn the left front (if looking out from the passenger compartment) idle screw ~ 1/4 turn inward (turning it righty tighty, which leans the mixture out). Did the vacuum gauge go higher? Idle speed increase? A yes to either question results in the same adjustment to the other screws *(IE go right front, left rear, right rear) another 1/4 turn each (or 1/8 turn for greater precision).

If during this process, idle speed goes up ~ 200 rpm, reset the idle speed to it's lowest stable setting again and repeat.

Just make sure what adjustment you make to one, you make to all.

If you want to make sure you can get back to the same starting point if so desired, turn one screw all the way in, and count how many revolutions it takes...turning it back out the same revolutions will get you to the same point, thus you can always start from square one by turning them all of them all the way in, and backing them out that number of turns you counted at the beginning.

When you get to the point that the idle quality starts to diminish as you turn inward, turn each screw back out 1/4 to 1/2 a turn each and your idle is calibrated for a "lean best" idle...and again, set the idle speed for it's lowest, stable idle...MIGHT have to double check the timing again just to be sure.

THEN see how the carb does with throttle response and off-idle hesitation. I'd be surprised if it was still there.

Here's an owner's manual that'll give you a good idea of how to do all this:
http://www.nosnitrous.com/data/Products/Technical/199R10219-3.pdf

requires adobe reader...this will give you pictures and show you exactly what screw is where, how to find what, etc, etc....

Your fuel mileage should improve substantially too

Of course, if nothing improves, and you know nothing else is wrong with the carb, then a bigger squirter can do the trick...why? Hesitation with sudden throttle changes means the car is going lean as the throttle plates swing open...neither the jets or power valve can begin to flow fuel quick enough, so things go lean and power cuts out...your accelerator pump should take care of this...but if IT itself isn't pumping enough fuel when the plates open, it'll hesitate, but more accurately bog until the other systems kick in...which is why I'm not sure their correct based on what your telling. I'd think you WOULD describe the problem as a bog, not a hesitation. But the bigger squirter would bring more fuel initially, and shorten the amount of time it's delivered ever so briefly, thus improving the AF ratio while the carb switches from primary jets to it's power enrichment system.

add set it for a lean cruise. try different vacuum springs to easily solve the question, does it want more fuel.

a 5.5-7.5 pv

stock jets are in the AF ratio for the butterfly diameter

also you try to get it to pull away , using the least amount of pumpshot. i disagree with his addvice.