Motor sputters mid throttle?

[rynophiliac]

I just picked up a 70 cuda with a 440 and a 4 speed.  The motor runs fine from about 0-1/4 throttle, then at about 1/4-3/4 throttle it sputters and slowly picks up speed then at about 3/4- full throttle it runs like a beast and roasts the tires.  Any ideas why the sputter at mid throttle?  It only happens under load.  If I rev it in neutral its fine.  My first guess is the timing because I also get some occasional dieseling when I turn off the engine?  Also the car has been sitting for awhile but has all new gas and the carb was gone through by a shop and tuned so probably not a carb tuning problem.
thanks

[67vertman]

I would also check your fuel filter!  could be a fuel starvation issue.

[Chryco Psycho]

I need more info , Timing should be 14-16* at idle & 36-38 @ 3000 rpm , I would not use the vacuum advance at all .
 what type of Carb & intake , sounds like a typical edelbrock carb , you need to check float levels , make sure the accelerator pump has no play & is working correctly , also need to know idle vacuum reading & power valve or springs used

[rynophiliac]

Carb is a holley 850 cfm, vacuum advance is hooked up.  I will bust out the ole timing light to check that tomorrow and see where its at.

[Chryco Psycho]

are you dumping the carb wide open from idle ?
 with a a double pump carb you either need a lot of pump squirt in the rear or you maybebetter to learn to snap the throttle 1/2 open instantly & either wait for some RPM or roll in the throttle from that point

[rynophiliac]

If I floor it from idle is hesitates for a seconds then it takes off screaming but if I slowly increase the throttle that is when I notice the sputter.  Basically the car runs great from 0-1/4 and from 3/4-full throttle.  I didnt get time to check the timing n stuff on it today but I will soon thanks for all the help so far.