Is it on initial tip in, or with aggressive acceleration? One of the things I would check is proper operation of the secondary air valve door. It is spring loaded and acts as a delay in the implementation of the secondaries to cover up a lean spot that will happen should you engage them without proper vacuum to pull in extra fuel. Now if it is every time you hit the throttle, I would also check the accelerator pump operation. This one is easy, as all you have to do it look down the throat of the primaries while rotating the throttle by hand. If it squirts, it works. It may have dried out and cracked while sitting. I would also consider bad gas if it has sat for a long time as it may have gummed things up depending on which of the 90+ fuel mixtures your area receives. If you suspect this, run the car out of gas and put some fresh in from a can. Other things would be a plugged fuel filter from rust that formed in the gas tank, a leaky float, or even corroded distributor terminals. Then again, it could be a bad brake booster too as it would cause a lean condition.
To adjust the mixture screws, try this: Attach a vacuum gauge to a manifold vacuum source. Next, bottom both screws while the engine is off, then back off 1.5 or 3 turns ( can't remember which), but make sure both screws are backed off the same amount. Start the car, and set the mixture screws for maximum vacuum at idle, with the engine at the warm idle speed. It is important to have a warm engine because fuel requirements vary with temperature, and idle speed will vary as well. Also, be sure to balance the screws so that they are set roughly the same. You may need to adjust the idle speed down once you are done. If this happens, verify your initial timing with a timing light once you are done. Be sure the vacuum advance is not hooked up while adjusting it as it could give a false reading.
