Vintage Cars: hesitation!, master mechanic, quadrajet

Question
hi.  i have a 66 gto with a 389 bored out to around 409, with a ram air4 cam, 750 quadrajet and headers.  runs great. only problem- hesitation from idle.   i have replaced the accelerator pump, have played with the timing till i am blue in face.  i have disconected the accelerator pumpjust to try it and car runs good, yet no pump.  when the car is cold, and is started, the choke is on high idle, there is no hesitation all during the period the choke is on, even when it is starting to come down from a high idle to normal idle.  at that point, the hesitation starts again.
help me, and thanx!

Answer
In my experience (and I don't claim to be a master mechanic) every hesitation issue like you mention in carburetor equipped cars comes from not enough fuel upon throttle tip-in.  That points to the accelerator pump.

How new is the Quadrajet?  Maybe you have a newe pump, but the bore may be worn out.  Are you missing a check valve?  Is the float level set?  Might the floats be leaking or slowly sinking?

Also, what manifold are you using.  If Factory you should be OK, if aftermarket, then what are the characteristics of that manifold...check with the manufacturer of the manifold to see if you have the right part for the cam/heads/carb.

Start by setting everything to factory specs including the timing and dwell.  Make sure the distributor vacuume advance is working and that the advance weights are working.  

Check the vacuume.  A RAIV cam is pretty wild, but should still make OK vacuume.  If not, check the gaskets, and check the various ports for leaks.  Check the carb base gaskets, sometimes you can pick the wrong gasket from a rebuild kit and mess things up while it still looks correct.  Check the main shaft for wear in the base plate, a major source of leaks.

Here are two articles from Hemmings Muscle Machines that should help with the carb, because I think that is where your problem is:

http://www.hemmings.com/mus/stories/2005/01/01/hmn_feature21.html

http://www.hemmings.com/hmn/stories/2007/01/01/hmn_feature29.html

Get the books they recommend and if all else fails, check Hemmings Motor news for expert carburetor rebuilders.

Also, make sure you have enough fuel line pressure.  A stock or high performance mechanical pump should serve your GTO just fine, no need to put in an electric pump at this build level.  But pressure should not be a problem as the car runs great otherwise, so it think it comes back to the carb.

Good luck.

Don