i just replaced my tired 307 in my 70 chevelle with a 350/330hp crate motor. timing has been adjusted, carb rebuilt, and vacuum leaks checked for. rpm's fuctuate at idle nearly 400 rpm and when i try to drive it surges at low speeds. it tends to die at stop signs and under loads. i unhooked all the vacuum lines and plugged them up. it seems to idle higher when i disconnect the trans modulator. could this be a sign of a vacuum leak at the modulator? any ideas? thanks for the help.

Rich

I think someone else had a similar problem on Chevelle Tech. Anyways my 67 did the same thing and it was because the fuel tank was clogged and not enough gas could get through.Hope this helps.

Here's the similar post http://www.chevelles.com/forum/Forum9/HTML/001334.html

Was the throttle shaft bushings checked when the carb was rebuilt. Qjets do get loose on the primary shaft causing inconsistent idle etc. If you grab the shaft and can move it, it is worn. I had a guy rebuild my junkyard Qjet and replace the shaft bushings for $75.
I will ditto Socalrat on the fuel tank, also rusty fuel lines, and perhaps the 307 used skinny lines as well. New fuel pump?? Distributor ok, etc.

Frank

------------------

figured out what it was.. the vacuum advance was kicking in too soon. i guess the springs are took weak. took it in to get looked at today and the mechanic said i could just run without the vacuum advance. it stopped dying and surging but seemed to have less torque. i'm assuming this is from the lack of advance. would i be better off getting an adjustable vacuum advance(accel, crane cams) or an advance spring kit? also, the mechanic told me that i should reduce my spark plug gap to .035 even though i have an hei distributor, is this correct? i currently have it at .045. thanks.

Rich