Join Date: Mar 2002
Location: Austin, TX, USA
Re: Motor stubles when I romp on it
Carb adjustments can be tricky because any one setting affects the others. As a result, it's easy to find a knob (like accellerator pump cams) and tweak it until you're off in the weeds - fix one problem and introduce another. That 670 carb should be a good match for your 350 motor. If this is a new carb, it should be set to factory specs. If you've tweaked stuff, put it back to factory specs.
Timing is a critical factor affecting off idle performance. So don't over look timing as a factor. And the comments above related to vacuum advance are very important as well.
Vacuum leaks are another prime culpret. Remove the carb and carefully inspect the base gasket(s) to make sure they have a good witness mark indicating a good fit between the carb base and the intake manifold. I have an Edelbrock Performer RPM intake and the holley accelerator pump lever was contacting the number #1 runner making a huge vacuum leak. After installing a 3/8" phenolic spacer the vacuum leak went away. Another good place for vacuum leaks is at the intake manifold gaskets. Of all the methods used to find leaks (propane, starting fluid, WD-40), I like to use plane old water. Squirt water around the gasket area with the engine running while listening for sucking sounds.
Use a vacuum gauge to baseline your motor and measure improvements as you make your adjustments. The vacuum gauge will tell you alot about timing, vacuum leaks, and valve adjustments that will affect off-idle performance. Take things one step at a time so you can measure / see the effect of a single change. It's doubtful you'll need anything above an orange accelerator pump cam and I wouldn't be surprised if the carb didn't come with one already.
My money is on the vacuum leak / timing issue.
68 Coupe, 350 w/ Edelbrock Performer RPM heads, cam, intake, 700R4, Dave's small body HEI