My 72 350 4spd is running good - starts up, no pinging, etc... however, when I get in to 4th gear at around 45mph it kind of feels like the car is jerking or skipping, any thoughts? Plugs are new and pretty sure gap was set correct?

Thanks

 

In 72 the vacuum advance kicks in when in 4 gear. Its sounds like the vacuum advance is over advancing, causeing your chugging. It may also be the static timing being over advanced, but more likely the vacuum advance.
Ed

 

Check engine timing per the sticky at the top of the C3 forum.Did you re-curve your distributor? (change springs,or weights inside the distributor)

 

If it only does this at full throttle (no such 'chugging' at that speed with lighter throttle), you might have a fuel flow restriction. Check fuel filter, kinked rubber supply hose at fuel pump, low float height in carb bowl, etc.

If you still have the emissions stuff that blocks off vacuum advance in lower gear ranges, get rid of it and retune/retime the engine again. (...after you find the cause of the chugging problem)

 

My 69 was acting similarly when I installed a newly rebuilt (by Lars) points distributor. A difference may have been that I experienced this at lighter throttle, say 30mph in 4th and when coasting in gear. I agree with Ed, its most likely the vaccuum advance. Simply disconnect it, plug the hose, and go for a drive to be sure. Lars thought that it was most likely modern fuel not being able to handle the additional 16 degrees of crankshaft advance, which is the minimum of most any can except for the adjustable ones which are junk.

Solution? Lars suggested shortening the slot on the can by welding / grinding a small bead into it. To determine how much material to add, simply measure the distance that the existing slot allows the rod to travel. Divide this by 16 (or whatever the spec is for your unit). That number is how much distance equals 1 degree of advance. To shorten the slot from 16 to 12 crankshaft degrees, take this number, multiply by 4, and shorten the slot by that amount.

With all that said, I happened to be running straight manifold advance when I was experiencing this problem. Engine is a bone stock 350/350 with a Qjet. I switched from manifold to timed or ported vaccuum off the Qjet, and my poroblem went away. This made sense to me because now during very light part throttle and coasting, I no longer have any vaccuum advance. I know there are pros/cons and lots of opinions out there on the manifold/ported vaccuum advance subject, but this particular 69 is happy running the way GM designed it.