something strange

at a steady speed my ole girl,with a rebuilt carter and a new hei distrib,would buck, but under acceleration it was fine and would haul, so i talked to dan and he sent me a rochester. finaly after a month of having it i felt good enough to put it on so i took off the air cleaner unpluged the vac adv then i decided to pull it out of the garage cause it was nice out and it ran great? so i put it back together and drove her, buck was back. so i opened the cover and unpluged vac adv while i was driving and buck went away, van ran great. i figure its because of the way the vac is from the carb i checked the vac adv with vac pump/gauge and its ok. pcv vac off manifold is fine off carb is erratic. im still going to change carb just thought it was funny.
jeff

It might also be too much vacuum advance. That's what those old distributor machines were for, to check and set mechanical and vacuum advance curves and totals.
Mopars are adjustable, Stock Chevys aren't.
After making sure the initial timing is good (starts without kicking back, idles good, usually around 5-10 degrees), check mechanical. It should be ~35 degrees total. This is the initial plus the added advance from the mechanical.
This can be adjusted by the springs (curve) and the slots (total). You have to braze the slots and file them for less, or file them out for more (usually not needed).
For the vacuum advance, you can see the arm that attaches to the breaker (points) or trigger plate (HEI). there is a slot that limits travel. Brazing isn't as good an option here, since the vacuum pot has rubber inside. JB weld will not hold, the pressure is pretty strong. You can try to block the slot with different things or even better go to the parts store or junkyard and find one with a shorter slot. GM cans are marked, the last part of the P/N is the max advance.


Check here
http://www.chevyhiperformance.com/howto/97438/index.html
and search "GM Vacuum advance" for more info.

Bowtie:

You HEI probobly needs a pick up coil. Very commun for the green or white wire to become broken inside the insulation right at the coil itself and not look brocken. Thats the pick up coil under the plate.

Some times you can prove it if you idle the engine and very slowly add vacuum to the advance unit with a hand vac pump. at one spot the engine may stumble or buck but you have to add vacuum real slow or youl skip right past the bad spot.

Sometimes you can hold the brake in drive and bring the rpm up slowly to load the engine and when you hit the right load it will begin to miss and buck, emediatly and without changing the load unplug the vaccum advance and it will smooth out.

I used to do tune ups profesionally in the 70,s that is a very commun problem with the HEI dist. (the only problem).

Unfortunatly you have to remove the distributer to replace the pick up coil.

M1D

just an update, the company i bought the dist from sent me a new adjustable vac advance, i set it in the middle and she runs like a champ.

Glad to hear it's running well and another thing is checked off the list!