Son and I are having problems, hope you can help....
1961 (Impala) Chevy 235 just rebuilt, added Clifford headers & water manifold, 4v Holley 390cfm (rebuilt by Larrowe and Sons), adjusted octone selector to 0, carb vaccum plugged, timing set to ~5deg BTDC....Idles great but studders when accelerating.

Dwell set ~22?

Headers running bare...have not had exhaust system completed yet.

What should we check? Appreciate any help.

Set the points at .018" & never mind the dwell meter. Then reset the timing. The vacuum advance needs to function, as well.

Re check valve adjustment.

Good luck.

PS: Don't run it to long with headers open that way.

I would say get rid of Clifford intake. I ran one for several years. Started with an Edelbrock carb. Got it to run then swapped for a 32/36 DGEV Weber (progressive). It ran great, but still hesitated on accelleration. A couple of weeks ago, I ditched the Clifford and put my stock intake back on. I built an adapter for the Weber, but swapped the carb for a 38/38 DGAS, which is a little bigger, and synchronous. Runs great, idles like a champ, but pulls like Bull!
Moondog

Don't "give up the ship" this soon.

Wait till you finish the exhaust system, break in and drive it some.

Happy trails.

makes me wonder how a carter 400 cfm 4bbl with a clifford intake would behave on a good 235/261? hmmm

Five degrees BTDC is insufficient with today's fuel. Try increasing the timing with the octane selector until the stumble goes away. That stumble is more ofen insufficient advance than a carb issue. I'd be willing to bet that the stumble will go away at between 7 and 10 degrees more advance on the octane selector. You will, of course, have to back it off if you get pinging under load or bucking against the starter.