Not sure if you have a road draft or PCV, but once had a similar situation at idle that was a leaking PCV line-mine is a combo hard line to vac hose. I would agree that it seems to be carb or intake related- There was a small crack that would expand with heat making it pretty erratic when it came and went. I have 3x Holley 94s with spacers so it drove me nuts taking everything on and off until I just got methodical and ruled out 1 vac source at a time.
Also had a blown diaphragm on an AT vac modulator that acted the same way and make the engine "seek"


Mike Rizzo

1963 F100 "Rudy"

Daniel Island, SC

Sounds to me you stil have a minor vacuum leak. One way of testing is to use starter fluid spray can with one of those extension plastic nozzles. ( WD40 will work too) . Try to gently spray around the carb area base, vacuum fitting, carb shafts, and see if engine rpm increases. I do it very little a time just a drop will increase rpm at a leak. Also check as best you can at the intake/head mating area. Also try plugging any accessory vacuum items. A 50 rpm surge should be adjusted out with idle mixture screws. If you have an idle dash pot. Back it off. It's for slowing down to idle with an auto trans and may not be necessary. Good luck.

56 Vic, B'Ville 200 MPH Club Member, So Cal.

Thanks Dobie and Ted
I've changed the gaskets either side of the spacer and the surge has reduced from 200/300 to 50/100.so I'm in the right area.
Idle = tick over. I just lapsed into my native lingo .......hoods and bonnets thing.
The idle mix screws seem to have a little more effect now previously they had none .
Thanks
stuey

Dobie's suggestion is a good one.  Not sure what ‘at tick over’ means but I’m assuming you mean at idle?  If the idle mixture screws don't cure the problem then I'll suggest that the float level could be too low but I'll put my vote towards a vacuum leak directly under the carburetor.  If you’re running a carb spacer, then be sure to check the gaskets on both sides of the spacer.

Lorena, Texas (South of Waco)

Sounds like a lean surge to me. Try adjusting the idle air mixture screws for best vacuum, then just a hair more.

point me in the right direction please
at tick over I have a 200/300 rpm surge. carb is a 4100. I've changed the power valve to no avail.
snapping the throttles open, response is instant.
would too much fuel pressure cause this, I have an FE type fuel pump with no regulator.
Thanks for any help
stuey