Engine surges at idle in or out of gear

[Bob & Sue 110]

When I bought this car a couple months ago engine ran fine at 800 rpm idle in gear 1100-1200 rpm in neutral, instant throttle response. I wanted to get the idle down to around 900-1000 rpm in neutral. Did all the correct steps from start to finish I thought set timing to 8^ no vacuum hooked up, set floats to just under halfway in the float window, warmed car up adjusted low idle have 14-15 vac while idling. Re-timed it to 8* at 1000 rpm, re-set low idle, hooked vacuum back up 3000 rpm 34 * timing.   

 

Now it seems while idling in neutral it surges between 900-1200 rpm when I put it in gear drops down to about 700 rpm then will surge & sometimes die. Took it for a quick test drive runs great above 1500 rpm good throttle response come to a stop 50% of the time it idles fine 50% rpm surges or dies.

 

Setup is 351W , FMX trans , Holley Street Avenger 620 CFM , Holley Dominator  manifold , Comp Cams 268 , Pertronix Ignitor 

 

Any ideas of what to check next? I'm going to rebuild carb what would be correct jetting & kit for this setup.

 

 

[Midlife 814]

That's waaaay too high a curb idle speed right now.  By lowering the idle speed, your car now surges, a classic indication of a vacuum leak or a bad carb.  It sounds like you set the idle speed so that the carb is using the intermediary circuits and not the idle circuits.  A vacuum leak and/or plugged idle circuitry is most likely, IMHO.

[Bob & Sue 110]

Worked on it before the SB started after watching a couple Summit videos I realized the accelerator pump clearance hadn't been touched since I've owned it after checking it it was open all the time probably -.030 to -.040 clearance set it to .015 clearance at WOT started it cold choke idle was 1500 rpm after a couple minutes idled back down to 850-900 rpm in gear 650-700 rpm running smooth no surging. Bumped the timing up to 10* runs great, can't test drive it right now there's about 5" of snow that fell today.

[stangnet33 88]

If you have power brakes check the vacuum line. You could even plug it off and see how it runs. Also check all other vacuum lines and do the same thing. Just an idea to try.