A few months ago I received some good advice here regarding what to do about an acceleration flat spot on my '56 TBird.

I ended up adjusting the accelerator pump arm and changing to a hotter accelerator pump cam; finessing the initial timing; changing the advance springs on the distributor for more advance; and finally, changing the power valve in the Holley from a #65 to a #75.

I had some success with that. So of course I figured, If a #75 is good, a #85 might be better. So I tried that, but the damn thing ran so crappy I quickly went back to the #75. That same day, I bought a Holley spring kit and took apart the secondary diaphram assembly and switched to a slightly stiffer spring. This was because the secondarys seemed to be kicking in too early and causing a slight flat spot. The heavier spring did smooth the transition out when under load.

But at the same time I've noticed that when I'm just cruising at low RPMs, the engine seems to "chug" a little. It did not do this before.

First thing I did was to clean and gap the plugs, make sure the timing was where it should be, and then the dwell. All those are dead on--they really didn't need much attention anyway.

Then, with engine hot I tried driving with the idle fuel mix screws everywhere from 1 turn out to 2-1/2 turns out. I could definitely feel the difference in A/F as I tweaked, but still this didn't eliminate the chug. Next I checked vacuum. The only SLIGHT variation my gauge showed was on the hose off the carb that powers the wiper motor. This is a convertible, I never drive in the rain, so I blocked that off completely. But it didn't help.

Then, I bought some SeaFoam and poured a 1/2 pint through the carb, because I thought that with all this carb work, maybe I dislodged a particle somewhere. But that didn't make any difference. I bought the carb new and it has only 7,000 miles on it 

I admit I am stumped. It's not a big problem, but geez, it was big enough so that my wife noticed it when we went out for a drive. I'm wondering, Why should it start chugging at lower RPMs when it never did before? Any suggestions will be much appreciated.

Joey