I don't know what carbs you have still... HS6? Could be that the throttle shaft is loose, which if the car is set to idle smooth could dump excess air upon acceleration and give you a stumble. You might try giving the shaft a jostling. It's been a while, but I think lateral movement is ok, but anything vertical or forward/backward parallel to engine means trouble.

Is it slow to accelerate when driving, or revving stationary? if the former, check your clutch clearance. Losen the nuts on the fork, move it so that the clutch is engaged. Holding the lever, move the adjusting nut 1/8" behind the engaged fork. All you need is that 1/8" of play. Tighten the lock nut afterwards, double check the play.

Another thing you might try is putting straight 30wt oil in the dashpots in place of ATF. I had a stumble like that in the 122 w/ HS6s and it made a difference.

B18D and B20B static timiming at 20deg BTDC has worked ok for me. Double check your dwell angle.

One thing at a time, by the way...

Cheers, -Sean
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'66 122s, '70 142s, '74 142e... Blue is beautiful.