'65 200ci 6 cyl with 1 bbl holley carb. Engine misses badly when I let off on throttle. Plenty of power when accelerating. Have replaced fuel filter, distributor cap, cleaned/rebuilt carb, and added a gas treatment to remove any moisture in tank. Still have problem. What's left?

 

Explain what you mean by saying it misses when deaccelerating. If that engine is pulling hard while accelerating, you don't have a problem. Is it popping and backfiring when you let off the gas?

 

It's not backfiring. However, it shudders and stalls violently when I let up on the accelerator or attempt a steady speed. It first occurred after filling the tank at Wal-mart which initially led me to believe it was simply bad gas.

 

Could be bad gas. Go out and shake the crap out of the engine and see if you have a broken motor mount. Put some "Heet" in the tank, then take her out for a 20 mile run and see if that clears it up. I had a very similar problem one time, and wound up draining out the entire tank and filling with new gas. Moisture had collected after years of storing it for 10 months out of the year, and only filling it partially with new gas just to top it off. Water will form at the bottom of the tank, so if you have a drain plug like I do, you might get by with just drainging a gallon or two.