Project 1964 Plymouth Valiant is moving along slowly, no time no money. In spite of the limitations, I have managed to rebuild the engine and install the Carter BBD, the two wire alternator, electronic voltage regulator, pertronix ignition module, pertronix flame thrower coil, Bluestreak cap, NGK spark plugs, and Accel Superstock ignition wires.
The Valiant is running great. On the open road it goes and sounds good thru a 2.25" exhaust pipe. The car also idles well in park or neutral. There may be a slight miss. However, when I put the car in drive or reverse and sit on the brakes it wants to run rough. It will idle under load fine for a few seconds then it seems to misfire then back to a smooth idle then back to a rough idle. It will almost cut off but doesn't. All of this is accompanied by a noise that could be a sucking or grinding. It also wants to make the sucking/grinding noise at initial acceleration, then it goes away. Other than this problem the car is running right and getting 19-20 mpg. Any insight from members about what is going on here will be greatly appreciated.

Sounds a lot like a vacuum leak(s). Could also be a fault(s) with the PCV system.

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Too many people who were born on third base actually believe they've hit a triple.

I suspected the PCV system myself. I replaced the valve and the hose running from the valve to the base of the BBD. What else should I check on the PCV system? I have the 1964 valve cover. The PCV valve pushes into a rubber grommet mounted in a removable metal cap. The rubber grommet looks like it has hardened and may not be making a good seal especially where the rubber inside the cap contacts the metal flange of the valve cover. I have doubts that I will be able to locate a new grommet. Should I look at changing to a later model valve cover with a different PCV setup? Also, is it possible that the inlet at the base of the BBD could be blocked inside the carb base? Thanks.