Classic/Antique Car Repair: 1970 Cuda 340, vacuum hoses, holley 750

Question
Have 1970 plymouth cuda. 340 / 727 torqueflight. Engine has 480 lift cam, elderbrock performer RPM intake, holley 750, factory heads, factory steel crank, .40 over pistons. Engine runs well, but does not idle in gear without reving up engine, especially when car is at operating temperature. This problem is much more noticable when trying to put into reverse. The engine, tranny, including torque convertor (2400 stall) have very few miles. Have had car 23 years (original 340 plum crazy purple/white interior car) and have never had this problem. Looking for any suggestions

Answer
As I say in bold type in my intro on this site, I know NOTHING about modified cars, so I really don't have any business trying to help here, but since this is a new problem for you, I can guess that you have developed a serious vacuum leak.  To prove this, set the idle speed so that the engine will keep running without you having to sit in the seat and hold the gas pedal down, then remove the air cleaner from the top of the carburetor, and with your hand or a piece of cardboard, very slooooowwwly block off the air flow into the top of the carburetor, starting from zero blockage and then slowly close over the air inlet bit by bit, while you listen to the engine.  If at ANY point, you hear the engine actually speed up as you do this, you know you have a vacuum leak.   

If you do, start pinching off vacuum lines wherever they run - if the car has power brakes, start with that, because that can be a big vacuum leak if there is a problem with the booster or the hose that goes to it.   If no help there, check all the other vacuum hoses by pulling them off the fitting on the intake manifold and capping the fitting.   If this doesn't help, get a can of WD-40 and start spraying it around the base of the carburetor and around the intake runners where they enter the head - if you can find a place where spraying WD-40 causes the engine to speed up, you've found your vacuum leak!

If you do not find a vacuum leak with the air flow blocking off test, I think you need to do a compression test to see what the condition of the valves and rings are.   If you know of a shop with an engine analyzer - get them to hook it up to the engine and run through the tests - it might pin-point the problem that way.

Dick.