[chiphead]

OK that's interesting.

The purpose of vacuum advance is to add timing at light load (high vacuum) and reduce timing at high load (low vacuum). When the throttle is opened, vacuum drops and vacuum timing goes to 0. This reduces the chance of detonation/pinging at heavy throttle. When throttle is reduced for cruising, the vacuum advance adds timing at light load to improve MPG and light throttle response.

Typically I like Ported vacuum from a carb source, so the vacuum advance only works above idle. This results in a more stable idle, because vacuum fluctuations at idle do not impact idle timing. Having to run manifold vacuum to the vacuum advance to get the engine to idle, indicates either a lack of base mechanical timing, or too-lean idle fuel mixture, or a cam that is entirely too large for the engine size and/or compression. The cam is marginally too big at 9.3 compression. This can be mitigated by increasing compression, advancing the cam 4* and/or using Rhodes lifters. The rest of the issue is tuning, adding base timing, adding idle fuel by modifying the Q-jet's idle feed circuits..

I would not recommend throwing another carb on it, unless there is something specifically wrong with your carb. The '77-80 electric choke and APT Q-jets are easier to tune, but not required. A good Q-jet tuner could make any '70s era Q-jet work well. Q-jets are notorious for being lean in the idle circuit, so the vacuum advance could be a crutch for lean idle mixture. Going from 750 to 800 CFM isn't going to do much on a mild street motor. Be aware that if you get the TV cable adjustment wrong, the 2004R transmission can fail due to low line pressure. So pay close attention to any throttle linkage changes.

Tune, tune tune. Throwing parts at it is not a shortcut.