[olddog]

I didn't trust my memory, so I did some research. The Carter Thermo-Quad was a mechanical secondary carb. It was mostly used by Mopar, but was also used by GM and Ford.

My first car was a 1966 Mercury Comet Cyclone GT with 390 HiPo. It came from the factory with a 780 CFM vac 2ndary Holley. At least the original purchaser told me it did. It stumbled when the 2ndarys kicked in and it also stumbled when the auto shifted from 1st to 2nd. Original owner claimed it did it from day one and Ford dealer could not fix it. I rebuilt it, tried several power valves, and practically every jet combination in the kit. After many months of frustration, I put a new Holley 650 CFM spread bore (adapter plate) with mechanical 2ndarys. I think it was a double pumper, but I am not certain any more. Anyway it solved the stumble and fuel mileage went from 8-10 to 12-13. Seat of the pants it made more power too. Drive-ability was perfect.

I know vac 2ndary allows you to get away with too big a carb. Too many people are believers they are great for me to say they are not. However I do believe mechanical 2ndarys get a bad rap. They do not forgive being sized too big, but when properly sized and set up they too will run very well. People who cannot get them to run well often times have too much carb for the engine and will not admit their mistake, rather they blame the carb. Just one opinion.