All of this can be confusing, I know. I have a buddy that likes to talk computers with me, he blabs on and on about gigabytes, RAM and other **** that goes way over my head. I tell him that my computer is a Dell and it works fine. I probably sound like an average guy when someone like ME starts talking cars....
Okay...First up: That carburetor! A 750 double pumper carburetor is too much for a street driven 360 automatic car. Unless you drag race the car, the double pumper throws TOO much fuel into the engine. Jetting it down is a crutch. The issue is that it is a "double pumper" design. These have mechanical secondaries and an accelerator pump for both the front 2 barrels and the rear 2. A 750 Vacuum secondary carburetor would have been about $150 cheaper and ran a bunch better. Here is why: For street use, 90% of your driving will be at low rpm and midrange. During this type of operation, the vacuum secondary carburetor acts as an "on demand" device. The engine only gets what it needs. Under about half to full throttle, the secondaries open and then you get the full 4 barrels. The double pumper (Even if rated at the same 750 CFM) dumps fuel whether the engine "needs" it or not. Over time, the excess fuel goes unburned and fouls spark plugs, forms deposits on intake and exhaust valves and causes carbon buildup. In the worst case scenario, the unburned excess fuel washes oil off of the cylinder walls and causes early piston ring scuffing and wear. The double pumper carburetors do work great in a drag race engine, since they operate at wide open throttle at almost 100% of the time.....down the track!
The reason that I asked about the gas tank? Most old cars like ours either have a rusty gas tank or a new replacement tank. Clean fuel is a must! A rusty tank will plug up the fuel filter and starve the carburetor...kinda. Some rust powder still gets through and clogs up internal passages, jets, air bleeds, etc. I have dealt with this and it sucks ***. A new tank can be had for under $100, new fuel lines for $80. Check CarParts.com I bought a tank for a Duster and it was $48 ! NEW !
A street car should also have vacuum advance. Chances are that your MSD setup does not. Vacuum advance helps you get a cleaner burn, helps the spark plugs last longer and gets you better mileage. I know that nobody builds these cars for commuting but get this: I did a distance road trip to test a setup. One way to Van Nuys without vacuum advance (450+ miles) and one way home WITH vacuum advance. In my 70 Charger with the 440/493, Mopar '509 cam....10.8 mpg without, 12.6 mpg with vacuum advance. The vacuum advance also results in less carbon buildup for a longer lasting engine.
There are parts and procedures that people push on you that are wrong. People often try to push race parts on people that only do street driving. They may deliver a slight power advantage at wide open throttle but for the majority of street use, they are not the best choice.
An intermittent problem can be difficult to fix. I would switch to a different carburetor eventually but your current problem may not be directly related to the carb. I think that Ignition issues are more likely to be causing the unstable and shaky idle. Fouled plugs, a bad spark plug wire, a loose fitting plug wire, (Either into the distributor cap or at the plug) a cracked distributor cap, a defective rotor, a bad coil, incorrect adjustments inside the distributor.... A word of note: Even NEW parts can be defective. Don't make the mistake of assuming that a part cannot be to blame because it is new.
My 75 Power Wagon used to idle fine but cut out and sputter at anything over 2500 rpms. I traced it to a bad rotor: The tip had broken off. Weird **** happens with engines, man...
Good luck. Feel free to print this page out and check everything out. There is a lot to digest and it may be hard to remember it all.
