Sick Six

I rebuilt my ’70 T/A’s 340 Six-Pack engine a while ago, and it’s never run right. Everything is stock, except the cam is a purple shaft 0.484˝/284°. I’ve put about 2000 unhappy miles on the thing thus far. The carbs were rebuilt, but the center carb was eventually replaced because of corrosion from sitting so long (the car has 27K original miles on it.) The outer carbs are still original. It has power brakes, too. The problem is that the vacuum is very low—at idle only 8 in. or so. On occasion it will surge at idle, other times it will die. It is always a 15 minute ordeal to get the thing to start, whether it’s the fast idle or cold temperatures, it is quite a task. I haven’t ever been able to get the secondaries to tip in‑ even with lighter vacuum springs installed. I thought that the cam was installed wrong, but after I ripped the front of the engine off, it appears to be aligned correctly (Edelbrock double roller installed at 0°). Do you think that the cam is too radical? I can’t imagine that is the case, but the lack of vacuum is a major concern.
The other culprit may be the factory dual point distributor. I replaced the points, but when using a timing light, the mark on the damper is all over the place. My first inclination after getting your blessing on the cam size, is to replace it with an electronic ignition conversion. Prior fixes have included blocking off the power brake port, but no appreciable difference was noted in idle quality. I also have sprayed carb cleaner around the bases of the carbs. No dice. Lastly, I have blocked off all vacuum ports for the carbs, but no difference. At what point do I suspect a cracked manifold? By the way, the six-pack linkage moves freely with the throttle pulled back manually.
What would be a normal vacuum reading for a cam that size at idle? This isn’t my first go around with Mopars, but I really am stumped on this one.

I doubt that the cam specs are at the root of the problem.
Checking that the camshaft/crankshaft sprockets are correctly installed just proves that the camshaft/crankshaft sprockets are correctly installed. You need to prove that the cam lobes are where they need to be in relation to the crankshaft. In other words: “degreed in” with a wheel. Until you to that, you’re just throwing darts trying to hit the target, and you’re blindfolded.
The spark timing problem is also something that needs to be addressed before going any further. Possible causes for this jumpy timing problem include: Sticky mechanical advance weights, and/or a broken Litz wire ground cable. When this cable breaks, all the ignition primary current flows through the breaker plate’s ball bearing, arcing and sometimes welding it. (This is a Prestolite distributor peculiarity). The hard starting could also point to an ignition problem (I will assume you’ve checked for proper choke operation accel pump shot, etc.) If the breaker points and ground wire are OK, then I’d suspect the coil or a wiring problem. The coil pretty much is a R&R/tryout deal. To check for hard-starting/wiring problems, hook a DMM, or, better yet, an analog VOM (DCV range) hooked from battery positive to coil positive. Have an assistant crank the car using the ignition switch, as in normal operation, while you observe the meter. At no time should it read greater than one volt. If it does, there’s a wiring, connector, or ignition switch problem. The other way to check for this—kinda the “reverse” approach—is to temporarily run a jumper wire from battery positive to coil positive and see if this fixes the cold-start problem. (Don’t run it this way for more than 2 minutes).
Next, with the engine off, hold the center carb throttle at WOT and attempt to open the end carbs by moving the linkage rods. They should open fairly easily with no stickiness. A common problem on the end carbs, especially originals, was the throttle blades kinda “digging in” when closed and then being too sticky for the vacuum diaphragms to open. If the vacuum is steady—doesn’t jump around—I wouldn’t be terribly concerned about the actual number.