Missing at 3,000 rpm

At the very least, I'd back the timing up until you get stuff sorted out. Did you regap the plugs to take advantage of the MSD primary circuit? Also... did you get the 6A or the 6AL? If it's the 6AL, don't overlook the limiter chip... if it's the 6A, make sure it's PN 6200 (not 6246).

I can't see required voltage at idle being very high (even if the plugs are gapped in the 0.060" range) but even so, I would have likely opted for something a little better than "chain store" wires. We've been using the MSD wires for a few seasons and they are very good.

One afterthought... is this a case of dropping cylinders randomly or a steady miss?

I'm going to swap the NAPA wires for MSD's super conductors.

Its the 6A box, PN 6200. I have the plugs gapped to .035 (stock calls for .034).

Also, it is a steady miss, not random. I can't pinpoint the cylinder, but it always happens.

With the MSD setup, you could probably go as wide as 0.070".... We runn

With an ice pick type electrical tester, hook the alligator clip to ground and poke the ice pick end into the spark plug wire at the spark plug one at a time. If it's always missing on one cylinder, the one that makes no change in the idle/sound is the one thats missing. I'd do this before you take the cheap wires off and put the expensive ones on.

If the expensive wires are on and it's still missing, pull the spark plug wires off the spark plugs one at a time WITH insulated gloves on. Again, if it's always missing on one cylinder, the one that makes no change in the idle/sound is the one thats missing. With a '72 351 you should have room to do this.

Too much timing. To set base timing, get rid of the timing light. I know, I know! That flys in the face of common wisdom and known mechanical rules. But just listen. The timing light is used to set the base timing of a stock car with stock parts to a setting that the factory engineers determined to be the best based on current fuels, technology, parts, expected use, yada, yada, yada. How many of these standards apply today? Answer - None! So forget that standard and come up with a new one. Also remember that base timing has absolutely no bearing on power output or efficiency - unless you measure those standards at idle, and who does that? What you need is a vacume gauge and a half a glass of water. Unplug the vacume advance line at the distributor, and plug the line in to the vacume gauge. With the engine full warmed up, turn the distributor until vacume is highest at the same idle setting. Now watch the glass of water. Fine tune the setting until the water ripples are the smoothest. Now shut the engine off and restart. If it starts easily, that's your best base timing. If it's seems to be hard to start, back off

Now it's time to set total mechanical timing. This has a direct impact on your total power output. Most SBF's like about 34-36* total advance, all in by

Finally, you can set vacume advance. All that does is provide a little fuel economy at light/partial throttle, like cruising. It doesn't add any power at all. If not adjusted properly, it will cost you some power. Start at about 8*. Drive around in your normal cruise mode and see how it does. If it starts to ping, back off. If not, add a little.

Also, check your plug gaps. Stock for '72 was about .032 or so (if I remember right). But with your high powered ignition system I'd open that up to at least .045". You NAPA wires may be the bottle neck keeping you from opening them up even further. But once you got the rest tuned correctly, you could give it a try.

Thanks! I'll give it a try.

To verify, though, using your vacuum gauge method, it is measuring ported vacuum just above the butterfly plates in the carb - it's not measuring intake vacuum. Right?

Since you're at idle, you can use either one. If you use manifold, be sure and unplug and block the vacume line to the distributor.