[Excaliber]

I think the vacuum advance and your HG readings at various speeds done when the engine is NOT UNDER LOAD, are a red herring. A snipe hunt. A,,,, well you get the point.

WHY do we even have vacuum advance? What purpose does it serve for PERFORMANCE? It doesn't have anything to do with performance, in that regard, take it out, block it off, throw it away. It's PRIMARY PURPOSE in life is to maximize mpg by enhancing cruising speed/rpm efficiency. That's it, thats all it does, all it was ever intended to do.

You will ever see a vacuum advance unit on a serious race engine, or even a serious street engine. Step 1. Maximize your timing on the base and mechanical side. I have found that most modified engines do indeed run better at low to mid range rpm with 18 to 20 degrees advance. That alone solves SO many problems, like stumbling, hesitation, flat spots, etc. But there are a couple of problems with running this much BASE timing. One is it may be hard for the starter motor to crank the engine, especially when hot. Two is it may be difficult to limit the MAX mechanical advance to maintain the desired total amount. Anywhere from 32 to 38, not generally more than that. Now ADD to that 32-38 the vacuum advance of say, 10 to 12 degrees. The new total may be (38+12) 50 degrees! That is reasonable!!!! Most motors could handle that, like it in fact, maximize your mpg. All though, 50 is pushing the outter limits in some cases. IF, your engine is under light load, like steady state cruise rpm with light throttle, it's amazing how much timing you can throw at it.

The reason is simple: As the HG (vacuum) drops, due to increased throttle opening, climbing a hill, accellerating, heavier load on the engine, the vacuum also drops. The radically advanced timing goes away, drops back to mechanical advance only!

It's unlikely your motor will be pulling anywhere near the HG at 2,500 your reading in the garage as it will at 2,500 rpm under load! Try this, get a really long vacuum hose and mount the vacuum gauge on the window or in the cockpit so you can see what the REAL HG is under various conditions, rpm, load, cruise, WOT, etc.

I run both mechanical and vacuum advance on my side oiler, with an MSD "ready to run" (does not require an MSD box) and comes with a vacuum advance canister. I run so much base timing that I HAD to find a way to limit the mechanical advance. MSD does not sell a stop bushing that offered enough "stop". I had to buy a special, bigger, stop bushing made by a speciallty racing shop just for this purpose. The fact that they are available is indicative of how many people are having the same problem, your problem! To much mechanical advance and no way to limit it enough.

The other thing you need to know is: When does the vacuum advance begin to advance? At what HG? Second is: How MUCH advance does it offer at what HG?

MSD starts at about 15" and offers about 10 to 15 degrees of advance (I forget the exact amounts).

For now, disconnect the vacuum advance, work on the mechanical advance, then go back to the vacuum part. Good luck.