[Old Henry]

I'll take a stab at your questions then offer some ideas:

1. I haven't had a blown power valve so don't know thos symptoms.

2. With a massive vacuum leak you would not be able to idle so you don't likely have that.

3. You have it right on how to adjust timing. It is best adjusted with a vacuum gauge for maximum vacuum. That setting will depend on your elevation. At 5,000 feet where I live I have mine advanced as far as possible. But, bad timing is not likely to produce the symptoms you describe.

Now my ideas:

Start with determining whether the problem is spark or gas. I do that by putting my timing light on the high tension wire coming out of the coil and watch it while I increase the RPM's. The light should be steady. If it misses at all there is a problem with the spark. Put the light on each plug and see if one or more is consistently unsteady. If so, check the plug and wire. If the spark is steady check your fuel pump by disconnecting it from the carburetor and see if good powerful squirts of gas come out of the line while cranking. Also check the pressure of the fuel pump. It should be between 1.5 and 3.5 lbs. If the flow is low but the pressure is right there is a blockage in the fuel line such as a plugged fuel filter.

I have the same engine as you and have some more ideas once you post the results of some of those tests. Or PM me for more help.