I'd agree with looking at the vacuum advance. I just finished converting to manifold vacuum and a B26 can. Idle vacuum went from 11" to 15" following Lars's instructions. I may have to go to a B28. Haven't driven the car yet due to the continual rain.

 

try ported vacuum and see if the problem goes away. If it does you have too much vacuum advance for the conditions that occur at high rpm and a closed throttle.

 

"Ported" vacuum is for idle or low RPMs only. Should not have any effect at cruise speed to conduct a test..

Let me think here 69L46. You said in post # 17 that a bushing limits your canister advance. I have not had a dizzy pulled apart in a while but I believe the bushing you are referring to, limits the mechanical advance only. Those limit bushings come in several color-coded sizes such as 16*, 18*, 21* and so on. In other words the weights of the mechanical advance fly out until the bushing stops them, or limits them.
On the Vacuum advance, as I recall, its limit of travel is directly related to the arm from the canister to a plate under the rotor. On some dizzys a bracket with four choices of limits controls advance depending on how the bracket is oriented. The maximum Vac adv can be set to your liking. The one I use is MSD # 84281 for $11 at Summit. It uses one hold-down screw which is already in the dizzy. (Google it for a picture)

You need to attach a vacuum tool with gauge to your dizzys Vac Adv port. With the cap & rotor removed apply vacuum, then take note of when the Vac Adv arm begins to move. That is your "Start Vac on the canister charts. Then note when it maxes out. That is your "Max Vac". How much vacuum does it take to hold it steady? Does it bleed down indicating a leak in the canister?

Maybe Lars will bump in here and inform us what limits the Vac Adv arm / pin.

 

First, I would like to thank all of you for your replies. Lots of good ideas that I would not have thought of. Today I reset timing to 35 all in and then checked again at idle with the can unhooked, Got 20 degrees initial. I mentioned earlier that I couldn't run at 36 total because of knocking. Well I checked the can at idle and it was pulling about 25 crank degrees. And this was with a bushing that stopped it about .032" from it's max.. So you can see why I had to retard initial for it to even be usable. I learned to not trust the can specs.

I welded the slot so that it was limited to 11 degrees at the crank. No knocking, idles better, runs a little cooler and performs great. It possibly can use a little more vacuum advance but it's runs so good now I think I'll leave it alone and use it for a while before allowing a little more.

Well now for the rest of the story. The intermittent jerking/trailer hitching was apparently being caused by the ground wire. It was broken inside the insulation near the terminal I guess from being 50 years old. It doesn't move much but after all that time it wore apart. Taking the can out a couple of times finished it off. It really did feel like an over advanced condition. Anyway, I wanted to follow up and also offer thanks for everyone's help.