If there is an air leak, WOULD it run rough, smooth out and run rough again? I don't see how, so IMO, NO Thanks
I'm not looking for an air leak, cuz you said the only time it runs rough is stopped, and idling in gear, I suppose like at a stoplite; and at this time the idle-timing is fixed at 5*Btdc and not wandering.(got-it lol).
So, in my opinion, this has nothing to do with timing, nor with a vacuum leak.
I'm suspecting the TC
or the powervalve,
or a combination.
with a possible problem in the low-speed circuit.
The vacuum to rpm readings will help me formulate an opinion as to what test to do next.
For instance; a 50 rpm drop between N/P and in gear tells me the TC is fine. And if the vacuum reading varies only a little, then I'll stop thinking about the TC.
But if the vacuum is low to start with, and then drops even lower, perhaps low enough to open the power valve, then the rough running is due to the leaking PV, but that PV is not the root cause, which would be the low vacuum reading or a bad TC.
So then we have to do a compression test...... because you said you already reset the valve-lash. Had you not said that,then the valve lash would have been the first thing on my mind.
And if the compression test is good, then I have to assume you did a good job on the lashing. But if not, then we have to do a LeakDown test to see if any valves are leaking......which leads to a valve re-lash. And that will lead to other tests.
The best place to plumb the vacuum line to, is anywhere on the intake manifold. Close to the firewall there is usually a vacuum-tree. If not, you can disconnect the choke pull-off and tap in there. Just don't use the spark-port that runs to the vacuum-advance cam.
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BTW, talking about the Vcan, with the engine idling in N/P, suck on that line; the rpm should go up, maybe as much as 100 rpm after multiple sucks. That proves both the can and the line are good. But if the rpm does not go up, then put the timing lite on and verify the 5*. Then switch the vacuum line over to the choke pull-off port. This should add about 9 to 19* of advance. If zero, then either the line or the can is bad, so check the line next. If the line is good then the can is bad. If the line is bad, replace it and retest. This has nothing to do with your problem.... unless the Vcan is currently connected to full manifold vacuum.......lol.I just thought this would be a good time to mention this.