I used a common propane torch to flow gas around each manifold inlet runner. Unfortunately I couldn’t detect any change in the engine idle at any point.
What I did discover was the cause of the engine shake at idle. At some point when i was swapping out the spark plugs and the spark plug wires I must have broken off the vacuum line to the heater controls.
This added a wee bit of air to the back side of the intake manifold and must have caused what I understand is called a “lean air misfire” in one of the rear cylinders. Once I fabricated a temporary fix the engine idled much better. The vacuum fitting is available for CDN$5.95
Now to get back to some nice summer driving 🙂