Most likely, you have some sort of a vacuum leak in the pathway to the #1/2 intake port.

If it were a motorcycle, the thing to do, would be to get it up to some load level, then cut the ignition, once stopped, pull all the plugs and look at the color of the center electrode insulator, to identify the mixtures of each cylinder.

There's a couple odd things that can happen to manifolds, one being a porous casting... the other is a porosity or crack in the segment of the manifold where exhaust and intake share a common wall.  On Allis engines, and most industrial inlines, the exhaust and intake manifold is either manufactured as a single assembly OR... they're bolted together such that exhaust heat can pass through a thin conjoined surface into the intake at an area fairly close to the carbeurator inlet flange.  The reason for this, is because airflow through the venturi and throttle plate cause a rapid reduction in pressure that will not only freeze the passage and bind the throttle plate, it will also inhibit good evaporation of fuel, and as a result, make for a very inefficient combustion cycle.

What happens at times, is a leak occurs from the exhast side, through that shared surface, into the intake manifold... and because of it's location, the natural accumulation of carbon soot, and the fact that they're always at room temperature when you're inspecting in there, a simple thermal crack can be downright invisible.

porosity through the same surface is basically just as invisible.

Since you've already ordered another manifold, you'll know soon-enough wether that is the culprit... but if you want to try something else, start it up, and carefully lift one of the plug wires JUST SLIGHTLY... make it jump from the distributor tower to the plug wire for a couple strikes- if the cylinder clears the miss, then what you're dealing with is probably carbon-fouling of the spark plug insulator.  If so, replace the plugs, and you MAY need to go up one heat range to keep them clear.  Don't make it a habit to leave them this way, especially on a magneto- forcing an arc gap at the distributor burns away the distributor and wire, it also forces ignition voltage way up, which can damage the winding insulation of the magneto's high-tension winding.

Oh, and if you replace the plugs, use a non-Champion plug... they're not what they used to be...

You could run a compression test, but I'm with the Dr.  If the manifold isn't sealed good, you probably have a vacuum leak.

AaronSEIA

So you sprayed carb cleaner on and around the exterior of #1 and #2 cylinders on the manifold. using carb cleaner you got a difference in how it ran? Just trying to clairify your test method. I have heard of tests like that but have never done it. And the Dr. says you should see no change?? I am sure not the expert. I did not see that you had checked the engine timing? Now that may not be a issue at all, but a good item to cross off the list too.

 And yes like Jim we need some pictures too!

Regards,

Chris

Thanks for the update on your WC! Would a compression check find or indicate that there is a leak between cylinders?

Regards,

 Chris

You may want to mark that old manifold with "BAD" on it. Time goes by and someone else might have issues.

I bet the surface could be flattened on an mill, or ground, and would be good as new. Unless there’s a crack somewhere a good flat mating surface would make it whole again. I save all kinds of stuff like that.