Thanks again to all the suggestions. I had already done most of them before my original post.

FYI, I purchased both of the engines this year with the plan to restore both of them. Both were reported as non-running. After restoring, the green 1955 Johnson ran perfectly. The red 1956 is the problem engine that won’t fire at all.

Some of the suggestions I had already completed:
* the fuel in the tank of both the running (green) and non starting (red) engine is the same and is new fuel (non-ethanol) this spring.
* The filters in both tanks were removed and cleaned in ultrasonic cleaner.
* Gas reaches the carburetors and drains out when high speed needle (or drain plug) is removed.
* Piston rings in both engines are free
* Reed valves in both engines are not stuck or corroded, and look identical.
* Compression in both engines is nearly identical: near 60 psi in both cylinders
* As stated in my original post: I swapped the carburetor and magneto between the two engines. The red engine still would not fire and the green engine still started and ran fine, proving both magnetos and both carbs are good.
* Tested spark on both magnetos before and after doing the swap, and had a spark jump of over 3/8”.
* Both engines have new spark plugs. Also swapped the spark plugs and it didn’t affect either engine – green ran fine, red wouldn’t fire.
* The points were set on both magnetos using a volt-ohm meter and the marks on the flywheel. All four point sets are dead on the marks. Not possible if the cam was upside down.

In response to the more recent suggestions, I’ve started to focus on whether I’m getting enough vacuum in the crankcase to draw air and fuel from the carburetor. I tried feeling with my finger if I could feel suction in the throat of the carburetor, when spinning the motor with a drill. I couldn’t really feel any suction, but the carb throat has some side channels that make it difficult to seal with your finger.

So I installed the silencer and then fashioned a small strip of paper next to the opening of the silencer on the red engine. I wasn’t able to attach a video showing the movement of the paper strip from the pulsing of the vacuum in the crankcase when the engine is rotated with a drill. I did the same test on the green engine, and the movement of the paper strip looks to be identical. My conclusion is that amount of vacuum created in the crankcase looks to be the same in both engines.

Needless to say, I’m stumped! I’ve checked and rechecked and tested just about everything I can think of. I’ve already swapped the magneto and carb and plugs with no success. The only other thing I haven’t swapped are the power heads. Currently contemplating doing that.