The valve to close off the air flow to the diesel inlet ports in the head is in the manifold. If you have the manifold off, you'll get suction in the diesel position because the decompression valves are closed and isolate the gas ports. If you open the decompression valves, both chambers are now connected together and you won't get any vacuum on the gas port unless you cover the diesel port.
Couple more things to check. Make sure the linkage that operates the butterfly valves in the manifold is working properly. If everything thing is switching like it should, then you're down to basically troubleshooting a gas engine.
Start with the basics. Is there spark? There's a contact on the changeover linkage that shorts the magneto on the diesel position.
Assuming you have spark, check for gas in the carburetor. I've had to clean the inlet port on my WD-6 (same engine) carburetor a couple times. One of these day's I'll get sufficiently ambitious to overall it. Also check to make sure that there's gas in the starting tank and the shutoff valve is open (yes, I know its a dumb thing to ask, but its happened before).
With the starting lever in the gas position, gas should drain out the bottom of the carburetor if you open the drain valve. If it doesn't, something is clogged, or there's a linkage problem. There's a mechanism that lifts up the float in the carburetor to shut off gas flow when you're running on diesel, and you won't have any gas in the carburetor if that's not working (or its in the diesel position). Took me a while to figure that out one time, as I didn't seem to be getting gas in the carburetor. Might be another place to check.
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