Zenith Carburetor on Yale Fork lift

[Paso One]

I am working on a old Yale Fork lift with a 6 cyl Flat head engine.
Do you know who supplied the engines to Yale?
This engine has a Zenith 12912 Carb on it.
I ordered a carb kit from Mcdonald Carb and Ignition. Great service and a nice kit they supplied.
There is so many unknowns as there no is markings on the anything on the fork lift other than the carb tag. Engine has PT casting on head.
I removed what appears to be a propane block above the carb.
After cleaning the carb and installing the kit it starts and idles very nicely.
However when you go to accelerate the engine it just boogs down gasping for air.
Major hesitation but will return to idle just fine.
What modifications were typically done to the carb when running it on Propane?
Not sure if this was a add on propane kit or stock from the factory.
I plan to run it on gasoline only.
What changes do I need to make to this carb?
What may have been done if it was a add on kit?

[mmcdonald]

Should be a Chrysler engine if original.

[dkamp]

From the photos you posted, I cannot identify this very well, but it may not be a propane mixer you've shown... it could be a Velocity Governor... or part of a VG system anyway...