Sounds as though could be an old L51c
With a 162 continental engine.
The carb is an updraft style. you can adjust
The carb and see if that helps.some had an adjustment for the vaporizer as well
It is possibly mid sixties 1967-69
Is this a Continental engine?
Such an older machine some things to consider if you haven't already: vacuum gauge reading should be 18-22 inches at idle, compression test, cylinder leak-down test, ignition system, LPG components.
Compression should be above 165 psi (fully charged battery, all plugs removed, throttle held wide open).
Tune up: points, plugs, condenser, cap, rotor, wires. Spark plugs can be gapped at.025" - 0.28" since propane is more difficult to ignite. If the distributor has a vacuum advance test it and the breaker plate movement.
LPG system: rebuild or replace.
Valve adjustment.
Timing. Set at part throttle while stalling the tilt function to put a load on the engine. Set for highest rpm then check for easy starting, no pinging under a load, good power, etc. After setting the timing adjust the fuel mixture.
If the engine is tired (low compression) you can try one- or two-step hotter spark plugs to see if that helps.
Good luck.