"Clearing the tar" from the LPG injector holder last chance filter, or from the LPG vaporizer (if that is what you refer to) should not have any bearing on how it operates on gasoline.
So, have you been trying it on gasoline as well as LPG?
However, if you have not cleaned that "last chance filter screen in the LP injector holder assembly, that could still be a fuel starvation point, and cause it to fail to rev to full RPM
You say you "have no dash lights or hour meter". If the engine MIL (Malfunction Indicator Lamp) is not lighting up when you turn on the key switch BEFORE YOU START THE ENGINE, the MIL not operating is also considered to be a "serious defective condition" by the ECU and the ECU WILL limit the RPM based on that until the MIL is i working order.
So, if you have some kind of loss of ground to the dash, or loss of a power circuit (fuse blown or bad wiring connection) feeding the dash/monitor, the ECU is probably keeping the RPM limited because the MIL is not functioning.
This RPM limiting effect would be present whether you selected gasoline of propane fuel