Typically , the propane needle valve is set to 1.5 turns. Do make sure this valve is installed after the solenoid. As for the issue of the motor spinning down prior to ignition, once you get the flow correct, some of this will go away. However, if you have the Fadec program and the data cable, you can go in to the autostart function and set the minimum rpm to connect the starter and rpm before ignition parameters to improve the start.
First parameter is starter voltage , and this should be 39%
Second parameter can be set to 2000 rpm
Third parameter set to 3000 rpm

Hope this helps!