Ok, open windows explorer and navigate to where your copy of dcs is installed. I don't have the 109 but the following is where the Default.lua for the P-51 is stored.....
<your drive>\DCS World OpenBeta\Mods\aircraft\P-51D\Input\P-51D\joystick
I've attached a screen dump of what my 51's Default.lua location looks like. Go to the 109's folder location and rename its Default.lua to Default.old - (right click and select rename)
Then run up DCS, go into control setup and reassign...check how the throttle behaves.