In this tutorial I will be telling you how to have a Start Engine button for your propeller engine

Step 1
Open the part settings and locate the Pitch Control

Step 2
Set the Ptich Control to manual and set the Pitch Range to 70%

Step 3
Turn On Throttle Governor and set engage at to 5%

Step 4
Locate Throttle Input and set Input to whatever Input you want

Step 5
Lastly, locate Propeller Pitch and set Input to Throttle

I hope this helps