These use vacuum from the motor to create a hole in the cylinder so that the motor is easier to pull through compression. It must remain open for easy starting and stay closed afterward for proper running.
If the decompressor is stuck open (motor not firing):
- It may be contaminated. They get a little oil in them and it can congeal. Remove the decompressor and clean with gasoline.
- The vacuum line may be blocked or have a hole in it.
The most common reason for not getting a spark is a bad plug. They can look fine but be broken internally. They’re cheap and easy to replace so start there. Other causes:
- Connector Primer bulbs are common culprits. Consider removing the bulb from the system and joining the fuel line with a simple nipple to see if that cures it.
- Fuel line cracks. These can be impossible to see. Usually they show up where the fuel line goes into a nipple or around a bend or gets chafed (rubbed enough to cut into the material).
- Vent: The gas tank must vented to allow air in as fuel is burned. Otherwise a vacuum forms that eventually prevents free fuel flow. Unscrew the cap and blow into the vent line. If you cannot blow into then either a one-way valve has malfunctioned (if installed) or there is something blocking it. Replace the vent line and/or one-way valve.
- Fuel Line: Make sure there’s gas in the tank, disconnect the fuel line from the carb and blow into (or pressurize somehow) the vent line. Fuel should push up and out of the fuel line’s end. If it does not, the filter (if installed) may be plugged or the fuel line may kinked or pinched.
Black Devil Decompressor
Replacing this unit is difficult because of the close cooling fins that prevent getting a socket around the main part. It may require a skinny socket.