Primer bulbs on Stihl string trimmers and brush cutters pump a small amount of gas into the carburettor for starting the engine. Without gas in the combustion chamber, the engine will not fire. Simply pulling on the starter rope will not draw fuel to the engine. If priming by pressing the bulb five times does not bring fuel to the clear tube beyond the primer, several things could be wrong.
Inspect the clear tube at the output of the primer bulb. If gas is present in the tube, the system may be flooded with gas already. Pressing the bulb only sends more gas to the carburettor. Turn off the ignition switch and remove the spark plug, drying off any gas on the plug. Open the choke and pull the starter rope several times with the piston chamber still open. Replace the spark plug and repeat the usual starting procedure.
Check the gas tank next if the clear output tube shows no fuel present. Fill the tank nearly full if the fuel tank is empty or nearly empty. The fuel level must be above the pickup body--the input of the fuel system--in the bottom of the tank.
Loosen and replace the fuel tank cap if the fuel level is normal. Occasionally, a lower air pressure in the tank compared to air pressure outside the tank can interfere with priming. Opening the tank equalises the pressure. Always replace the cap before attempting to start the trimmer.
Inspect the primer bulb itself. Punctured or torn primer bulbs will not prime the motor. The priming system must be airtight to function properly. Leaking primer bulbs could cause fires during operation.
Check for obvious leaks in the supply side of the fuel system. Do this only with the ignition off and the engine dead cold. Two-cycle engines should operate in any position without fuel leaks. Rotate the machine to different positions and check for leaking fuel. Leaks in the supply side could interfere with priming.
Check the fuel itself for debris. Bits of grass or dirt in the gas could block the fuel pickup. Discard dirty gas and replace with clean gas and oil mix. Stihl recommends service by a qualified technician if the fuel system or pickup is faulty.
When working with fuel problems, test the machine in an open well-ventilated area. Work on high ground, not in a depression. As spilt fuel evaporates, explosive fumes pool in low-lying areas.