Shared Knowledge


Small Gasoline Engine Primer Bulb


11 November 2006






I suspect what I am about to write is known to everyone already or is well defined in your small gasoline engine device user manual but for what it is worth….


On many small gasoline engines, like those a part of chainsaws, string trimmers, leaf blowers, etc, there is a starter primer bulb, which you are supposed to push 4, 5 or X times before you try to start the engine.


What does the primer bulb do on small gasoline engines?  It actually brings a small amount of gasoline to the throat opening of the engine carburetor (carb). The problem is that any small gasoline engines have the carb mounted on the side of the engine and any gasoline pumped up to the carb throat may or may not actually make its way down the throat.


So here is what I have found that actually works:


-         Set the choke in the off position or no choke. If you set the choke on, it closes off the throat of the carb and any gas you pump to the throat via the primer bulb will not make it down the carb throat.


-         If your carb is located on the side of the engine, roll you small engine device over so the carb is sitting vertical. With the carb in the vertical position, any gas you pump to the throat will, by gravity, go down into the carb, where you want it.


-         Now use the primer bulb, pushing the recommended number of times. Note here that I have never “flooded” a small engine by pressing the primer bulb too many times. The amount of gasoline actually pumped to the carb throat on each pump is very, very small and so really not much chance of over doing it. On my Stihl leaf blower I pump something like 20 times.


-         After you have primed the carb and waited a couple of seconds for gasoline to move down into carb, move the choke to the closed position, which is going to seal off the carb throat to enrich the fuel\air mixture in the first couple of piston fuel compression cycles.


-         Now rope pull or electric start you engine and when it begins to “fire” or try to start, move the choke about half-way open or whatever works on your engine and try starting again.


The key thing I am trying to get across is that you should make sure the carb is vertical so that primer bulb gas actually runs down into the carb.


Finally, small engine primer bulbs are plastic and thus good for only so many "pumps" before they crack, begin to leak and not pump gasoline anymore. These bulbs are easily replaced and the manufacturer of your small gasoline engine equipment should have the replacement bulb.




Ron - Shared Knowledge Home