Why Does the Primer Bubble Collapse on My String Trimmer?

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There could be several reasons the primer bubble collapses on your string trimmer.
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The priming bulb on your string trimmer supplies fuel to the carburetor when the engine is cold. It can't perform this function when it collapses; if this happens, you may find your trimmer impossible to start. That's because the collapse, which may be due to the bulb itself, can also signify a blockage in the line leading to the bulb from the fuel tank. The cause is often related to gummy varnishes left by old fuel. Replacing the bulb when it fails is a relatively straightforward procedure.


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A string trimmer's priming bubble can collapse after varnish left by old fuel collects on the sides of the bulb causing the sides to stick together when you press the bulb.

Function of the Priming Bubble

The primer bulb or priming bubble for a string trimmer functions in the same way on most trimmer models. It's often located just behind the starting handle, and you should press it 10 times when cold-starting the trimmer. Each time you do, it forces gas and air into the carburetor, and each time you release it, it draws more fuel from the gas tank and more air through the air filter. Loosely sprung check valves in the fuel system prevent the backward flow of fuel.

When to Fix the Bulb

A bulb may collapse simply because it's old and worn out. Bulbs are made from rubber or flexible plastic that can become soft after repeated use, especially if you use fuel with a high ethanol content. If you're in the habit of leaving the tool in storage for extended periods, varnish in the fuel can collect on the sides of the bulb, and this can make the sides stick together when you press the bulb. The solution is to replace the bulb. This is usually a simple procedure, although it involves removing the carburetor.


The Faulty Check Valve Problem

When you release the primer bulb, it creates a suction force as it regains its shape, and this force is sufficient to draw fuel through the check valve located in the fuel line that leads from the tank. Another consequence of leaving old fuel in the trimmer for an extended period is that it can gum up this valve and prevent it from opening. If you do manage to start the trimmer, the fuel pump may develop enough suction to open the valve, but it's more likely that the trimmer won't start. You'll probably have to drain the fuel, clean the fuel lines and refill the trimmer with fresh fuel.


Maintaining the Trimmer

Because primer bulbs do wear out, it's a good idea to replace the bulb on your string trimmer every season if you use the tool frequently. You should also regularly clean or replace the air filter. A dirty one can contribute to sluggish fuel delivery, and that can lead to poor performance of the priming bulb. If the priming bulb on your trimmer is slow to pop back to its original shape after you press it, try removing the air filter. If the bulb then responds normally, it's time to clean or replace the filter.

You should also consider winterizing your trimmer in order to preserve the integrity of its parts during the off season. During the last few uses of the trimmer, avoid overfilling it with fuel that will sit over winter and deteriorate the rubber primer bulb. Knowing how to fix a primer bulb on a string trimmer also means you'll be prepared to replace it each spring before the lawn maintenance season begins for best results. This is also the perfect time to inspect the air filter and fuel lines for optimal performance.



Chris Deziel is a contractor, builder and general fix-it pro who has been active in the construction trades for 40 years. He has degrees in science and humanities and years of teaching experience. An avid craftsman and musician, Deziel began writing on home improvement topics in 2010. He worked as an expert consultant with eHow Now and Pro Referral -- a Home Depot site. A DIYer by nature, Deziel regularly shares tips and tricks for a better home and garden at Hunker and Family Handyman.