Replacing a String Trimmer Carburetor

My string trimmer would no longer idle. If I let off the throttle, it would die. First, I inspected the fuel lines and replacing the air filter, fuel filter, and spark plug. This was long overdue maintenance anyway, but it didn’t fix the issue. Thus, the remaining culprit is the carburetor. It turns out that replacing a string trimmer carburetor with a remanufactured one is cheaper than I thought.

Backing up, I was planning to take it to the small engine repair shop near my house. It turns out that it had shut down. I started looking into rebuilding the carburetor myself. Most likely, with a problem like this, one of the jets in the carb has gotten gunk built up to the point where it doesn’t flow properly. However, it turns out that an entire new carb was only $17 on Amazon. It also turns out that the EPA mandates hiding most carb adjustments. I’ll do a teardown of the old carb to show how to repair it in another article.

Old vs New Carburetor

Old vs New

Old vs New

I had a second reason for replacing rather than rebuilding. It turns out my idle problem was caused because the idle screw assembly had broken off at some point, unbeknownst to me. You can see the missing part if you compare the two carbs pictured above. Replacement is the only practical option in this case, although I’ll keep the old one as a spare.

Removing the air filter

Unscrew the orange screw to remove the air filter cover

Unscrew the orange screw to remove the air filter cover

The air filter will almost certainly be sitting on top of the carburetor, so it has to go first. On this one, there is an orange thumb screw holding the filter and a cover on.

Air filter and cover removed

Air filter and cover removed

Then, the cover and filter just pull off. If the filter is dirty, this is a good time to replace. I had already replaced the filter, so mine is brand new.

Remove both long nuts

Remove both long nuts

Done

Done


With the filter and cover off, you can see the filter housing. There are two long bolts that hold the filter housing and the carb to the trimmer. Undo both of these, and the filter housing will come off. The bolts are either a T25 or T30 torx, I don’t remember which.

Removing the carb

Undo the fuel lines

Undo the fuel lines

The next step is to undo the fuel lines from the carb. Be aware that gas may spill. Have something to catch it with.

The throttle cable slides out

The throttle cable slides out

This was hard to get a picture of, but the throttle cable can just slide out. You can manually open the throttle at the carb by twisting it, and pull the throttle cable out.

Cable is now free

Cable is now free

This is easier to explain by picture than with words. Here’s the end goal. If you do the step above correctly, the throttle cable slides right out.

Undo the lock nut holding the cable

Undo the lock nut holding the cable

Finally, there will be a lock nut holding the cable to the housing. Be sure to loosen this with a wrench, then lift up.

Don't lose track of this gasket

Don’t lose track of this gasket

When you pull the carburetor off of the trimmer, there will be a gasket between the two, as pictured. Be sure to keep track of this. If it is in rough shape, it may need to be replaced. If this gasket leaks, it will likely allow too much air into the combustion chamber and lean out the trimmer, making it run rough and stall.

Wrap-up

Once the old carburetor is removed, the new one goes back in the same way, in reverse. The job really isn’t very difficult. At least in my area, a $17 carburetor is cheaper than paying the local shop near me to rebuild one, much as I dislike how disposable things have become these days.

If you have an issue after replacing a string trimmer carburetor, be aware that the problem may be the remanufactured carburetor. The first time I put one in, the trimmer would die when I revved it. This is actually what prompted me to replace the fuel line. When that didn’t work, I realized it was the new carb at fault. I got a free replacement, and it has worked fine since.

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