Posted by docmirror on May 15, 2019 at 16:48:51 from (107.213.165.255):
I've bought one trimmer carb over the years, as it was clogged up. Tiring of this, I decided to get out the tools and stop messing with this.
Abstract: Trimmer carbs in the US no longer have adjustable mixture screws. They have a cast surround shoulder, and the screws are put in with a special cylinder tool with a notch. One can buy a set of carb adjustment tools for $8-20 with a set of common heads, or one can make their own slot for free.
I chose to make my own. Mainly because I wanted it working right away.
Tools: metal cutting coping saw, or a bare hacksaw blade, with open end. Small screwdriver. Can of carb cleaner with red straw.
Procedure: Locate the two dark gray mixture screws. There will usually be an embossed letter next to them with "L" and "H: corresponding to 'low speed' and 'high speed' circuits of the carb. See picture. The H or high speed needle is on the right. Note they are recessed in the casting. The feds don't want us changing the mixture in our carbs to spew more CO, NOX in the air.
When one of the circuits gets clogged, usually with congealed oil and plastic and Ethanol, the engine will die at low, or high speed or maybe both. A cleanout is needed.
Using the coping saw, cut into the casting, right over top of the gray head of the mixture screw. Try to cut with the saw blade in the center of the mixture screw. This will be the slot for the screwdriver. Repeat for the other screw as well. See the red lines in the picture. This is the cut line.
Keep cutting through the casting, and the top of the screw until you have a good deep slot in the dark gray screw head. Turn the carb so the mixture screws are facing down, and spray off the casting and screws with carb cleaner. Flush out all the metal filings so they won't go down the mixture screw holes later.
Using the small screwdriver, screw the needles in to the stops first, and count the number of half turns. Recall these numbers. Now screw the needle all the way out, but note if it has a spring and o ring, or any kind of sealer for the threaded hole. Keep the needles screws and any other parts together.
Now use the small screwdriver and remove the plate on the opposite side of the priming bulb. This is the fuel pump and inlet needle and seat. Under the seat, you will find a diaphram. Carefully remove this by using a exacto or razor blade to pry it cleanly off the carb body.
Set the diaphram aside, and now use the carb cleaner, to spray into all the various ports on the body. Spritz down the holes in the well where the diaphram was removed, spritz inside the carb jet holes. Keep going until you get good flow from the opposite end of where you are spritzing from. The well where the mixture screws go should get plenty of attention. Make sure every port is clear, and flows through the body, and comes out somewhere.
Hopefully, the goo will all come out. I've never had to soak one, but have used a couple cans of carb cleaner to finish up.
Now that everything is all clean, put the mixture screws back in, and set them to the depth they were when you started. This will onyl be the starting point for mixture. If the low mixture was out 2.5 turns, start at 2.5 turns out from the stop.
Once more spritz in the well of the carb, and if you can out of the main jet. Install the diaphram, and the cover plate on the body of the carb opposite from the primer bulb.
Put the carb back on and attach the hoses from the tank. Make sure you put the right hoses in the right port.
Prime the carb with the bulb. Start the engine, using choke and then let it warm up. It may not need choke of the mixture is now richer than before.
Once it's warmed up, set the low mixture screw to best running with engine just above idle speed, or about 1000 RPM. Do not rev it up now, as we are setting low mixture only.
Now, rev it up. It should go full speed. Run it at about 6-7000RPM, near full throttle. Carefully turn the H high speed mixture screw in, very gently. If the engine dies, turn it back out a full turn and start again. Set the H mixture so it runs smooth at high speed, and doesn't blow a lot of smoke. This wil be about 14:1 or a bit richer.
Go trim something, and get it running for 20 minutes. Stop it and remove the spark plug. It should be a copper, dark tan color on the insulator. If it is nearly white, the engine is too lean. If it is chocolate dark, or worse the engine is too rich.
Re-start the engine, and final adjust the mixture screws. They are more sensitive than tractor mixture, so go 1/8 turn at a time. Once set, adjust the idle screw to 1000 RPM.
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