Post by rowdy235 on Feb 9, 2019 16:12:30 GMT -5
Hi all, just thought I would write up a quick how-to on rebuilding a Walbro WT carburetor. This is a WT-220 off my ST-145 string trimmer but others should be similar.
Step 1 is removal of the carburetor from the machine
Step 2 is disassembly. I like to lay out all parts on paper in order, as it helps with reassembly. If you need help, look below as I go into more detail with the reassembly section.
Step 3 Put the main body, top cover, and bottom spacer into the ultrasonic cleaner. I do 10 minutes in solvent (I like to use simple green as its good for grease/crud but also won't hurt metal and most plastics) and 10 minutes in water to rinse. If you don't have an ultrasonic cleaner you can substitute a good cleaning with carb cleaner through all the passages.
While the parts are cleaning I usually take time to review the carb kit and take out the gaskets I need. Since they build these kits for a wide variety of carburetors, not all the parts will be used.
Step 4 Remove the main body and blow out all the passages with compressed air. Nothing serious maybe 10psi will do fine. I then wipe down the unit and make sure all the gasket surfaces are clean.
Step 5 Reinstall the Choke and Throttle shafts. On my carb, the choke shaft just sits in the opening but the throttle shaft has an E-clip. Both plates are held on with a screw, make sure to tighten securely.
Step 6 Working on the top of the carburetor, install a new screen as shown. I find the male end of an air hose coupler is the perfect size. Clean-up and reinstall the top cover with a new gasket and mesh piece.
Step 7 Turning you attention to the bottom, install the new metering level and needle valve. I probably don't need to explain this can be a PITA. I like to hang the needle on the lever, and drop the whole unit down on top of the spring. Install the screw (pictured) to hold it in. You want to make sure that the lever is setting flush with the carburetor body, but mine did not require any adjustments.
Step 8 Install a new diaphragm with gasket
Step 9 Clean up and reinstall bottom spacer piece and primer bulb. Make sure you tighten the four screws evenly, not super tight, about 1/4 past snug should be good. Reinstall Hi/Lo adjustment screws (not pictured)
Step 10 Reinstall on machine and tune. There are more detailed threads elsewhere but I will explain briefly.
Before beginning make sure you have a full fresh tank of fuel. I set Hi/Lo screws out 1.5 turns each. Start up the machine and adjust the screws to where it will stay running. Allow to reach normal operating temperature.
Lower idle speed as low as you can or if you can get to a good speed stop there (my unit doesn't have a clutch so I normally run the idle a little higher than usual). Begin richening low speed about 1/8 turn at a time allowing a few seconds for engine to react. The engine speed should increase/smooth out but will eventually begin to decrease. Once it begins to decrease, lean out the screw 1/8 turn at a time until it runs smooth again. Then test acceleration by "flooring" the engine from idle, it should accelerate without any hesitation. If hesitation exits, richen 1/8 turn and try again.
Then go to the high speed needle. Run the engine at WOT without any load and begin slowly leaning out the high speed screw. Eventually the engine will smooth out and increase in speed. Once it does richen it just enough to where it will 4-cycle (will sound like a slight misfire). Put the engine under load (eg into weeds for a string trimmer or into wood for a saw) and verify that engine clears up and does not 4-cycle. If still 4-cycling under load lean out hi speed a bit and try again. If the equipment cannot be removed from load (eg a blower) just tune high speed to be just lean enough where the engine does not 4 cycle.
My result was very good, the hard starting and hesitation issues I had last year are gone. Looking forward to a good year of trimming!
Step 1 is removal of the carburetor from the machine
Step 2 is disassembly. I like to lay out all parts on paper in order, as it helps with reassembly. If you need help, look below as I go into more detail with the reassembly section.
Step 3 Put the main body, top cover, and bottom spacer into the ultrasonic cleaner. I do 10 minutes in solvent (I like to use simple green as its good for grease/crud but also won't hurt metal and most plastics) and 10 minutes in water to rinse. If you don't have an ultrasonic cleaner you can substitute a good cleaning with carb cleaner through all the passages.
While the parts are cleaning I usually take time to review the carb kit and take out the gaskets I need. Since they build these kits for a wide variety of carburetors, not all the parts will be used.
Step 4 Remove the main body and blow out all the passages with compressed air. Nothing serious maybe 10psi will do fine. I then wipe down the unit and make sure all the gasket surfaces are clean.
Step 5 Reinstall the Choke and Throttle shafts. On my carb, the choke shaft just sits in the opening but the throttle shaft has an E-clip. Both plates are held on with a screw, make sure to tighten securely.
Step 6 Working on the top of the carburetor, install a new screen as shown. I find the male end of an air hose coupler is the perfect size. Clean-up and reinstall the top cover with a new gasket and mesh piece.
Step 7 Turning you attention to the bottom, install the new metering level and needle valve. I probably don't need to explain this can be a PITA. I like to hang the needle on the lever, and drop the whole unit down on top of the spring. Install the screw (pictured) to hold it in. You want to make sure that the lever is setting flush with the carburetor body, but mine did not require any adjustments.
Step 8 Install a new diaphragm with gasket
Step 9 Clean up and reinstall bottom spacer piece and primer bulb. Make sure you tighten the four screws evenly, not super tight, about 1/4 past snug should be good. Reinstall Hi/Lo adjustment screws (not pictured)
Step 10 Reinstall on machine and tune. There are more detailed threads elsewhere but I will explain briefly.
Before beginning make sure you have a full fresh tank of fuel. I set Hi/Lo screws out 1.5 turns each. Start up the machine and adjust the screws to where it will stay running. Allow to reach normal operating temperature.
Lower idle speed as low as you can or if you can get to a good speed stop there (my unit doesn't have a clutch so I normally run the idle a little higher than usual). Begin richening low speed about 1/8 turn at a time allowing a few seconds for engine to react. The engine speed should increase/smooth out but will eventually begin to decrease. Once it begins to decrease, lean out the screw 1/8 turn at a time until it runs smooth again. Then test acceleration by "flooring" the engine from idle, it should accelerate without any hesitation. If hesitation exits, richen 1/8 turn and try again.
Then go to the high speed needle. Run the engine at WOT without any load and begin slowly leaning out the high speed screw. Eventually the engine will smooth out and increase in speed. Once it does richen it just enough to where it will 4-cycle (will sound like a slight misfire). Put the engine under load (eg into weeds for a string trimmer or into wood for a saw) and verify that engine clears up and does not 4-cycle. If still 4-cycling under load lean out hi speed a bit and try again. If the equipment cannot be removed from load (eg a blower) just tune high speed to be just lean enough where the engine does not 4 cycle.
My result was very good, the hard starting and hesitation issues I had last year are gone. Looking forward to a good year of trimming!
