Rebuilding and Cleaning Roundslide VM Mikuni Carbs
||October 5th 2007 at 6:17 PM
This is a short guide to help you rebuild and clean the carbs that are commonly used on a wide variety of 2-stroke engines. There are a few different styles of Mikuni carburetors, but this article will work for most roundslide style Mikuni carbs. Please read through this article in its entirety before beginning this job on your own.
To remove the carb from the engine, you will need to first shut off the gas with the inline shut off valve which is found on most sleds. Next remove the air box from the snowmobile. It is usually held on with a few screws or rubber straps. When pulling the air box from the snowmobile, check for things still attached to the box such as vent lines, and disconnect them as necessary. Once the air box is out of the way remove the fuel line, the choke plunger assembly, and the throttle cable/slide assembly. The fuel line is held in place with some sort of a clamp that will need to be moved in order to remove the line. The choke plunger assembly can be removed from the carb body with an open end wrench of the appropriate size. It is important that you do not move the choke cable adjusting locknut unless you know how to and need to adjust the chokes. The throttle cable/slide assembly can be removed by simply unscrewing the cap from the carb body. Once all three of these items are disconnected from the carb, loosen the clamp holding the carb into the rubber boot. This usually allows you to remove the carb from the engine. Take care in case other things are still attached and remove as necessary, noting where cables, hoses, and other items are routed and located throughout the removal process.
Next we will discuss a few terminology words on the external surfaces of the carb. NOTE: Click on each image to enlarge
In the above picture you can see the carburetor removed from the engine. In the center of the carburetor in this picture you can see the following
1. Idle Speed Screw
2. Fuel Inlet
3. Float bowl
In this picture you can see the following
1. Idle Air Screw
2. Choke Plunger Location
3. Float bowl
When removing the carburetor cap from the carb, DO NOT
remove the throttle cable or move the adjusting nuts on the cable. If the engine is equipped with multiple carbs, the synchronization of the carbs will be affected and the engine will not run efficiently. All of these pieces seen here will come off as an assembly and not require any maintenance for a simple carb cleaning. The cap simply unscrews from the top of the carb and all of the pieces seen here in the picture will be held together with the throttle cable. In this picture the parts are as follows:
2. Throttle return spring
3. Mixture needle retainer
4. Mixture needle
5. Throttle slide
6. Carb body
This is the bottom view of the carburetor. This is where the majority of the carb cleaning will occur. Remove the 4 float bowl screws shown to gain access to the inner workings of the carb.
This picture shows the float bowl removed from the carb. Parts inside the float bowl should be clean and corrosion free. A can of carb cleaner and something to loosen up the varnish/corrosion are really all that is needed to clean with. Fishing line and an air compressor also are handy items to have before beginning this project. More carb part names are as follows:
2. Float Arm
3. Fuel Inlet needle and seat (view obstructed by float arm)
4. Main Jet
5. Pilot Jet
6. Choke Feed tube
Finishing disassembly will not take much. To remove the floats from the bowl, carefully pry the float retainers off their respective pins. The floats should slide freely up and down their entire range on the pins. When removing the floats, note the position of the pins sticking out of the side of the floats for reassembly. To remove the main jet, simply unscrew it, taking care not to lose the washer that is underneath it. To remove the pilot jet, a small flat headed screwdriver is needed to unscrew it from the carb body. To remove the float arm and pin, get a small punch and carefully lightly tap the punch out of the carb body. With the float arm removed, the fuel inlet needle and seat can be removed with a wrench. To finish up disassembly, the air screw should be removed. Before removal, turn the air screw in until it lightly seats, counting the number of turns it takes to bottom out. Write the number of turns down, as you will need to know this when reinstalling the air screw back into the carb body. Clean the tip of this screw with a rag and some carb cleaner until it is once again clean again. When it is time to install the screw, thread it in until it lightly bottoms out and then back it out then number of turns you counted earlier. In my opinion it is not necessary to remove the idle speed screw as it will also affect carb synchronization. Unless it has already been messed with, its current setting will be correct 9 times out of 10 after the carb has been cleaned properly.
This is a picture of the carb completely disassembled. All parts are as listed below.
2. Throttle slide return spring
3. Mixture needle retainer/Spring Seat
4. Mixture needle and e-clip
5. Throttle slide
6. Carb body
7. Choke plunger
8. Needle Jet
9. Fuel inlet needle and seat
11. Float arm and Pin
12. Float retainers
14. Idle speed screw
15. Air screw
16. Main jet washer
17. Main Jet
18. Float Bowl
19. Float Bowl Screws
Now that everything is totally disassembled is it time to start cleaning. Everything in the carb should be cleaned. Remove as much of the varnish and corrosion as possible from every piece available. Spray carb cleaner through all orifices and make sure the flow of cleaner is not obstructed. Be sure to spray cleaner through both the main jets and pilot jets as these are the two most important pieces we are after when cleaning this style of carb. This is where I like to take a piece of fishing line and use it like dental floss, running it through the jets to be sure they are cleaned. This is also a good time to inspect or replace the fuel inlet needle and seat.
To reassemble, simply reverse the process. Replace any seals or gaskets as needed to keep the carb sealed properly. If you have multiple carbs simply repeat the process for the remaining carb(s).
With the carb reassembled and cleaned, your problems should be solved. In this process, making sure everything is clean is probably the most important thing that can be stressed. Good Luck with your carb rebuild, and if you have any questions or things you feel should be included in this tutorial, please feel free to let me know on the message board or via PM system on snowmobilefanatics.com