Installation instructions for Honda XR650L
Mikuni TM40 Conversion

NOTE: This manual is provided as technical reference tool to assist experienced mechanics in the installation of the Niche Cycle, Honda XR650L, Mikuni TM40 carburetor conversion kit. This manual in no way replaces the experience and skill necessary to safely and property install a motorcycle carburetor. It is assumed these kits will only be installed by professionally trained motorcycle mechanics. Niche Cycle, The Vintage Bike Builder and all associated parties accept no liability for any damage, or injury resulting from the incorrect installation or maintenance of any carburetor. Please use this installation manual at your own risk.
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Introduction
 
The Mikuni TM40 kit is a very straight forward carburetor upgrade for the Honda XR650L. With these instructions, anyone with basic mechanical skills and a few tools can tackle the job. The TM40 features a built in fuel pump which provides unparalleled throttle response and eliminates the need for the troublesome, constant velocity Keihin carburetor, which is stock on the XR650L. Like all Mikuni’s, quick starting and easy maintenance make the TM40 an easy choice for this carburetor conversion.
 
Fitment
 This kit fits all 1993-2019 XL650L models. Over the years, owners have modified these bikes with performance exhausts, custom tanks, seats, handlebars and controls.  Our test bike for this conversion is a 1994 model.  It has some non-stock components like a large capacity tank as well as a high performance exhaust.  Other than that, the bike is basically stock.

PictureA inexpensive socket set like this will set you back less than $20 at Harbor Freight and can be ordered online if there isn't a store near you
The Tools you’ll need
Make it easy on yourself and use the right tool for the job.  If we’re very specific about a tool, it’s because using anything else will eventually cost you time and money.  If you don’t have the right too, get it and save yourself a lot of aggravation.  If you try to use tools like an adjustable wrench or a vice grip, you’ll round-off the nut or bolt and have to replace it. With metric tools so cheap and available on Amazon, eBay or Harbor Freight, it’s just penny wise and pound foolish not to get the right tool.

One of the best tools you can use is a cell phone or digital tablet. Professional mechanics used to draw a lot of diagrams, but these days, they take lots of pictures. Some keep a spiral bound notebook handy for taking notes.  These notes and pictures save a lot time and frustration on projects – especially when you are disassembling something you are unfamiliar with.

If you own a motorcycle, you should have all of the tools listed below and more to maintain your bike.  Rather than list the individual sockets or wrenches, I have put them down as kits, because that’s how they are normally sold. You won’t need all the wrenches or sockets for this just, but it makes more sense to buy the kit than buying these tools individually.


Here is a list of tools and items you will need:
  • A  flat blade screwdriver set
  • A  Phillips-head screwdriver set
  • Metric Hex Key (Allen wrench) set
  • Metric combination wrench set (6mm-19mm)
  • Metric socket set (6mm-19mm)
  • Razor knife (for cutting hose)
  • Plastic prying tool  (see note below)
  • Needle nose pliers
Primary disassembly

  1. Using a number 2, flat blade, screwdriver, or your fingers, turn the cam locks on the right side cover to gain access to the air box and battery (IMAGE 1).
  2. Using an 8mm wrench or 10mm wrench (depending on your battery), loosen and remove the negative battery terminal.
  3. Push the rear part of the air filter retainer clip forward. Lift and unhook the front clip from the rear clip. This will separate the two halves of the air cleaner retainer and allow the air filter element to be removed (IMAGE 2).
  4. Remove the two 12mm bolts under the rear fender to remove the seat (IMAGE 3). Note: the black section of the mudguard looks like it is a part of the fender and not a part of the seat.  Once these bolts are removed, lift the seat up and reward to remove it.  To avoid confusion with other bolts, place the bolts you removed back into the frame, or place them in a baggie and label them.  They will be needed for reassembly.
  5. Using a pair of needle nose pliers, squeeze the fuel hose clip to release if from the hose and slide it down the hose several inches.  If your bike has another type of clip, such as a hose clamp, loosen it and move it a few inches down the fuel line.  With the clips removed, twist the fuel line left and right and then backward to remove it. 
  6. Using an 8mm wrench or socket,  remove the fuel tank retaining bolt at the base of the fuel tank.  With the bolt and cushioning grommet removed, lift the tank slightly and pull back.  Be sure to keep track of the rubber grommets and then can easily fall to the ground and bounce away.  Bag and label the grommet or replace on the frame after the tank has been removed.  Although they usually don’t fall off, be sure the keep track of the two back tank rubbers on the front of the frame.  Note: If you using an oversized, aftermarket tank (like the one on our demonstration bike), you may need to loosen screws in the front of the tank as well.
  7. Looking at the left side of the carburetor, you will see several small tubes connected to the carburetor (IMAGE 4). Some of these are breather tubes and others are drain lines. They can all be removed with the carburetor. On our demonstration bike, one of these lines had already been plugged with a screw. However, if any of the lines from the carburetor attach to the air box, disconnect them from the carb, trim the line close to the air box and plug it a suitable plug. One way of plugging the line is to insert a tight-fitting screw, bolt or a small wooden plug into the line. Make sure the larger breather tube running from the crankcase to the air box is intact and not blocked.  Note: "Uncorking" the XR650 involves removing the smog pump fitted on some years, rerouting and plugging crankcase vent lines, as well as creating some new breathing holes in the top of the air box. This is a fairly straight forward operation and greatly compliments the TM40 carb installation. Done correctly, you will see a significant difference in performance. We do not go into that process in this manual. It is suggested you google "uncorking the 650L" and compare several sources before attempting this modification.  
  8. Use a 14mm wrench to loosen and remove the cap keeping the choke cable in place on the carburetor (IMAGE 5). This cap is plastic. Take care not to break it as it will be reused during the installation process.
  9. When the cap is unthreaded, gently pull up the starter (choke) valve up to remove it from the carburetor.  The starter valve assembly will come out attached to the cable. The assembly includes the rubber cover, clip, retaining cap, spring and plunger.  The Honda choke cable and cap may be retained for this installation. Note: If  you would like to retain the handlebar operated choke, this can be done by simply swapping the choke plunger and spring from stock Keihin carb and replacing them with the plunger and spring from the Mikuni. Since this isn't necessary for the successful installation of the carb, it's probably a good idea to do the initial testing with the stock Mikuni choke in place. This will eliminate any variables which could cause problems. When you are ready to swap the choke components, just remove the choke (button or remote cable) from the Mikuni carb and attach the plunger and spring to the stock Honda cable in the opposite manner in which you disassembled it. Insert the plunger back into the orifice in which it was removed and secure the plunger and spring, using the plastic nut on the stock Honda cable. This is the same size as the choke plunger securing nut on the Mikuni carb. IMAGE 6 shows the Mikuni plunger and spring on the Honda cable. 
  10. To remove the plunger and spring from the cable, pull up the spring and slide the cable nipple out of the recess on the plunger.  Place these small parts in a baggie to keep from losing them. In the photo to the right, the arrow is pointing to the cable nipple in place in the recess on the plunger.
  11.  Use a 10mm wrench to loosen both the push and pull cables at the cable bracket attached to the carburetor (IMAGE 7).  With the nuts loosened, lift the cable out of the cable bracket on the carburetor (photo below right).
  12. With the cables out of the bracket, remove the end of the cables from the bellcrank on the carburetor (IMAGE 8), and lift the cables out of the way.
  13.  Using a #2 Phillips-head screw driver, loosen the hose clamps securing the air box boot and the rubber manifold adapter.
  14. Gently grasp the air box boot, pull it back and out to rest if on the frame (IMAGE 9).  This will allow you a little extra room to remove the carburetor. 
  15.  If you’re not removing planning on replacing the carb right away, put it back in place and then pull it back against the frame when you are ready to replace the carb. This keeps the boot from becoming permanently distorted. If you’re worried about scratching the frame on your bike, you can place masking tape over the frame member to protect it from the boot and clamp.
  16. Check to make sure the top of the master cylinder is screwed on tightly. Then, using a 10mm socket; remove the bolt securing the rear master cylinder to the frame (IMAGE 10).  Gently pull the master cylinder back behind the frame member and remove the hose from the clips (IMAGE 10).  Use a wire tie to secure the master cylinder out of the way. This will provide even more room for removing and replacing the carb.
  17. With the hose clamps loose, pull the carb back and away from the rubber manifold adapter. With it dislodged from the manifold adapter, turn the carb on its side and remove it from the bike (IMAGE 11).  Stuff a clean shop towel or rag in the manifold to protect the intake from getting debris in it while the carb is off (IMAGE 12).
  18. Pull the boot covering the throttle cables back where it enters the throttle housing.
  19. Remove the two Phillips screws in the throttle handle housing (IMAGE 13).  Lift the top part of the housing off. The bottom half of the throttle housing will remain with the cables still screwed in. Place the top and screws in a baggie or container to keep them from getting misplaced. 
  20. Lift the white, plastic throttle slider (guide) from the bottom of the housing and place it in a baggie with the screws and bottom half of the housing (#2 in IMAGE 15).
  21. With a 10mm wrench, loosen the lock nut on the cable housing and unscrew the throttle cable adjusters (#1 IMAGE 15). Remove the cables from the housing.
  22. Remove the round barrel cable ends from the twist throttle handle and set the handle aside (IMAGE 16).
  23. Spray the twist end of the throttle cables with silicone spray or WD40, and gently pull the double boot off the cables.  Store this with the other throttle components for later use in the assembly process.
  24. Release or remove any cable ties and remove the throttle cables from the bike.
This completes the disassembly process
During the disassembly process, keep the components you will reuse in a separate container. Drain any remaining fuel from the old carb. It is a good idea to remove the float bowl and blow out the old carb with compressed air. You may need this carb for some reason in the future and removing the old fuel will keep the carb from being mucked up with old, smelly fuel.
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IMAGE 1: Removing the side cover on the right side provides access to the battery and air filter
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IMAGE 2: This clip releases the air filter element. It is always a good idea to install a new air filter with a carb kit
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IMAGE 3: These nuts remove the seat. Be sure to loosen the correct set.
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IMAGE 4: Several vacuum and drains lines will be removed with the old carb.
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IMAGE 5: Loosen this plastic nut to remove the choke cable.
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IMAGE 6: The Mikuni plunger and spring on the stock Honda choke cable.
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IMAGE 7: Loosen securing nuts to remove throttle cables
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IMAGE 8: With nuts loosened , remove cables from brackets and bellcrank.
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IMAGE 9: After the air box boot clamp is loosened, the boot can be carefully pulled back and allowed to rest on the frame member.
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IMAGE 10: The reservoir for the rear brake master cylinder can be removed and temporarily put out of the way while the carb is removed and then installed.
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IMAGE 12: Rotate the carb and pull it out from the left side.
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IMAGE 13: It's always a good idea to place a clean cloth or shop towel in the intake after removing the carb. This keeps costly mistakes from happening.
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IMAGE 14
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IMAGE 16: Remove the barrel ends to free the twist throttle from the cables.
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IMAGE 15