2.16. Carburetor

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See also: 5.03. Carburetors


2.16.00. Carb removal

2.16. Carburetors

Disconnect the gas line, vacuum hose and throttle cable; remove the gas tank, air boxes/filters and side covers; loosened the carb clamps. Also remove the cam chain adjustment cover nut. Stand on the left side of your m/c facing back;

Grab each carb on the intake side and pull up and then quickly push down, repeat if this doesn’t free them from the boots;

Once free from the boots, push the carbs against the center frame and let them slowly fall intake side tilted down. (see images 1 & 2)

Image 1


Image 2


As they come down, you want to start to pivot the carbs around the center frame so the left side carb is moving back. As you do this, the right side carb’s intake throat on the engine side must be clear of the right side carb boot and lifted over the camchain adjuster. (see image 3)

Image 3


After the right carb clears the camchain adjuster, the right side carb’s intake throat on the engine side must be fitted between the camchain adjuster and the intake boot for the left carb so the carb goes below the boot and out. (see images 4 & 5)

Image 4


Image 5


The carbs are out (see image 6);

Image 6


Installation is just the reverse. I usually put some liquid soap on the boots and on the carbs so they slide into the boots.

This is for an 80’s XS650, and should work for earlier models but will have less clearance,.

John In WL, CT

2.16.01. Carb cleaning

This Carb cleaning procedure is taken from: http://home.earthlink.net/~sidskids/carbs/spraycarb.htm****THIS LINK APPEARS TO BE BROKEN****


Crud can migrate in the passages kind of like arteries and heart attacks (sorry for that analogy). I wouldn't bother disassembling the whole set of carbs for a soak in carb solution unless absolutely necessary. Instead, I would clean them without full disassembly as follows:

You need:

about three or four cans of carb spray cleaner.

shop vac with a hose that fits the carb inlet.

rubber gloves

eye glasses

glass jar

caliper for float height check

care so you do not mess up the floats (or just remove them)

Bob's edit - Work on a tray or some other form of table top with a lip around the edge. This is so that when (note, when not if) you drop something small it stays on the tray or table. These little pieces easily bounce into dark corners and get lost. - Bob's edit end.

Step-By-Step Procedure:

1. Remove the bank 'O carbs.

2. Remove the tops and the diaphragm assemblies with springs and keep them in order.

3. Remove the float bowls.

4. Remove the main jets and toss them into a small glass jar.

5. Push the nozzles (the brass emulsion tubes that the mains screw into) upward into the bores that the diaphragm cans were resting in, and pull them out. Do not pull them out with needle nose pliers as shown in the Clymers manual, or you will mar them and the main needles will not slide in the nozzles smoothly! Toss the nozzles into the glass jar.

6. Remove the screwed-on caps that cover the pilot jet if your model vintage is a '78 or '79. If your model is '80 or '81, they will not have caps.

7. Remove the pilot jets with a good-fitting screwdriver. I had to grind the sides of a screwdriver to get it to fit properly. Tap the handle a few times with a hammer to help break them free. If your mechanic did these already, they will be bright and clean-looking and will not be stripped or buggered up. Toss them into the glass jar. If your jets are suspect, get new ones. They are cheaper from Honda than Yamaha. but you must take one in for the style match-up.

8. Put on some rubber gloves and glasses to protect your eyes.

9. Spray out the holes in the carb bowls with the straws on the cans of carb cleaner. This is where eye glasses might save you!

10. Spray enough liquid cleaner into the glass jar to submerge the brass parts and let them soak.

11. Remove the pilot screws (aka air/fuel mixture screws), springs, washers and orings (80 and 81's). Note that 80 and 81's have covers over the pilot screws, which are recessed, when in factory configuration. You must remove the covers to properly clean the carbs.

12. With the carb body upside down, spray into the starter jet (aka choke) tube. It is that one-inch long brass straw that fits into the hole in the float bowl. While doing this, open and close the choke to flush it out. Don't worry too much about the pinhole in the base of that brass tube at the base of the carb body, it is supposed to be there to entrain air into the fuel.

13. With the carb body right side up, spray down the pilot screw bore and it should spray down the pinhole beneath it into the ventouri bore by the throttle plate.

14. Cover that pinhole with your finger, open the throttle plate to expose the other three pinholes, and force carb spray into the idle circuit passage. It should at least drip out of the three pinholes, and will run out of the opening for the pilot jet and if you have a 78 or 79 model, it may also run out of the main jet bore.

15. Spray into those three pinholes from the inside of the ventouri bore. You will have to hold the throttle plate open to do this.

16. Now turn the carb body so that the bottom of the body is toward you (for reference) and the inlet of the ventouri bore is up (this is the side with the holes around the perimeter). Spray the hole at 10 o'clock. This is a hole that vents the float bowl to equalize pressure. Cleaner liquid will run out of the hole on the lower-left corner (relative to the position you are holding it) of the float bowl gasket seat of the carb body.

17. Remove the air pilot jet and put it in the glass jar. Spray the perimeter hole at 8 o'clock. This is bore for the air pilot jet. Carb cleaner will run freely out of the pilot jet hole, next to the main jet hole, and some may run out of the pinholes and pilot screw bore.

18. Spray the pressed-in main air jet in the perimeter hole at 4 o'clock. This bugger is difficult to get clean. Fluid should run freely out of the main jet bore, but on '78 and '79 models, it may also run out of the pilot jet bore because of the interconnecting drill hole. Note that on those models, the main jet size has a direct effect on operation of the idle circuit because of that drill hole (near the main jet location).

19. Only '80 and '81 models have perimeter holes at 2 o'clock. This hole serves to vent the float bowl area (yes, another one). The '78 and '79 models instead had hoses to ran up to the air box to serve the same function. I disconnect and spray out the inside of the air hoses on those models.

20. Note that while spraying the air pilot jet bore (step 18), you may have forced particles back into the pilot circuit. Repeat steps 13, 14, and 15.

21a. If you removed the floats and fuel valves, clean the valve needles (I wouldn't soak the ones that have the rubber tops used in the later models and provided in some rebuild kits). Make sure the spring action of the needles works as it should. Clean the valve needle seat area with a q-tip wetted with cleaner. Do not soak the valve assembly in cleaner because the screen retainer will weaken and may fall apart. If you have the later model carbs, replace the O-rings of the seats of the valves.

DRYING: Now, use the shop vac to dry the passages in the carb body. This is important to suck out any loosened particles and to suck out any liquid that could be ladened with gasoline tars. If you do not do this, the tars may "puddle" and re-solidify in troublesome places. Note that the carb body has little brass balls pressed in at various places. They are there because Mikuni interconnects straight holes to make angular turns in the passageways of the carb body. The unused openings to the outsides are sealed off with the pressed-in balls. The problem is that the portions of the lengths that are dead-ended are crud accumulators. Vacuum-dry the carb before it has a chance to do this. If you have the earlier models, you can replace the fiber washers but you should take care in finding some that are the correct thickness, or your float height will be way off.

22. If you removed the floats (recommended), place the vacuum hose over all of the fuel inlet openings on the bottom of the carb (pilot jet opening, main jet opening, brass straw, fuel valve opening, and work the choke back and forth. Give it a good amount of drying time.

23. Hold the shop vac’s hose to the inlet of the ventouri bore (the side with the perimeter holes) with your right hand. Hold your palm against the outlet of the carb. While sealing top of the carb against your belly (because you are now out of spare hands!), work the throttle plate back and forth to open all of the pin holes. Hopefully, any particles that are left will be sucked out as it dries. Give it a good amount of drying time.

24. Clean up the nozzle tubes, that have been soaking, with a wooden tooth pick.

25. Hold each pilot jet carefully with forceps and spray into the threaded end. The carb spray should be about equal for each hole in the side and should stream out the other end. If they are questionable, buy new ones.

26. Reinstall all of the brass into the body.

27. Adjust the float heights with the gaskets removed from the carb body. I use a caliper that has a slider in the handle for this measurement and measure the height (carb upside down) from the gasket mating surface (no gasket) to the top-most part of the float. Do each side and use the average. Adjust if necessary. I have never used the manometer method described in Clymers for the 1981 models.

28. Reassemble all other parts, except for the bowls, and prior to putting the diaphragms back in, inspect them up to a light for holes. Repair holes with RTV Ultra Black, rubber dip, or similar. Never clean them with carb cleaner. Wipe off the cans with cleaner on a rag. You might want to clean the main jet needles at the bases if necessary.

29. Test the float valves for leakage by blowing into the fuel inlets with the bowls off and the carbs upside-down. There should be no leakage. I am informed that, while blowing, as you tip the carb bank, the fuel valves should release all at the same angle (if they are balanced).

30. Replace the bowls onto the respective carb bodies. Turn them upside down again and blow into the fuel hoses to test for leakage. The reason for the retest, is that floats do not have much clearance in the bowls, and if they were bent, they may stick against the sides.

31. Install on to the bike with a clean air filter and air box, inline fuel filters, and non-rusty gas tank.

2.16.02 Carb Pilot Screw Turns Out

Model Turns Out
XS1 & XS1B 1/2 turns
XS2,TX650, TX650A, TX650B 3/4 turns
XS650C, XS650D 1 1/2 turns
XS650E, XS650F 2 1/4 turns
XS650G, XS650H Factory Set at 2 1/4

2.16.03 Carb Float Level Settings

Model Float Level Inches Millimeters
XS1 & XS1B 0.94 24.0
XS2,TX650, TX650A, TX650B 0.94 24.0
XS650C, XS650D 0.98 25.0
XS650E, XS650F 0.94 24.0
XS650G, XS650H 0.98 25.0

2.16.04. Carb Balance Technique

Farrell Hope

Farrell's articles


It is quite easy to balance the carbs on a '81 XS650 without any special balancer. However it is not done by checking whether the butterflies are moving the same, but by checking whether the cylinders are running the same. Because of differences between various things regarding the individual cylinders, this may actually only happen when the butterflies are not in the same position exactly.

An XS650 will tune very close by simply feeling the output of the exhausts, place your hand about 1/2" from the exhaust, and the pressure and sound must feel the same. Obviously if you have a two into one exhaust this can't be done.

To begin just start the bike, let it warm up, and adjust the idle up to about 1400 RPM using the normal idle adjustment screw. Then pull the plug lead from the RIGHT side. (It is actually better to ground the lead than pull the plug, because pulling the plug can cause damage to some electronic ignition system, but it wont on an XS650. It can also cause carbon tracking on the coil, but again this has never happened to me in years of doing this on these bikes, so I would just do it by pulling the plug lead. In later years learn how to make a little grounding rig and ground the lead. But for now just pull the lead off the plug.)

Adjust the idle to about 700 rpm while running on the LEFT side only, using the normal idle adjustment screw. Then reconnect the plug lead, and pull the LEFT hand plug. Set the idle to the same 700 rpm, but this time use the screw in the linkage between the carburetors. Then reconnect the plug lead, the bike will idle somewhere around 1200 to 1400 rpm. Adjust the idle to your desired level using the normal adjustment screw (should be around 1200 rpm). Fine tune the balance using the screw in the linkage between the carbs, and feeling the exhaust outlet as described above. This will only adjust the running of the right hand cylinder, move it to run either faster or slower, until the sound and feel with your hand covering the exhaust feels the same for both cylinders. Then finally readjust the idle using the normal idle adjustment screw.



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