5.03. Carburetors

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See also: 2.16. Carburetor

See also: 12.19. The $1.55 Carb Syncronizer

See also: 1.03. Tools


Drain Float Bowls

If you've got a manual petcock turn it off. If it's vacuum it'll be turned off when the engine's stopped. Open the drain screws on the float bowls until they empty then close them. Open the petcock or turn the vacuum petcock to "prime" to refill the float bowls. The dirt should be dislodged by this proceedure. If it's not, repeat. If that doesn't do it then the float needle or seat is probably damaged and needs to be replaced. One of those drain screws is hard to get to since the other crb is in the way. Another way to do it is to close the petcock and run the engine until the float bowls are empty but don't do this if you suspect raw gas in the cylinder or crankcase. I knew a guy who bent a rod trying to kickstart a bike with a cylinder full of gas!



If you bike runs fine at idle and dies at poor throttle make sure that the plastic spacers are on top of the c-clip on the jet needle.

If the brass tubes that are used to connect the tubes between the two carbs leak use a 2 part epoxy such as JB weld to secure them in the carb body


Idle Mixture

'I wanted to pass this on, since it solved a problem I was having'

After starting my 80SG, I had to leave the choke on until the engine warmed up, otherwise it would die at idle. My idle mixture screw springs were crusty and the o-rings were flat. I bought a couple of new screw assemblies from Mike, plugged them in and now it idles smooth immediately after start, with the choke off.

Nick Maximowich, Ohio

O-Rings for idle mixture screw

Sometimes it is worthwhile checking Yamarobber. I just bought two of those idle mixture o-rings for $3 each (for 34mm CV carbs)without the mixture screw.


Mixture Adjustment

Farrell Hope

Farrell's articles


The mixture adjustment screws are preset at the factory and permanently capped. They are not set for optimal performance, but set to run slightly lean to ensure the emission requirements are met. They only affect the bike significantly during idle. They can be adjusted by removing the gas tank to get access, drilling the caps and prising them off, and then adjusting with a little screw driver. The screws are in the little vertical towers dead center on the carb just ahead of where the carbs go into the rubber boot of the engine side manifold. The caps do not need to be replaced. When drilling the caps be very careful not to let your drill plunge through deep enough to destroy the head of the soft adjustment screw, this is a common error. On top of this the screws are often seized, and attempt to adjust them destroys the screw head. But if you insist, here is how to adjust them.

Set as follows, for each cylinder. When the bike is running on one cylinder only, back it off on the idle adjustment screw to where it is running as slow as it will run reliably. The turn out the air/mixture screw out (anti-clockwise) till the motor starts to rich stumble (run badly and shake). Then turn it in (clockwise) counting the turns until the motor starts to lean roll (run badly and shake). You then have two choices:

(1) Turn the screw back half the number of turns counted so you are half way between rich and lean.

(2) Turn the screw back one quarter anti-clockwise turn from lean roll. This is my preferred method. Turn the cylinder up to about 700 RPM, and repeat above adjustment to the air/mix screw.



Carb Sync adapter barb

For those bikes that do not have the vacuum barb on the carb mount & for those that want to use the actual vacuum prot that do, I came up with a way to make your own "barb" to screw into the port on the carb.

I took a 1/4" steel rod, drilled a hole thru axially, tapered the end a little on the grinder & then used a 6x1 mm die to cut the threads for about 20 mm. Cut it with a hack saw a about 25mm behind the last thread. Do this twice & you have one for each carb.

You can then use a vacuum gauge to sync the carbs for a smoother running bike.



Where are those hidden air jets?

Hidden jets, Wish I had a nickel for every time I heard someone say: "My 650 runs like a clock once I found and cleaned those hidden jets ”.

I discovered this little known secret two years ago when I bought my first XS. No matter what I tried I could not get her to run well.

Image:jet_1.jpg Image:jet_4.jpg

I have been riding since the 70’s and know my way around motorcycles, especially the Mikuni carburetor. I have to admit I was a little surprised when I tried to adjust the air jets on my SJ and there were no air jets to be found. After asking the guys on the 650 bulletin board I was told “Oh those are hidden” WHAT. Who was the egghead who came up with that concept? Uncle Don, God bless him, told me how to get to those little buggers out and clean them up. I am now just passing on his words of wisdom.

Image:jet_2.jpg Image:jet_3.jpg

Mikuni Air Jet removal and cleaning

1. I used a drill press for this operation. You have to be very careful when drilling or you could ruin the upper portion of the air jet.

2. Using a 7/64” drill bit carefully drill through the center of the lead plug (see illustration). If you do this very slowly you will feel when the bit cuts through the lead plug and drops into the air gap. At this point STOP.

3. Using a #2 Ez-out remove the lead plug. Make sure your Ez-out just fits the 7/64” hole you drilled. You don’t want to damage the air jet. If you do this correctly the lead plug will spin out and the Ez-out will not come in contact with the top of the air jet.

4. Once the lead plug is out turn the air jet clockwise until it seats. Take count of the number of turns. If you are like me you will want to write it down in your shop manual. Then remove the air jet. On the air jet is a very very small washer and o-ring. If you loose these I don’t know where you will find a replacement.

5. If the o-ring and or washer stay inside the carburetor use a paper clip to remove them. Fashion a small hook on the end of the paper clip to assist in the removal.

6. Then shoot the air jet orifice with carburetor cleaner and a health dose of compressed air. I have a nice air chuck with pointed rubber tip. This is great for directing air into small openings. I have been spoiled and will not work on a carburetor without it.

7. Another word of caution: Carburetor cleaner makes rubber parts swell and can cause them to crack. If there is a chance that some of the carburetor cleaner could get on other rubber parts in the carburetor then you might want to disassemble the entire carburetor.

8.This was submitted by mrgizmo. For those who haven't yet found the hidden air screw. A word of caution please remove the float bowl before directing compressed air into any carburetor. Most newer 650's have plastic floats that are pretty durable, but some newer bikes and most older ones have brass floats. Directing compressed air into the carburetor can raise the pressure in the float bowl enough to collapse the float, which then becomes a sink

I have recently been told that what I have been calling an "air jet" is in actuality a "pilot screw", hmmm go figure. That's what I get for beleiving everything I read in Clymer's.

That’s it any questions PM Kibokojoe.


Ken Kibokojoe


Carb Removal

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.


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