12.10. Hidden air screws

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Ken French's Hidden jets fix:

Hidden jets. I 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.

I’ve 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.

Mikuni Air Screw 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” (3 mm) 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” (3 mm) 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 healthy dose of compressed air. I have a nice air chuck with pointed rubber tip. This is great for directing air into small openings. I’ve 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 carbureto, 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 believing everything I read in Clymer's.

That’s it any questions email me.



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