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Old 11-16-2010, 08:08 AM
TADracer TADracer is offline
XS-XJ Guru
Join Date: May 2006
Location: Kenner, Louisiana USA
Posts: 1,415
Vacuum advance with ZRX Carbs.

For those of us trying a carb upgrade to a rack of ZRX 36mm Keihins, the problem of correctly operating the vacuum advance rears its ugly head for the earlier machines. Here is what I did to solve the problem. It was fairly easy but it requires a steady hand and a bit of patience. Best part is if you screw one up, plug the hole and you have three more tries. I made it on the first try.

If you look at the bottom of the carb there is a cast-in port ready made for a vacuum tap. I used a digital caliper to take several measurements and determined that a small hole strategically drilled in the bottom of this well will exit right on the rear edge the butterfly when it is seated. The hole needs to be very small and drilled at an angle towards the rear of the well.

In this picture the hole is already drilled but the wire indicates the angle the hole miust be driled to exit in the right location. I wedged the butterflies open with a scrap of soft wood and placed a light in front of the carb spigot so I could see when the point of the drill started to break through. I used a 1/16" drill in my best drill motor that provided the slowest trigger speed. I would drill in a bit with very light pressure and then check for light in the bottom of the hole. As soon as I saw a glimpse of light I stopped drilling. Then I took a 1 mm airbrush cleaning wire and worked it through the hole to open it up a bit.

Here is a view of the hole location after drilling.

Here is another view

Then I took a common 1/4" to 3/16" hose barb fitting available at any auto store and the 1/4" side is a tight fit into the well. The original vacuum hose attaches easily to the 3/16" side. The barb is easily removed to replace if it should break. A 90 degree fitting would allow a bit more room under the carbs for the hose but this one is what I had at the moment. Also, it does not interfere with reaching in for an idle mixture screw adjustment.

Here is the finished product. I installed the carbs and the vacuum advance operates exactly like it is supposed to in that there is no advance at idle, a ramp up in advance as the butterflies are cracked, such as when cruising and then no advance when the butterflies are opened wider like under hard acceleration.

Have not had a chance to road test it much other than a trip or two around the block but it seems to be working as intended.

Thats a wrap.
Mike Giroir
79 XS-1100 Special

Once you un-can a can of worms, the only way to re-can them is with a bigger can.