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-   -   Vacuum advance with ZRX Carbs. (http://www.xs11.com/forum/showthread.php?t=36055)

TADracer 11-16-2010 08:08 AM

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.

TADracer 12-02-2010 07:31 AM

Did a bit of tweaking
It is getting cold here around New Orleans. I know, cold is relative and the 40's we have been having here might seem balmy to some of you guys up North. But I ride year round so I use a two spark plug system to help keep my motor temp where it likes it.

I decided to switch to my hotter plugs and while checking things out I noticed that when the throttle is cracked open the timing plate bounced back and forth a bit before it started a smooth advance. I put on my timing light and in that 'shake zone' the timing was all over the place. I took a ride with the cover off and it did the same thing under load. I did not notice this before so I figured that sone adjustment should be done here.

I took an old pilot jet (one of the funky ones from a K&L carb kit that dont work) and then put two layers of shrink tube around the body of it to make it want to fit tight inside the vacuum tube from the carbs and to close off the holes in the side of the jet leaving only the small hole in the end open. Then I shoved it about an inch into the tube at the dashpot end and gave it a try.

With the jet restricting the vacuum pulses in the line the advance now works smoothly throughout the range and elimated the 'shake zone' on the timing. I don't really notice any difference in rideability when just off idle when the bouncing was happening but I figure that concerning timing, smooth is better than rough.

The more I ride with these carbs the more love them and I would recommend them for someone looking for an easy and reliable upgrade from the 30 year old OEM carbs.

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