Drilling Out Main Jets

[49er]

There is a spring under the needle that always holds upwards force on the needle. There is also a piece that goes on top of the needle that the needle gets pushed up against from the force of the spring underneath. If you add washers to the top of the needle the needle will be lowered (and the spring will be more compressed) because the needle hits the piece that goes on top of the needle at a lower position because of the extra washer on it. I hope that makes some sense..

Nick

Thanks for the clarification Nick.
On the manual slide carbs (VM's) there should be 2 nylon/plastic washers. From memory the thicker one, about 2mm thick ,sits between the clip and the slide. The other thinner one sits on top of the clip and is closest to the positioning/tensioning spring. If you remove or fit a thinner washer between the clip and the slide, the needle will be lowered deeper into the jet, causing a leaner condition.
There is some damping done by the washes as the top one causes a sideways/offcenter condition at the needle when the throttle/slides are fully open. This is designed to reduce wear on the needle and jet surfaces during normal operation.
Ian

[trippivot]

all I say is if you drill any jet . sand the original number off of it .

that may help a fellow mechanic after the bike is sold a time or two and the mod has been forgotten.

[madjack57754]

Why bother drilling. There are some good supplier very close to you. Just go to www.sudco.com or www.psep.biz or my favorite www.carbparts.com to get everything you need plus the technical support is invaluable.
I used to drill jets until I found these guys, now I have complete EBC jet selection boxes just like a dealer has. It's amazing how cheap these selection boxes are compared to what the dealers get for a single jet or needle.

[49er]

all I say is if you drill any jet . sand the original number off of it .

that may help a fellow mechanic after the bike is sold a time or two and the mod has been forgotten.

I agree and do just that.

As for buying an assortment of jets instead of drilling, there is a good case for that too, if you're tuning lots of different combinations on your machine, or are tuning many others.

A comprehensive set of drills allows you work on any carburettor. You can confirm that a 102 main jet does in fact have a 1.02mm oraface or any other size for that matter. Having confirmed that, you have a starting point for future tuning, especially if you have just bought a new to you GS.

I have drilled mains on Dellorto carbs only to find that I was now too rich. I then soldered the hole up and re-drilled to an intermediate size and tried again. Once the right jet size was found, I would buy an OEM set for competion. Delortto's have push on main and air correction jets that attach to each end of emulsion tubes.

For my last carb rebuild, I bought 4 GS 750 K&L kits. I was impressed with the apparent quality until I measured the replacement mains. They were all stamped with 102. Two were correct the other two were 0.98mm. If I had fitted these to a stock 750, I would have wondered why two pots weren't performing correctly, at mid to top end.