Vacuum advance hole on ZRX Carbs

[motobrix]

So I started the bike up and looked at the advance. It was pulsing way too much, so I added a restrictor in line, and now it seems to be working better. It still pulses a bit at idle, but not very much. Is that normal? It doesn't advance at idle, just pulses around zero.

The bike isn't running perfectly, but I think that's more because of the jetting than the advance. I'm at 135 mains, and 35 pilots, with 2.5 turns out on the mixture screws. The mains seem decent, with lots of power on full throttle, but the revs take a while to come back down after releasing the throttle.

Also, anyone shim the needles on these carbs? Or is the needle height best left alone?

[MaximPhil]

Hi Motobrix,
I wonder if you have the vacuum advance connected to the correct carb nipple ? Carb Guru please advise.
Phil

[TADracer]

I'll chime in since I did it.

Back to the hole topic, I would just plug the one you did with JB Weld and try it on another carb as it doesn't matter which one you tap. I admit drilling the hole with a drill motor is a hard thing to accomplish because of the bulk of the tool and the small size of the target hole. Perhaps it would be easier (but a lot longer job) putting your drill in a pin vice and making the hole manually. Drilling would be very easy to control then but it would take a long time to drill.

The hole must be very small and positioned exactly in the right spot in relationship to the butterfly (look at your stock Mikuni #2 carb for a reference). When the butterfly is closed there should be no part of the hole exposed at idle and therefore no vacuum is drawn. When the butterfly is cracked open slightly (like in cruising) vacuum from the airstream before the butterfly is drawn to advance the timing plate and when the throttle is opened (like in acceleration) vacuum drops off to retard the timing plate.

The main thing I notice about all this is if you do not have your vacuum mechanism working (like you just did not do it al all) your overall mileage will drop off a bit at a steady cruise and that's about it.