YICS, Carb Swaps, and Vacuum Advance

[crazy steve]

Here's an interesting thought for those who try carb swaps....

One issue with swapping carbs to non-XS units is the usual lack of a place to hook up the vacuum advance on a carb, requiring some finicky machine work to install a hose barb. You can run without the vacuum, but will take a pretty big hit on fuel economy if you do.

So here's an idea... how much vacuum is available at the YICS port on the cylinder? Would this be an alternative place to pick up vacuum to operate the advance? It would be pretty easy to drill and install a hose barb into that passage, and might even give you a 'smoother' vacuum source rather than the single-carb source usually used....

[BA80]

The vacuum port has to be ported Steve. The vacuum is supplied as the throttle plate opens. There is no, or very little, vacuum at idle to the advance pot.The YICS is constant manifold vacuum.

[crazy steve]

Yes, I understand that you don't want much if any vacuum at idle, but the YICS isn't as directly ported into the intake as the stock set-up is. Someone would have to check to see just how much vacuum is there at idle, as well as how much you got under various loads. I'm not advocating that this will work, just that it might...

This is just a 'think piece', maybe somebody will look into it and see if it's feasible....

[BA80]

The YICS ports connect all the intake ports in order to make the vacuum constant between the cylinders. Like a single plane intake on an old 4 or 6 cylinder carburated engine. It makes for a smoother idle and more even acceleration but it makes it a bitch to tune multiple carbs.It will be constant manifold vacuum.

If you were to hook all the vac nipples on the boots together you would have the same thing. Steady manifold vacuum.

[TopCatGr58]

I'm a bit confused.

Steve,

I thought the problem was putting NON XS carbs onto XS bikes, but your statement didn't say it that way. Okay, I just reread it, it's a little ambiguous to me...but I can now see that you meant changing the XS carbs "FOR" non-xs type carbs, but still on the XS...it just sounded different when I first read it!

But like Greg said, the vac. adv. pot needs the controlled and metered vac from the carbs, that it's essentially non-existent at idle, and then builds some with rpm, but also is more with very little load engine load, but actually drops when throttle is opened fairly wider to actually allow for retarding/releasing the vac. adv. so that it can come back down from the max 52 deg. btdc cruising lean rpm low load level to the ~30 degrees more power timing setting for richer A/F mixtures and heavy throttle load.

The intake boot ports, or yics ports can't provide that type of metering affect, and would provide too much vac. adv overall. Even with linking the UNUSED synch ports together on a non-yics engine, even hooking them up to a plenum type chamber and a smaller metered orifice, it would still be providing a constant vacuum, and actually more or increasing with increasing rpms.

Now after saying/typing that, I was thinking about the synch ports and how they behave during actual vac. synching, how the vac. level raises or lowers as we adjust the synch screws on the carbs which is just opening or closing the butterflies a little. SO...as the throttle is opened, the overall vacuum level should drop because there's more air being allowed thru the carb throats, and as the throttles are cut back like in cruising mode, then the overall vac. levels should rise because the carbs are not letting in as much flow when the butterflies are almost closed.

SO...perhaps the synch ports could be used, but like suggested, linked together and connected to a smaller plenum type chamber, and with a metering device(pilot jet) in line. We know how much vac. the vac. adv is supposed to have for full adv. mode, it might be a matter of having the plenum rig set up, and then a vac. gauge, measure the vac. levels first from an OEM carb set at varying rpms and engine loads..could get tricky trying to do this while actually riding the bike....will need a pillion helper! Then measure the vac. from the ports/plenum device and see IF similar vac. levels can be obtained at same rpm/load levels with perhaps different sized pilot jets needed to reduce or increase the vac. signal dependent on what is actually seen during use!

Okay, Steve, you've got the experiment layout, go for it!!

T.C.

[BA80]

You can't control the vacuum betwwen the butterflies and the engine. There HAS to be a valve operated by the throttle to control the vacuum going to the pot. That is what the oraface at the #2 carb butterfly does. There isn't vacuum to the advance port until the edge of the butterfy moves past that oraface and exposes it to the manifold vacuum and then it is controlled by how far open the butterfly is until it moves completely past it.

[TADracer]

From my point of view.....

My thought is that the sync ports or YICS ports (though I know little of the YICS system) are not suited to use on a vacuum advance mechanism. You have to deal with two parts of the vacuum curve so what you want is zero vacuum at idle AND on deceleration. These ports will always be at high vacuum at idle and thus will advance the spark to over twenty degrees when it should be at the static setting of 5-10 degrees depending on if the bike is a standard or special model. On deceleration, the vacuum should immediately drop to zero to retard the spark to the point of the centrifugual mechanism and that will never happen on a direct vacuum source. About the only way to get the correct vacuum curve is through a ported tap slightly behind the butterfly where the butterfly acts like an on-off vacuum switch when at idle and on deceleration.