Drilling the slides.

[KEITH KRAUSE]

You guys have it basically figured out. A larger hole increases vacuum and improves throttle response time. Sometimes different springs are needed too.
What's missing is the reason you need to enlarge the holes.
CV's depend on a certain amount of restriction from the stock airbox. Without this restriction, like if you take off the box just out of curiosity, the bike runs poorly. By running pods, you lose much of the restriction in this "closed system" and the result is less vacuum going through the passage and acting on the piston/diaphragm assembly. By enlarging the hole, you regain much of what was lost. That's how I understand it.

[Steve]

That's how I understand it.

And who do we think we are that can doubt Keith's word on carbs?

Looking at the situation just a little more, I'm not sure that there is any more vacuum that gets there to lift the slide, it can just get there faster through the larger hole.


.

[TheCafeKid]

Ok i have a question. With what little i know of physics, isnt the air actually drawn OUT of the slide by the vaccum pull of the motor, thereby causing a low pressure area inside the slide causing it to lift? Or is this completely absurd?

[chef1366]

There is an oval port at the top of the filter side of the carbs. If you suck on that port the slides lift by letting air in through the port that was drilled. The larger the hole the quicker the slide lifts.
Without an airbox and with pods there is less air pressure for the vacuum to draw the air through the oval port and the slide air port so you need to drill it to allow air to enter at a quicker pace to make up for the lack of pressure from the air box.
Also there are aftermarket slide springs that also speed up this action.
I don't have a degree in Physics so I hope I explained this OK.
I understand how it works but communicating it is a different animal.

[Steve]

On my carbs, that oval port is connected to the bottom side of the diaphragm. :shock:

The only way I can see vacuum getting to the top of the diaphragm is to go through the hole in the bottom of the slide. That is exposed to the faster-moving air in the venturi area of the carb, so it will have the lowest pressure. Most of the vacuum will be on the engine side of the throttle butterfly valves. About the only time the hole in the slides would see the same amount of vacuum would be during wide-open throttle, when there is no restriction caused by a partially-closed butterfly.

The vacuum seen by the slides is directly related to the amount of air passing through the carb, so it will lift accordingly. The amount of vacuum in the intake tubes (where you connect the gauges to sync the carbs) can change depending on engine speed at any given throttle opening.


.

[KEITH KRAUSE]

Jeez! You guys are getting too scientifical for me.
I'm going out to play with the ZX now. New hugger to install and a couple other goodies!

[chef1366]

On my carbs, that oval port is connected to the bottom side of the diaphragm. :shock:

The only way I can see vacuum getting to the top of the diaphragm is to go through the hole in the bottom of the slide. That is exposed to the faster-moving air in the venturi area of the carb, so it will have the lowest pressure. Most of the vacuum will be on the engine side of the throttle butterfly valves. About the only time the hole in the slides would see the same amount of vacuum would be during wide-open throttle, when there is no restriction caused by a partially-closed butterfly.

The vacuum seen by the slides is directly related to the amount of air passing through the carb, so it will lift accordingly. The amount of vacuum in the intake tubes (where you connect the gauges to sync the carbs) can change depending on engine speed at any given throttle opening.


.

So you think you need to drill the slides because there is less restriction of the air flow with the pods?
I see your theory on the faster moving air in the venturi finding it's way through the slide air port and to keep the air flowing with the oval air port at the rear of the carbs.

[Steve]

Actually, let me back up just a bit here. I had never thought of drilling the slides until I helped someone install a DynoJet kit, which suggested drilling the slides, but did not say why. I had never heard of drilling the slides, so it caught me by surprise.

Now, my (new-found) idea on why they need to be drilled at all:
Part of it might be due to less restriction at the air filter, but the larger hole in the slide will allow faster transfer of air pressure differences between the venturi area and the space above the diaphragm. A normal-sized hole will eventually equalize the pressures, but the larger hole will get the vacuum to the top of the diaphragm quicker, and will lift the slide sooner, making the mixture a bit richer, sooner.

Oooohh, too much thinking, and it's raining here, so I can't go for a ride. ](*,)


.

[salty_monk]

Yes, you are spot on. You're getting more air quicker so you also need more fuel quicker.

Dan

[darkbane]

Sorry if this question is too specific on the more broad range thread...



I'm running my '83 GS650g with K&N Pods, a wrapped 4 into 1 exhaust. My main jet is 135 at the moment, and I'm still not running 100%. The Previous Owner said that he rejetted the bike, but I don't think he did it correctly, so I'm trying to run through it myself. I don't believe he drilled the slides, but I'm not sure.

Couple Questions: regarding the Mikuni bs32ss carbs.

What would be the stock hole size for this carb?

Why does the slide guide holder cover the air hole?

If I were to redrill this hole, how much larger should the hole be?

Would it be easier to just drill a new hold on the other side of the slide instead of redrilling the existing hole (this would make it easier to go back to stock later, by filling the new hole instead of filling and redrilling to stock size later)?



Mikuni bs32ss
Mikuni bs32ss

[doctorgonzo]

Wow, this was in informative carb thread!
I have some parts on the way from Chef, I bet I get good effect from replacing my one slide spring that is "sprung" and the one that is mismatched and smaller. Not to mention the diaphragm that has a tear.

[KEITH KRAUSE]

Actually, let me back up just a bit here. I had never thought of drilling the slides until I helped someone install a DynoJet kit, which suggested drilling the slides, but did not say why. I had never heard of drilling the slides, so it caught me by surprise.

Now, my (new-found) idea on why they need to be drilled at all:
Part of it might be due to less restriction at the air filter, but the larger hole in the slide will allow faster transfer of air pressure differences between the venturi area and the space above the diaphragm. A normal-sized hole will eventually equalize the pressures, but the larger hole will get the vacuum to the top of the diaphragm quicker, and will lift the slide sooner, making the mixture a bit richer, sooner.

Oooohh, too much thinking, and it's raining here, so I can't go for a ride. ](*,)


.

Yes. The reason for drilling the orifice to the diaphragm chamber is to improve throttle resoponse that MAY be lost when you install free flow filters on CV type carbs. Depending on your particular set up/bike, you may find no significant loss of throttle response and won't benefit from increasing the orifice diameter. Much more often than not it does benefit the throttle response, unless you screw the job up which happens too.