Drilling the slides.

[Steve]

I understand the basic operation of the CV carbs and all the various circuits in them.
When a Dynojet kit is installed (and I understand why), it is recommended to drill out the holes in the slides.

My little pea-pickin' mind tells me that intake vacuum pulls the slide up, and the hole in the slide is a bleed hole so the slide is not lifted as quickly or as much, with a given level of vacuum. In my mind, drilling a larger hole would increase the bleed rate and require more vacuum to lift the slide, delaying its action.

Can someone please confirm/deny this or otherwise enlighten me? Thanks. 8-[


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[salty_monk]

I believe it speeds it's action in both directions but I'm not 100%....

Dan

 

[trippivot]

I asked a tech fellow at Dyno-Jet on the tech line about this very thing last summer to quell a bet.

See me and the guys at the shop were bickering about getting the GSXR to throttle right, so I phoned dyno right up, to proove my piont.


"Does the larger hole keep the slide from lifting as quickly?" I asked him on speaker phone.

His reply was, NO. By drilling a larger hole in the slide it actually makes it lift quicker with less vacuum to assist the transition onto the needle jet earlier in the rev range.

A lot of those kits have stiffer slide springs included too.

I take great pride in getting the jetting perfect. I like carbs and carb tunning.

 

[Nicholaschase29]

So if you wanted to delay the slides from lifting as quick, you could either stretch the springs out a little or fill the holes with epoxy and then drill a smaller hole correct? Is there any special type of epoxy that would be best for this? or jb weld?

Nick

 

[trippivot]

So if you wanted to delay the slides from lifting as quick, you could either stretch the springs out a little or fill the holes with epoxy and then drill a smaller hole correct? Is there any special type of epoxy that would be best for this? or jb weld?

Nick

in theory that is true. aluma bond 2 part epoxy.

you are not doing yourself a favor if this is your plan.

are the slides already drilled? there are much better ways to tune in CV carbs

 

[Nicholaschase29]

there are much better ways to tune in CV carbs

Could you enlighten me please? what, in your opinion would be a better solution?

 

[trippivot]

I need more info on what the machine is or is not doing and when the problem exists.

idle ,just off idle ,midrange , high range

0-1/4 throttle, 1/4-1/2 throttle, 1/2-3/4 throttle, 3/4- WOT

current jetting, float height, mixture settings , actual engine modifications (no last owner stories) , state of tune, compression, state of electrical system


air leaks? rust in the tank? plugged fuel tap?

 

[salty_monk]

Mine are drilled & I'm going to JB Weld & put them back to stock. JB weld worked for Chef1366

Dan

 

[trippivot]

Mine are drilled & I'm going to JB Weld & put them back to stock. JB weld worked for Chef1366

Dan

if the damage is done why not put in bigger pilot jets, lift the needle a .5mm? or both?

what do the plugs look like? black brown grey white?

 

[salty_monk]

The idiot who drilled the slides did the above actions along with oversize mains, result is not good!

Mine are 4mm - that's oversized even for a dyno jet stage 3 kit! Without a 4:1 & pods there doesn't seem to be any point in doing this.

I first came down to stock mains - better, then dropped the needles, probably slightly worse if anything but it runs good when it's cold.

My next move is to re-drill the slides to stock & try again, it's the last thing as everything else is stock.

In answer to how else you could tune it for Nicholas, everything I've read tells me tweaking the float level would be a better way to make your bike richer but would affect all throttle positions to a point.... lifting the needles would make the mid part of the throttle richer too (more fuel flowing for a given vacuum if you like...)

Plenty here can give a better diagnosis than me but you need to give out all the info as requested above

In old race bikes it seems they also shaped the bottom of the slides as well as altered the vacuum hole size & other mods but it was largely experience & trial & error, I don't think you want to get into that... have plenty of slides ready if you do!!

 

[Nicholaschase29]

I first came down to stock mains - better, then dropped the needles, probably slightly worse if anything but it runs good when it's cold.

If you dropped the needles it would make the bike leaner. A leaner bike runs worse when it is cold, compared to bike that is running rich.

I would say that if it runs good when it is cold with the setup you have, the mains are too large and to compensate you lowered the needles which helped midrange. This seems unusual to me because the jets would probably be a little lean stock for emissions and to claim a higher mpg.

Unless you mean the bike runs well now over all even when it's cold... then nvm

 

[Steve]

By drilling a larger hole in the slide it actually makes it lift quicker with less vacuum to assist the transition onto the needle jet earlier in the rev range.

Evidently I have a wrong concept of how the slides are lifted. I was under the impression that vacuum lifted the slides. If you enlarge the hole, how does that lift them any quicker? :-s

 

[salty_monk]

Nah, better when it's cold... I am almost convinced the drilled slides are at the root of my problem, I have *stock* airflow but as soon as I get on the gas it lifts the needles way too quick & it goes rich...

That's why it runs great on the pilots.

The reason you drill them with pods is to lift the needle quicker as proportionally you get more air through the carb for each butterfly position I think.

Dan

 

[Guest]

The larger hole allows more air to enter the diaphragm and lift the slide quicker.

 

[Steve]

OK, I think I have it figured out now. 8-[ My rememberer wasn't working quite right (again). ](*,)

Somehow I remembered a vacuum passage coming around the diaphragm to the top of the slide which then pulled the slide up. I just looked at a spare set of carbs and realized that such a passage does not exist. The vacuum that pulls the slide up does, indeed, come through the hole in the bottom of the slide, so it makes sense that a larger hole would lead to quicker operation.

Thanks, Chef, for steering me in that direction. \\/


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[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. :-s


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[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?

 

[Guest]

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.


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