Slide lift hole confusion

I have a 81 kz750H I have been tuning and slowly lighteneing for a cafe/rat bike theme, I have a dynojet kit. The vacuum slides I have, have two lift holes, The diagram for the dynojet kit shows only one. In the directions, Im told to drill it out. Should I drill out both the holes? are these the stock slides? will drilling them out affect driveability? I will post a picture soon enough.

Yeah, post a photo before drilling. Someone else here will probably have better specific info.

There's usually only one hole for vacuum, and it's usually off center. (Naturally, there is a hole in the center for the needle.)

I haven't seen one with two vacuum holes, but they certainly could exist.

If you add the cross sectional area of the two holes, do they add up to the cross sectional area of the drill bit?

If they are anything like my CV carbs there are 2 holes. The one in the center is the hole that the jet needle goes through...Do Not drill center hole, the hole that is offcenter is the one that gets drilled...If it needs to. I did drill out the slide hole but now am not sure if I really needed to.

There is a small hole on the bottom of CV carbs to allow the slide to drop after being raised. The purpose of "drilling the slide" using the supplied 1/8" drill bit is to allow the slide to drop quicker. I have personally experimented with doing this modification and found it to be of no value as there was no appreciable difference in "throttle response. It makes more difference installing fresh return springs. There is only one return hole on the BOTTOM of most any constant velocity type carburetor. It will be offset from the middle. If you have two holes and are not sure which should be drilled, I recommend you drill neither as it is a waste of your time but if you really want to get your money's worth from the jet kit, take a pic of a slide and post it and drill away.

wiredgeorge wrote:

There is a small hole on the bottom of CV carbs...

The hole we're talking about is on the bottom of the slide piston, not the bottom of the carb... just to make sure we're all talking about the same hole.


wiredgeorge wrote:

There is a small hole on the bottom of CV carbs to allow the slide to drop after being raised. The purpose of "drilling the slide" using the supplied 1/8" drill bit is to allow the slide to drop quicker.

The offset hole (on the bottom of the slide) is what lifts the piston. Vacuum in the main bore pulls air out of the hole which creates a vacuum above the piston and diaphragm. That vacuum pulls up the piston. It's often explained in the FSM's out there.

Making the hole bigger may allow the piston to drop faster, but more importantly, it allows the piston to rise faster (or sooner). It's not something you'll necessarily be able to detect unless the conditions are just right for A/B comparisons. That is because you would only drill the hole larger to when you make other changes in the motor or air-intake tract.

The air filters tend to affect the function more than anything else. Typically you drill the hole larger to make up for the effects of a less-restrictive air filter... like going to pods instead of the airbox. Less-restricive air filters reduces the vacuum in the bore which reduces the tendency for the piston to raise. In some bikes, the piston won't even go up, at all, with the filters removed.

Post edited by: loudhvx, at: 2007/10/11 13:45

I was talking about the slides... Loud, I am sure you have personal experience with slides not rising if there are no filters installed... I have never seen this. In fact, I generally observe all slide operation without installing filters or an airbox to ensure they are working correctly. I look at about 100 sets of the things a year without an airbox or pods before installing the same. I also stand behind my statement regarding "drilling the slides"... I wonder if you have ever done any actual experimentation on slide drilling? I think the engine vacuum is what raises the slides and vacuum is created by the engine... if more air is pulled into a combustion chamber, I truly doubt (based on my personal experience) that small changes in the hole size will make a difference with either the lift of the piston or its falling rate. This is a tuning trick that has been used by the Harley crowd for MANY years and while it is written about, I have never seen any factual data that would convince me it works. Well, it has been an interesting discussion but I guess it has been getting off-track. The original poster had best put a .jpg up that shows a slide as there is still some confusion regarding which hole he is referring to...

WG, my contention was the statement "There is a small hole on the bottom of CV carbs to allow the slide to drop after being raised" which implies the only purpose of the hole is to let the slide drop. Without the hole, the slide would not raise.

If making the hole bigger allows the slide to drop faster, then it would also raise faster. It's just a matter of air transfer and the hole is the determining restriction.


On most KZ's I've seen with CV (mostly the 400's), the slides will still work without filters. But I've come across some CV GPZ's where the filter removal really hampered the slides from raising... to the point of not running without the slides drilled. It seems to me (from experience) the motors that use carbs that are bordering on too-big for the motor are the ones affected by slide hole changes (and airbox changes) the most. When the carbs are too big, apparently there is simply not enough vacuum when you take off the airbox. The first time I came across this problem was on a Yamaha Radian. That bike would not run at all without the airbox. As soon as the box was back on, it ran like brand new.

I would assume Dynojet has done far more experimenting and tuning than I ever will, so I would follow their advice. But, I don't always just take their word on things, I like to find things out for myself, and I've found their jetting instructions to be right-on, through countless hours of my own testing, with a stand-alone EGA, and on-the-road testing.
So as long as your bike is not greatly different from what Dynojet tested on, then I would drill the slides as they recommend.

.

Post edited by: loudhvx, at: 2007/10/11 15:49

You all have een a monstrous help, and I do have to slide lift holes, twoeads the outside like described, one on eash side, along with the hole in the middle for the needle. I have modified the airbox similar to one of the instructions received on this site and installed a k&n filter in the airbox, I also ahve the factory silencer baffles drilled out, I was thinking going to a midrange jet between what dynojet considers stage one and three, and drilling out both holes, as I do have a spare set of slides. Your response was far more than waht could be said at the kawasaki forum, and from dynojet on this matter, I will post pics as soon as I can.

ALso as far as the performance is concerned with it as It is with one slide drilled the stage one kit, and the other mods mentioned above, the bike pulls hard above 6000rpm, between four and six, if I were to give it full throttle, it almost bogs, and if I roll out the throttle is will start to pick up speed as if were toleanBut the plugs and co's indicate that it is running correctly, but I can't test the eg's under load down the road, as I dont have a dyno to work with.

Post edited by: BadgerChips, at: 2007/10/14 19:36