Air Hole In Vacuum Slide:Size Matters?

When the rubbers on my vacuum pistons rotted away on the BS-38 carbs on my 1979 KZ750 B4, I bought some from another twin but learned they are slightly different. For one thing, the small air holes in the pistonse (near where the needles come out) are different size. On stock, the holes are maybe 3/16" and the new ones are maybe 1/16".

Why does Kawi tweak the size of the hole in the pistons?

What effect would that have?

As far as I know, it just lets air in from the venturi so that one side of the rubber diaphragm has the lower pressure air which means the piston will rise.

Does the size of the hole affect the damping of the vacuum piston?

That hole controls the rate of slide rise per velocity of air at the intake.

Its really important, and part of a jet kit/pods modification for carbs with throttle plates/CV models.

I wish i could offer more info/correction info...

They should work ok though...

Try n' see.

GL.

I am running the new slides with the smaller hole. They do work sorta OK, but not exactly right. Maybe I'll try enlarging the holes to match the originals.

For the record: the holes in the stock slides are about .150", the new slide's holes were about half that. I worked the holes out on the new ones to about .130", reset the needle clips to the stock slot and it seems to be running wayyy better.

Sounds like the slides were rising much to slowly causing lean hesitation and lack of power. Is this what you had? As the carb tuner kits always drill these out larger than stock for mods that have increased airflow or lessened the signal in the venturi, which lets the slide lift faster.
Glade to here that its running better, I've seen replacement diaphragms for most BS series carbs, but may be that they cost more than a used set of carbs.

bountyhunter wrote:

For the record: the holes in the stock slides are about .150", the new slide's holes were about half that. I worked the holes out on the new ones to about .130", reset the needle clips to the stock slot and it seems to be running wayyy better.

bountyhunter wrote:

Does the size of the hole affect the damping of the vacuum piston?

Yes. That's about it from what I could tell. It affects how fast the piston opens or closes as you move the throttle. It will also affect how much it will flutter when the throttle is held at an intermediate opening.

By the time you are near half throttle (opened slowly), I think the slide is usually all the way up. Beyond that it's all butterfly valve.

This is based on experiments on a 400, but the 750 seems similar.

Motor Head wrote:

Sounds like the slides were rising much to slowly causing lean hesitation and lack of power. Is this what you had?

Yes, and they just would not tune out. If I got the needle clip in the right place for good throttle hit off idle, they didn't run right at full throttle at gearshifts. The needles just didn't track because the slide position wasn't tracking across throttle position.

Motor Head wrote:

I've seen replacement diaphragms for most BS series carbs, but may be that they cost more than a used set of carbs.

I haven't seen the diaphragms for the BS-38's, saw some for the smaller carbs on the KZ-440. I saw a place in England that says they will install diaphragms for about $120/set plus shipping. Not a bad deal, but I was reluctant to ship that far for fear of losing the slides.

I thought the whole point of the CV's was to make the mixture richer by opening slowly. If it opened up too fast (as in a mechanical slide) it gets too lean.

Too slow going up makes it rich from what I've seen on the EGA, but different carbs could be different.

Remember if the slide opens too slowly, yes,it restricts the fuel in the needle jet, but it's also restricting the air too, acting like a choke.

On a 400, we had the slide stick in a lower position and that cylinder ran super rich, then gas fouled the plug. We re-aligned the slide cover and watched it move properly, then the mixture went back to normal.

loudhvx wrote:

I thought the whole point of the CV's was to make the mixture richer by opening slowly. If it opened up too fast (as in a mechanical slide) it gets too lean.

Too slow going up makes it rich from what I've seen on the EGA, but different carbs could be different.

That sounds right, the new slides with smaller air holes did run a little richer for the same needle position. When I enlarged the holes, I put them back where they had been running sort of OK (which was 1.5 needle slots to the lean side) and it ran way too lean. It had run way too rich at the standard (center slot) setting with the smaller holes. So, I then moved the clip back to the standard middle slot and it runs right again with the larger holes.