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Air Hole In Vacuum Slide:Size Matters?
- bountyhunter
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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?
1979 KZ-750 Twin
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- TeK9iNe
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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.
Motorcycle Shop Owner/Operator
79 Kawie Z1000 LTD
81 Kawie Z1000 CSR
83 Honda VT750C A
85 Kawie GPZ900 A2
86 Zukie GS1150 EG
93 Yamie XV1100 E
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- bountyhunter
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1979 KZ-750 Twin
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1979 KZ-750 Twin
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- Motor Head
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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.
1982 KZ1000LTD K2 Vance & Hines 4-1 ACCEL COILS Added Vetter fairing & Bags. FOX Racing rear Shocks, Braced Swing-arm, Fork Brace, Progressive Fork Springs RT Gold Emulators, APE Valve Springs, 1166 Big Bore kit, RS34's, GPZ cams.
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- TeK9iNe
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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.
Motorcycle Shop Owner/Operator
79 Kawie Z1000 LTD
81 Kawie Z1000 CSR
83 Honda VT750C A
85 Kawie GPZ900 A2
86 Zukie GS1150 EG
93 Yamie XV1100 E
Lucky to have rolled many old bikes through my doors
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- loudhvx
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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.
1981 KZ550 D1 gpz.
Kz550 valve train warning.
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- bountyhunter
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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.Sounds like the slides were rising much to slowly causing lean hesitation and lack of power. Is this what you had?
Motor Head wrote:
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've seen replacement diaphragms for most BS series carbs, but may be that they cost more than a used set of carbs.
1979 KZ-750 Twin
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- loudhvx
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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.
1981 KZ550 D1 gpz.
Kz550 valve train warning.
Other links.
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- bountyhunter
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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.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.
1979 KZ-750 Twin
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