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CVK Vacuum Slide Hole

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Does anyone know what drilling out the hole in the slide on a CVK carbuerator does for performance? My JD Jet kit tells me to drill out the hole but I wanted to see what the benefit was first.

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It lets the vacuum "build" above the diaphragm faster. The vacuum is what "lifts" the slide.

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Drill the hole. We all did it. We all had smiles on our face on the first ride after doing it. Walk on the wild side! :confused: :confused:

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I won't challenge the wisdom of drilling the hole, imagining the DJ folk have calculated or trial-and-errored resulting benefit.

As to effect, I'd imagine the hole slightly increases airflow volume, correspondingly reducing flow velocity, in turn reducing the "vacuum" (pressure differential) and the lifting force of the slide, for a given throttle (butterfly) opening.

Whatever, the modulation of the slide opening as a result of drilling the hole, I'd think, is good for performance with the DJ installation.

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I agree that opening an orifice within the venturi could reduce airflow (thus vacuum), but I think what DJ figured out was that by drilling the slide where they suggest, it actually creates more velocity/vacuum on the engine side of the carb slide, which is also where the diaphragm gets its vacuum. The improved throttle response at low rpm is because there's a bigger leak of air when the slide is closed, so the available vacuum occurs earlier and in greater quantity at or just off idle. I took 4 turns off the slide spring too, just to give me as little resistance as possible in getting the slide to move early.

I ain't no engineer, but I did a lot of figuring when I had the carb apart and before I started drilling, and it seemed to make sense to me. Like many things in life, you can really screw up if you drill the wrong orifice.

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Good point.

A hole in the slide would mean less restriction; maybe increased flow velocity results and additional "vacuum" is available to lift the slide.

Regardless, I'd follow DJ's guidance and drilll the hole.

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Drilling the hole on the CVK was around a long time before DJ ever made a kit for the KLX.

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Drilling the hole on the CVK was around a long time before DJ ever made a kit for the KLX.
No DJ kit, but . . . what's the hole diameter and placement?

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Enlarging the vacume hole (not the hole the needle goes thru) has been around long before DJ started making kits for the KLX. Lots of guys cut the slide spring 3-4 coils and drilled out the vacume hole. Don't have the spec's in front of me, but if you need them, PM me and I'll dig them out of the boneyard.... :crazy:

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Don't have the spec's in front of me, but if you need them, PM me and I'll dig them out of the boneyard.... :crazy:
Thanks . . . when the carb's on the bench in front of me, I'll get in touch.

"Til then, I'll live with Mr. Keihin's arrangement, improving it next carb take-down (if I don't go whole-hog and get a pumper, like you did!).

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Nothing you can do to a CVK will make it 1/3 as good as an FCR. :crazy: I just PM'd the specs to a new KLXr. Let me know if you want them. bp

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Nothing you can do to a CVK will make it 1/3 as good as an FCR. :D I just PM'd the specs to a new KLXr. Let me know if you want them. bp
Not now, thanks; but maybe . . . "one of these days!"

Since typically I don't ride on the edge (nowadays), truth be told, the CV pretty much "helps me make it, through the night."

Yet,

Breathes there a man with soul so dead,

Who never to himself has said,

"My! That extra performance would be COOL!"

:crazy:

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I dream of an FCR. Still keeping my eyes open for a reasonable price. I know Jerry sells them new, but am wondering what bikes come stock with the 35mm? If I know what to look for at the wreckers, I may be able to get a break on the price.

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