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TOPIC: Drilling slide lift holes (CV carbs)- what does it do?

Drilling slide lift holes (CV carbs)- what does it do? 8 years 6 months ago #54294

  • Trav
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In the Dynojet kit they provide a drill to increase the size of the hole in the bottom of the slide (the slide lift hole? I believe).

Anyway, what is this going to do? It tells you to drill the slide for both the stage 1 and stage 3, so whatever it is, I assume it is something of an improvment over the stock setup, whether stock or pods and 4-1?

Does it maybe have something to do with operation in conjunction with the slightly different shaped adjustable jet needle that they include? To clarify, it is not the hole that the needle is installed through in the slide, it is a seperate hole next to it. The needle installs into the slide without modification.

Post edited by: Trav, at: 2006/06/13 18:27
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Drilling slide lift holes (CV carbs)- what does it do? 8 years 6 months ago #54345

  • The Milkman
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This is a complete guess, but I would think that it would let air into and out of the chamber so it would modify how fast the slide moves up and down.
78 650-C2, Stock engine, Jardine 4-2 Exh., 17-38 sprockets, dyna ignition and coils, coil wiring mod, carb mod.
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Drilling slide lift holes (CV carbs)- what does it do? 8 years 6 months ago #54436

  • Trav
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I guess the title should have been "enlarging slide lift holes" as there are already holes there.

But yea I assume it's something like that, but I'm not exactly sure. Also why do they want to change that?

I'm just wondering really.
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Drilling slide lift holes (CV carbs)- what does it do? 8 years 6 months ago #54443

  • dtkid
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Drilling the slide lift-holes allows the vacuum to have a greater and quicker effect on the lifting of the slide.

This will increase the responsiveness of the slide and therefore increase air-flow to the intake. This is usually necessary to match the increase in fuel delivery allowed by the new needle and main jet.
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Drilling slide lift holes (CV carbs)- what does it do? 8 years 6 months ago #54499

The hole in the slides is 3mm... they give you a 1/8" drill bit. If you drill the slides, you are supposed to get "more throttle response" but it is essentially a waste of time. Try it and find out for yourself. This practice started with the Harley folk on the CV40s on their big twins... Vulcan owners used to tout the effectiveness. Another "trick" is stretching the return spring and clipping it a bit shorter to also "increase throttle response". That is a waste of time as well but you can try that as well if you are intent on messing around with your carbs.
wiredgeorge Motorcycle Carburetors
Mico TX
www.wgcarbs.com
Too many bikes to list!
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Drilling slide lift holes (CV carbs)- what does it do? 8 years 5 months ago #56422

  • Trav
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Frankly, I'd rather not fuck with the carbs at all.

However, I have had them apart as the carbs I picked up for a new slide actaully ended up being in immaculate shape compared to the set I was running, so I was making sure everything was set before I put them on my bike.

I used to mess around with my car in high school doing stupid shit like that (if that's all it is) because I had nothing better to do.

Then I got this bike, and sometimes I wish I didn't have to mess around with it at all.
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Drilling slide lift holes (CV carbs)- what does it do? 8 years 5 months ago #56493

the bigger hole complements the needle taper. it makes the slide require more vacuum to lift. in all it really makes the slide slower to lift increasing the intake port velocity 300 feet/sec= optimum port air velocity
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Drilling slide lift holes (CV carbs)- what does it do? 8 years 5 months ago #56503

  • Trav
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Interesting. From the shape of the needle, I had thought it might be something like that. I forget if I mentioned the difference in size and length of the stock vs dynojet needles already or not.

While it may be a budge hot-rodder trick (or not) in general application, I would think dynojet knows what the hell they were doing when they disgned these kits.

How else do you explain guys throwing them in their GPz550's and have them running great?

btw, where did you pop up from man? I've seen alot of precise intelligent posts from you in threads around here. The kind of helpful stuff from somebody who seems to know, but with an explanation as well. B)
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Drilling slide lift holes (CV carbs)- what does it do? 8 years 5 months ago #56522

Trav, My opinion of Dynajet jet kits is that they are miserable. But that is only one persons opinion. If I were putting together jet kits, I would use different size main jets, put pilot jets in and use DIFFERENT jet needles for different applications which at least matched the needle jets in the different bikes. Dynajet doesn't. How can the same jet needle be used with same recommended clip position for a 76 KZ900 VM26 with an 0-6 needle jet and use the same jet needle for a 79/80 KZ1000 VM28 with an 0-4 needle jet? Sort of like trying to fit a 5'8" 250 lb. guy AND a 6'2" 180 lb. BOTH with a 42R suit. It might fit SOMEONE but surely not either of these guys... If you want to try drilling your slides with a 1/8" drill bit, go ahead and give it a shot. I have done this and experimented with doing this on a variety of bikes... I could tell NO difference. Perhaps you can. Try DJ jet needles when you do it and then try stock jet needles. Perhaps there is a difference I can't feel... btw: I have a BIG supply of DJ jet needles that have been taken out of customer's bikes who couldn't get them to run right...

My opinion on jet kits? Well, most folks are mystified by rejetting. They put the noisey pipes on and pod filters and the bike falls flat on its face at 5K rpm or has a huge flat spot. Putting on huge main jets cures that problem. SOMETIMES, a skinny jet needle with the clip in the right position will cure that problem. Dynajet fills that market space... folks who don't have a good idea what rejetting is necessary benefit from spending $130 (jet kit) for four main jets, four generic jet needles and a drill bit. Unfortunately, the over-rich top end and mid-range and lack of rejet for the pilot circuit cause as many problems as they solve. It would be far better if most folks looked at other bikes set up as theirs is and followed the advice of the owners of these bikes as far as rejetting goes and use original main jets and their original jet needles and used original pilot jets but in the appropriate sizes or clip positions. Just my opinion again.
wiredgeorge Motorcycle Carburetors
Mico TX
www.wgcarbs.com
Too many bikes to list!
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