Drilling the slide holes

2250 Views 12 Replies 9 Participants Last post by andyauger, Feb 27, 2009

localgod11

Discussion Starter · Feb 25, 2009

What exactly does drilling out the slide holes do?

My guess: Allows more air to flow into the engine by allowing the slide to to go up quicker.

Sorry for the basic question, just trying to get me an ejimacation

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andyauger

· #2 · Feb 25, 2009

The theory is that it pulls the slides up faster because pressure changes get to the vacuum diaphragm faster. In reality, it doesn't do anything beneficial, and you can cause troubles. It's a misunderstanding of how vacuum operated slides work and what they do (in other words, why are they used at all).

Did you ever notice that there are strong warnings about opening the passage up too much? That's because it's a dynamic system. If the hole is too large it changes the natural damping of the slide, so it can overshoot in operation. Then it's out of time closing down again. The pattern repeats. The venturi vacuum oscillates, fuel delivery gets erratic, most of the acoustic boost goes away.

At wider throttle openings the slide doesn't oscillate much (which makes the change ineffective). At smaller throttle openings changing the dynamics of the slide can lead to poor running and loss of power. If the drilling made any difference at all it would be during throttle position changes, and then only for a few thousandths of a second. Next time you synch your bike observe the fluctuations in the slides when the engine is at idle. The slide easily manages any transitions necessary.

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localgod11

Discussion Starter · #3 · Feb 25, 2009

Great info, thanks! why is it that the instructions on a jet kit tell you to do it?

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avc8130

· #4 · Feb 25, 2009

Hmm...never heard an argument against it before. Catpoopman had some dyno results to support the drilling. Never seen a complaint either. Not saying Andy is wrong...just interested.

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ac

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dan46n2

· #5 · Feb 25, 2009

contact Zoran at twinworksfactory and see what he has to say on it.

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andyauger

· #6 · Feb 25, 2009

My skepticism about stuff like this is that the carbs react very quickly stock. At idle they have no problem swinging quite widely 600+ times a minute. Once you're at, say, 3K the slides aren't moving much anyway so the reaction speed is not a factor in performance. Dyno tests are done WOT, at which point the slides oscillate very little if at all. Constant vacuum carbs are designed to improve fuel vaporization and delivery at part throttle conditions.

Also, why would Suzuki (or any other maker) intentionally give up performance when they could have designed in that bigger hole?

Changing the diameter won't change how high the slides open. A given vacuum signal will open the slides a given amount.

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imdying

· #7 · Feb 26, 2009

andyauger said:
Also, why would Suzuki (or any other maker) intentionally give up performance when they could have designed in that bigger hole?

Well, why would someone need to change the jets from what Suzuki installed at the factory?

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andyauger

· #8 · Feb 26, 2009

You may not have to change jets. My bike was jetted fine from the factory. I did have to adjust the idle circuit screws. Right now the front is 1 1/2 turns out, the rear is 1 3/4 turns out. I also massaged a section of the needles to correct a lean spot around 5K RPM. Other than that the mixture is perfect across the range. Most SVs are just a tad too lean, but not all.

I tried putting an extra shim under the needles and wound up too rich over most of the off-idle to midrange. If I did the jets, needles and idle mixture screws per catpoopman I'd be running badly indeed.

You want to drill the slides, go ahead. Don't expect miracles.

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wacky_woodchuck

· #9 · Feb 26, 2009

I did drill mine out and noticed an improvement in part throttle response. It "seemed" the driveability of the bike was better. Throttle responded much faster and smoother going from closed to open or part open to more than part opened.

As Andy stated, it really doesn't help when you have the throttle opened up (half way up to full).

For me, I feel it's a worth while mod. Simple to do, the jet kit gives you the correct drill bit size (#54 IIRC). Yes, it's irreversable...but it's a worthy addition to the carbed SV.

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Northwind

· #10 · Feb 26, 2009 (Edited)

localgod11 said:
why is it that the instructions on a jet kit tell you to do it?

Because they actually tested it, and found benefits. Worth pointing out here that Renegade, JHS, Dynojet and Factory Pro all indepentantly developed their jet kits, and all recommend the exact same thing, while not one jet kit producer for the SV recommends against this. It's clearly not a money-making scam, as the jet kits cost the same as equivalents for other bikes with less parts.

Andy's question "Why would suzuki give up performance" is a straw man, you might equally ask why Suzuki don't sell the bike with a race system and hot cams. What's appropriate for the stock state of tune isn't automatically appropriate for a modified state of tune.

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Veee

· #11 · Feb 26, 2009

andyauger said:
My bike was jetted fine from the factory. I did have to adjust the idle circuit screws. Right now the front is 1 1/2 turns out, the rear is 1 3/4 turns out.

****, you would think they can figure out mix screws considering they are perfect with slide hole. I don't think you should have changed it, you made it worse

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DibDubDoo

· #12 · Feb 26, 2009

Good thread. I was wondering about this

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andyauger

· #13 · Feb 27, 2009

I fooled around with the mixture because it wasn't dead nuts. It was pretty close and it ran fine, I'm just kind of anal with carburetors. The mixture screws were pretty close, but I decided to drill the caps and fine tune when I disassembled to hone the mid sections of the needles.

I have done a couple of VStroms and SVs that were pretty lean. However, I verified on a dyno with EGA before doing anything.

Like I said, drill the slides if you like. My suspicion is that the recommendation is common because "people say it works". And also like I said, it only makes a difference in transitions, and makes less a difference when the throttle opens wider. Take a look down the throats and you can see what they're doing as the revs go up, with or without a load.

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