CV Carb Mods

not a good idea.....it will cause the slide to flutter. i did the 1/8th hole and couldnt tune the carb....put a stock one in and all is good.

if you can put a 1/8 drill bit in the hole....somebody has already drilled it......good luck and have fun!!!!!

It was done to help the slide get up quicker, usually combined with a lighter spring, and flutter was the issue.

If it's done be sure the carb has a new stock spring inside the slide, that can help keep it moving up and down smooth.

CV is a good carb.

That info posted on that site is really outdated. Many years ago it was a common practice and popular among those building cafe racers (which that site specialized in), but for regular street riding which applies to just about everyone else those methods are flawed.

This response will be lengthy, but bear with me as the points are valid.

There are several flaws with the original thinking that drilling the slide works correctly. Just because 20 years passes and some tuners still refuse to let go of techniques that have been proven wrong, doesn't make them correct.

I've been tuning this carburetor for 21 years (since it came out in 1988) and other carburetors for many years leading up to the CV. I don't own nor sponsor any dyno shops so I have no motivation to hand out advice and then push people toward dyno tuning when things don't work out for them. Having said that, here is my own advice based on my experience and those I have helped.

#1 - DO NOT drill your slide with a 1/8" bit. The vacuum hole is designed to allow the slide to open (move up) as well as close. To quote Isaac Newton, "for every action there is an equal and opposite reaction". Drilling the hole larger allows the slide to open faster, but also allows it to close faster. Add to this mix the tension and length of the spring. A shorter spring or one with less tension will also allow the slide to open faster but close the slide slower. A stronger/higher tension spring will close faster but open slower (defeating the purpose of the drilled vacuum hole). Add yet another factor, a shorter/lesser tension spring combined with a larger vacuum hole will allow the slide to pulse or "search" as it bounces open and closed. Long story short (too late) there is a very fine balance between having the right spring and correctly sized vacuum port in the slide.

While there is no dispute that a 1/8" drill bit has not changed over the years, continuing to leave articles online supporting its use as if the information was still current and factual doesn't make it right. Drilling the slide that large has been proven otherwise for years and what you have read here is nothing new. Recommendations for drilling the slide hole with bits smaller than 1/8 are meant to clean up the hole casting and bring it to the optimal size... but let me repeat, DO NOT drill your slide with a 1/8" bit.

#2 - Needle taper, length, and location of its tapers throughout the slide travel are imperative to correct fuel delivery. The tapers, width, and positions along the needle's length dictates how much fuel is delivered as the needle lifts out of the needle jet. Adding washers to change the height of the needle impacts the fuel delivery across all bands or the tapers, whether off-idle or nearing WOT.

The needle and slide work hand in hand, and despite popular belief the slide is not controlled by throttle, rather by vacuum pressure. This relates back to #1 and the drilled slide. Change the action of the slide and you directly impact the fuel metering of the needle. A bouncing slide produces erratic fuel delivery while a restricted slide hampers changes in fuel delivery in response to vacuum conditions. Back to the needle itself... the needle has its tapers and widths designed to meter fuel perfectly at given positions. Move the needle's position with a washer and you shift all ranges along the needle's length. Throwing more fuel into the intake does not produce better performance.

Only by changing the width and taper at given positions can you achieve a corrected air/fuel mixture for the engine condition you wish to effect. IMHO this has been the biggest drawback to clip-style needles over the years, no one position is ever optimal, only a compromise to find the closest match.

As you can see there is far more to proper tuning than washers and drill bits. The CV carburetor uses a fine balance between idle, off-idle, mid range, and WOT, all while responding to engine and riding conditions. Probably why the CV went practically unchanged for 20 years while all others were forced to adopt fuel injection.

To quote from the movie Rush Hour, "Don't nobody understand the words that are coming out of your mouth man."

Seriously, I just wrote that long-ass explanation based on actual carburetion theory and experience, and now this suggestion to "glue a piece of lead inside the slide"? REALLY? ARE YOU SERIOUS? JFC!!!! :witsend: :roflmao:

Sorry, normally I keep a lid on things and don't go off when I read things that I dissagree with, but the previous post is either a joke, an elaborate spam-bot automatically writing posts from Wikipedia entries, or just a horribly misinformed individual writing posts just for the sake of participating.

I'll step down off that soap box now :soapbox:

WOW, someone needs to clean out their bong water!!!

I would continue to comment, but I can't understand a word that guy typed! If you want me or anyone else here to take you serious try putting your thoughts into small meaningful segments. Start be writing your post, then before submitting read it back aloud to see if it makes sense.

If there is someone here that speaks jibberish please help me out by translating this. Then I would be happy to comment.

Not purposely trying to be mean, but give me something to work with here. Otherwise have a nice day.

BTW, that youtube video is not even a CV (H-D) carb so now I'm even more confused.

Well he's gone now, I don't have patience for that nonsense.