Honest question - No can-o-worms! - Kawasaki KLR 650 Forum
1987 to 2007 Wrenching & Mods For maintaining, repair or modifications of Generation 1 KLR's. 2007 and earlier.

LinkBack Thread Tools Display Modes
post #1 of 23 Old 08-26-2016, 05:39 PM Thread Starter
3rd Gear
shinyribs's Avatar
Join Date: Aug 2016
Posts: 529
Honest question - No can-o-worms!

Cliff notes: Gonna be putting in a KLX needle, curious about drilling the slide.

I've got an '03 with about 40k on the clock. Maybe 500 miles are my own. I primarily ride dirt.

When I first got the bike I noticed it pecking on the highway at cruise, so I went in to the carb when I got home.

I was hoping to find an adjustable needle ( I knew i didn't have any kehein jets big enough for a single), but obviously did not. I was surprised to find such a FAT needle, though! Anyway, a little shimming got her cruising down the highway happily, but I've since learned about the KLX kit that you can get from Eagle Mike. Actually, I have it on hand now. Looking at the parts, it just makes sense. And since I'm having to run 93 to keep the dreaded pings at bay , it's obvious this baby is wanting more fuel. And while the kit seems awfully pricey, carb parts ain't cheap. Oh, idle mixture is currently about 1.75-2 out. Don't really recall exactly where I ended up that, but the bike seemed happy with it.

Anyway, my question is about drilling the slide. I'm on the fence about this because my brain isn't comprehending the "why?" of drilling the slide. I'm not opposed to the idea, I just want to understand it mechanically.

The internet forums teach us that it will improve throttle response. And I've got a pretty pretty decent concept of how CV carbs operate.

Here's where my mind is:

-Below the slide is engine vacuum

-Above the slide is a chamber connected to the vac ports present at the throat of the carb inlet

- A diaphragm used to lift the slide separates those two vacuum sources ( I understand all vacuum ultimately comes from the engine, but for the sake of conversation...)

OK, so if I drill the hole in my slide larger, how does that cause the vacuum to pull harder/ lift the slide quicker? By enlarging that hole aren't you effectively , kinda sorta, creating more of a balance by reducing the restriction?

Honestly, I'm just trying to figure this out. Since this is not an easily reversible thing I don't wanna jump it with both feet "just because".

Looking for personal experience, opinions and a little knowledge.

Would you? Did you? Would you do it again, or no?


Edit: oops, I forgot: stock exhaust, stock air box, standard foamy type air filter( hi-flo brand, I think it was)

Last edited by shinyribs; 08-26-2016 at 05:54 PM.
shinyribs is offline  
Sponsored Links
post #2 of 23 Old 08-26-2016, 09:36 PM
4th Gear
CheapBassTurd's Avatar
Join Date: May 2009
Location: Indiana/ Michigan line at the Lake
Posts: 1,586
Some experience the diaphragm "float"

The bike surges on and off during steady cruise as the slider floats
up and down. The people who do it now only go with 7/64ths.
It's unneccesary but brings up the slider faster with the larger vent.
Mine wakes up right when I whack the throttle. I didn't know it could
get much more responsive in the real world riding. Ya might get a few thousandths at the dragstrip.
The existing hole pops it up pretty fast as is.

The aggressive taper of the KLX needle is very popular here, and retains stock jetting but
lets them flow freely. I have a Sporster needle stacked with a #4 brass washer and 150 main.
Opened intake and exhaust.

This is my son, with whom I am well pleased." ----God
CheapBassTurd is offline  
post #3 of 23 Old 08-26-2016, 10:27 PM Thread Starter
3rd Gear
shinyribs's Avatar
Join Date: Aug 2016
Posts: 529
So you did drill your slide? And you think it does make the throttle snappier?
shinyribs is offline  
post #4 of 23 Old 08-26-2016, 10:50 PM
5th Gear
Join Date: Jan 2011
Posts: 4,705
The hole is the VACUUM PORT, shinyribs!

Venturi vacuum (pressure differential) is connected to the mixing chamber above the diaphragm through this port (aka, "hole"). With a larger-diameter hole, less restriction on passing dynamic pressure differential ("vacuum"); thus . . . more responsive action on the diaphragm lifting the slide, uncovering the main jet, blah-blah-blah . . .

HOW MUCH increased slide response? Hard to measure, and . . . a CV carburetor will ALWAYS have an inherent delay in slide opening awaiting vacuum (pressure differential) build-up. A CV carb will NEVER match the throttle response of a straight slide carburetor, but . . . a slide carb might need an accelerator pump to prevent fuel-lean operation upon rapid throttle twisting . . .

TOO MUCH slide hole enlargement can cause "flutter" in pressure transfer between the venture and the space above the diaphragm, resulting in surging and erratic running.

I commend to you, "Care and Feeding of the CVK40," link on this website, or . . . simply Google the title.

As to your "pinging," a richer mixture might not be the answer. Pinging is unusual with a KLR650, given its relatively low compression ratio and its rather conservative ignition timing. If higher-octane fuel quietens the pinging, the pinging may be a consequence of higher compression ratio from carbon build-up. Speculation only for the cause.
Damocles is offline  
post #5 of 23 Old 08-26-2016, 11:42 PM
Join Date: Aug 2016
Location: WNC
Posts: 6
Several years ago I installed the KLX needle in my '01 without drilling the slide. Not gonna. I'm very, very content with the way it runs. I remember some guys who did drill theirs also ended up filling the hole with JB Weld and redrilling it back to stock.

Just curious... what does your spark plug look like?
catamount is offline  
post #6 of 23 Old 08-27-2016, 12:02 AM Thread Starter
3rd Gear
shinyribs's Avatar
Join Date: Aug 2016
Posts: 529
Thanks for the replies, guys.

catamount, I haven't had a look at the plug yet. I've been swamped with work and haven't felt like fiddling with the bike much in the dark once I finally get home. It is on the agenda for tomorrow, though. The bike hasn't pinged much since I shimmed the stock needle up. It only pings when I load the engine heavily offroad. It was horrible about pinging with the 16T front sprocket, but the 14T has helped tremendously. I used to drag race a lot and pinging ( death rattle, we called it) is a horrible sound to my ears.

Damocles, re:

"Venturi vacuum (pressure differential) is connected to the mixing chamber above the diaphragm through this port (aka, "hole"). With a larger-diameter hole, less restriction on passing dynamic pressure differential ("vacuum"); thus . . . more responsive action on the diaphragm lifting the slide, uncovering the main jet, blah-blah-blah . . .

The port/hole you mention here ^^^ is this port, right?

The elongated, ovalish hole at the top of the carb throat

The hole/port people suggest to drill is this one:

Marked as "A"

So, what you are saying makes sense to me, and it's also why I'm a little suspicious of this drilling, as port "A" is on competing sides of the diaphragm compared to the port on the carb throat. So, like you said about the restriction, isn't drilling on "this side" of the slide sorta backwards? I mean, if you want the slide to raise quicker it would seem you'd want more vacuum topside of the diaphragm, not the bottom ( that would pull it back down....over simplified,I know lol). Or, is enlarging the hole in the slide what causes there to be more vacuum present topside to pull the slide up?

I'm not arguing either side, honestly just can't seem to wrap my head around this....and that annoys me lol
shinyribs is offline  
post #7 of 23 Old 08-27-2016, 08:32 AM Thread Starter
3rd Gear
shinyribs's Avatar
Join Date: Aug 2016
Posts: 529
shinyribs is offline  
post #8 of 23 Old 08-27-2016, 08:51 AM
5th Gear
Join Date: Jan 2011
Posts: 4,705
Doubt you've looked at, "Care and Feeding," probably won't look at "Bernoulli Principle" either!

I'll save you some research:
Bernoulli's principle, physical principle formulated by Daniel Bernoulli that states that as the speed of a moving fluid (liquid or gas) increases, the pressure within the fluid decreases.
Air flow through the venturi creates a low-pressure situation; this low pressure is "piped" above the diaphragm through the VACUUM PORT. Thus, low pressure is above the diaphragm, while higher ATMOSPHERIC PRESSURE is on the underside, via a half-moon orifice visible when looking into the carburetor air inlet from the air filter side.

O,K. We have ATMOSPHERIC PRESSURE under the diaphragm, and VENTURI "VACUUM" piped to the upper side of the diaphragm through the VACUUM PORT. In true analysis, atmospheric pressure lifts the slide, although some view this function as venturi vacuum raising the slide; no matter--pressure differential is the agent of change.

DRILLING your vacuum port reduces the restriction of the "pipe" conveying the dynamic pressure differential between the venturi and the top side of the diaphragm, thus more quickly raising the slide when the throttle is opened.

Take it or leave it. That's how it works, or . . . Bernoulli was just funnin' us.

Regardless--don't want to drill your slide? Then, DON'T!


Above the slide is a chamber connected to the vac ports present at the throat of the carb inlet
Not so.

The vacuum port is the hole in the slide, "A" in the image you post of the slide, not visible from the view of the carb throat.

That's NOT the vacuum port you show in your image of the carburetor air intake. Yet . . . see the curved hole at the top of the carburetor intake in your picture? That's the pathway for ATMOSPHERIC PRESSURE on the underside of the diaphragm . . . with atmospheric pressure under the diaphragm, and venturi vacuum piped through the vacuum port to the upper side of the diaphragm, the diaphragm raises the slide when the throttle is opened . . . drilling out the vacuum port enhances dynamic transfer of venturi vacuum to the upper diaphragm surface, resulting in faster slide opening when the throttle is opened . . . believe it or not!

I found this commentary accompanying the image you posted above (of the slide and the drill bit) on the Internet:
The slide moves up into the carburetor body as vacuum is applied to a hole "A" at the bottom rear of the slide. Enlarging this hole causes the slide to move up faster because vacuum is applied faster to the diaphragm that is attached to the top of the air slide.
Pretty well tells the tale.
pdwestman and shinyribs like this.

Last edited by Damocles; 08-27-2016 at 10:11 AM.
Damocles is offline  
post #9 of 23 Old 08-27-2016, 10:18 AM
Join Date: Nov 2009
Location: Lander, Wyoming
Posts: 6,471
Originally Posted by shinyribs View Post
There is the problem!
The air hole/vacuum port to the throttle slide diaphragm shown in the video is Incorrect!

The vacuum is created under the lower forward lip of the slide. Hole "A".

The curved 'eye-brow' vent in the bell mouth of the carb is what allows higher atmospheric pressure to 'push' the diaphragm to raise the slide as Damocles has posted correctly.

This is why even a tiny pinch or pin-hole in the diaphragm screws it all up.

Modify at "YOUR OWN RISK"!

Still riding my 1987 KL650-A1. 85,000+ miles & counting
pdwestman is offline  
post #10 of 23 Old 08-27-2016, 12:04 PM Thread Starter
3rd Gear
shinyribs's Avatar
Join Date: Aug 2016
Posts: 529
Ah, so I was looking at the system in reverse! That would explain why I thought drilling the slide would operate oppositely than what it does.

Cool. Thanks!

On a side/ yet related note, I just came in from a ride with the KLX stuff newly installed. I'll attach a pic of the spark plug as I found it before putting the new needle in, but it's far from an accurate plug reading. I've been riding around the farm doing chores, regardless, there definitely didn't seem to be any lean condition present.

With the KLX needle and all associated parts installed per the directions ( clips and such) the first difference I saw is that it's harder to wheelie. It requires more throttle input and it comes up less smoothly.

Power below 2500 IMO is boggish now. If you whack the throttle there's a hesitation followed by the bike coming back to life. It runs no better than before, it just bogs lightly then perks back up. Perhaps this is the "snappiness" that others are feeling? Power above 3500 rpm seems unchanged IMO. Previously I had the stock needle shimmed up and the issue mixture richened to 1.75 turns, so I wasn't exactly starting from scratch, in all fairness to the KLX kit.

These symptoms describe a rich condition to me. Yet, my fan is running more often than before. However, in all fairness regarding the fan, I am really riding the 14T sprocket extensively for the first time, so that may be related to that. I'm not riding at higher rpm now, but i am able to travel slower and ride the more technical areas that I life, so I may just be putting more strain on the engine due to that. Temp needle is glued in the center like always, ever consistent, so there's been no rise or fall in operating temp.

So, maybe I do need to drill that hole, but I'm not sure a sluggish slide is my issue now. Perhaps I'll drop the needle a bit and see how she responds.

Thanks again for the help on the CV stuff, guys. Damocles, yes, I did read the things you asked me to read. I've been playing with carbureted racing engines ( American V-8's)for almost 20 years now and all of my personal vehicles have been carbureted with non-stock engine combinations. Carb tuning isn't new to me, or intimidating. It's actually something I enjoy! Getting 22mpg out of a four speed '66 Impala SS was a fun adventure But the vacuum slide mechanism is definitely something I never experienced before a couple years ago when I ended up with a dohc CB750. Every day is a school day....

Oh, and I was happy to see a respectable number with this compression test. Although, with the auto decomp installed on this engine I was not really sure what to expect. Thoughts on that? I guess I could ask my manual, huh? lol
Attached Images
File Type: jpg 1472316864432.jpg (60.0 KB, 9 views)
File Type: jpg 1472316878285.jpg (82.3 KB, 12 views)

Last edited by shinyribs; 08-27-2016 at 12:10 PM.
shinyribs is offline  
Sponsored Links

Quick Reply

Register Now

In order to be able to post messages on the Kawasaki KLR 650 Forum forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Please enter a password for your user account. Note that passwords are case-sensitive.


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:


Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page
Display Modes
Linear Mode Linear Mode

Similar Threads
Thread Thread Starter Forum Replies Last Post
KLR250 jetting and other cans of worms tractorking Other KLR's - 250, 600, Tengai, C Models, KLX... 4 03-25-2014 02:27 AM
OIL Screen and gummy worms Grinnin 1987 to 2007 Wrenching & Mods 19 01-31-2014 09:52 PM
New Members.. Ask a question.. Any question.. Paper The Off Topic Lounge 85 05-23-2012 11:43 AM
An Honest Vendor efigalaxie KLR & Other Motorcycle Related Discussion 0 06-19-2011 07:28 PM

Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

For the best viewing experience please update your browser to Google Chrome