air screw adjustment - Kawasaki KLR 650 Forum
1987 to 2007 Wrenching & Mods For maintaining, repair or modifications of Generation 1 KLR's. 2007 and earlier.

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post #1 of 14 Old 12-24-2013, 11:28 AM Thread Starter
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air screw adjustment

can any one out there tell me what the airscrew adjustment is on a 1993 klr 650
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post #2 of 14 Old 12-24-2013, 01:38 PM
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The pilot screw's starting point is about 1.75 out from a soft bottoming, IMHO, for a stock bike. Service manuals may list lower numbers, but . . . men say the fuel-lean metering is more for emissions compliance, rather than optimum operation.

(The conventional name for an idle mixture adjustment screw on the engine side of the carburetor is, "fuel screw;" for one on the intake side of the carburetor, "air screw;" although I think Kawasaki has "air" somewhere in its parts list to describe this component.)

Actually, I think the CVK40 idle mixture screw meters an air/fuel "blend" into the venturi at idle.
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post #3 of 14 Old 12-24-2013, 01:40 PM Thread Starter
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thanks
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post #4 of 14 Old 12-24-2013, 11:37 PM
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There is no way to determine the best adjustment for a particular bike excepting by use of an exhaust gas bench but one can determine a very satisfactory setting quite easily.

An engine runs best with a slightly richer than ideal air/fuel mixture. For normal gasoline blends, this is 14.7 parts of air to one part of fuel, by weight which is a good number to quote to impress the ladies at parties but otherwise just a number.

The most simple way to find an efficient setting is as follows:
1) Engine operating at normal operating temperature and other adjustments correct.
2) Maintain curb idle speed as specified but not necessary to be continually anal about the speed.
3) Adjust idle mixture to lean (turn idle mixture screw in) and out until one finds the idle mixture adjustment which requires the least throttle opening to maintain the correct idle speed. Don't be too picky about this as close is OK.

*NB. for those who use different terms, the idle mixture screw is the one which adjusts the amount of fuel delivery and is located in the screw well in the bottom front of the carb. and adjusted by use of a small slot screwdriver bit. The curb idle screw/idle speed is the one located on the left side lower which is adjusted by finger turning of the knurled knob.

4) Now take care and using ear, tachometer (or better a test tach which gives better indication of differences in speed), adjust the mixture screw slowly inward (clockwise) until the engine begins to stumble.

5) Turn mixture screw out (anticlockwise) slowly until the idle speed stops increasing.

6) Correct idle speed to desired curb idle.

7) Now slowly turn the mixture screw in (clockwise) until the idle speed (RPM) drops about 50 RPM.

8) Readjust idle speed if needed.

Here's the reason for the procedure:
An engine runs better because it produces more power with mixture slightly richer than the ideal air/fuel ratio. We find the ideal air/fuel adjustment by making a rough adjustment by setting idle speed, then adjusting air/fuel mixture to about where the engine runs best. Now we find confirm by leaning out until the engine begins to suffer from lean mixture because it will react more noticeably to an over-lean mix than to an over-rich one. We need to be able to tell that we are about right without going too rich.

Now, we find the real "sweet spot" which will be a bit rich, then lean a bit to find the ideal mixture by going a bit lean. Many people simply look for the idle mixture adjustment which gives the best running which is the shown by the smallest throttle opening.

If you lack an air/fuel or gas bench, this will give a very satisfactory idle mixture adjustment.

This all is much easier with a "thumb knob" type idle mixture screw or a 90 degree adjustment tool but a 1/4" hex drive slot driver bit will do the job. Buy a 1/4" Ratchet wrench as a driver and you're set.

HIH

Norm
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post #5 of 14 Old 12-25-2013, 12:18 AM
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Quote:
Originally Posted by Normk View Post
There is no way to determine the best adjustment for a particular bike excepting by use of an exhaust gas bench but one can determine a very satisfactory setting quite easily.
The "1.75 turns out" was previously mentioned as a starting point for adjusting the fuel screw; the setting definitely can be refined. However, the idle mixture (pilot) screw dominates the mixture only at idle:



(Image from, "Care and Feeding of the CVK40" (I think).)

Beyond idle, other carburetor components significantly influence air/fuel ratio.
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post #6 of 14 Old 12-25-2013, 12:32 AM
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I don't understand your post. Are you saying that my description is incorrect or ?

He asked for idle adjustment so unless he adjusts from some confirmed reference, some arbitrary number of turns is just that, arbitrary. There are significant differences between carbs, locations, altitudes, and jetting changes so was attempting to provide a procedure which we used in the shops.
All modern carbs introduce an air/fuel mixture into the air stream at idle in the form of bubbles because of the air introduced by the emulsion ports in the tubes.

I know you never post poor information or without reason so wonder if I misinterpreted the OP?





Quote:
Originally Posted by Damocles View Post
The "1.75 turns out" was previously mentioned as a starting point for adjusting the fuel screw; the setting definitely can be refined. However, the idle mixture (pilot) screw dominates the mixture only at idle:



(Image from, "Care and Feeding of the CVK40" (I think).)

Beyond idle, other carburetor components significantly influence air/fuel ratio.

Last edited by Normk; 12-25-2013 at 12:36 AM.
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post #7 of 14 Old 12-25-2013, 07:42 AM
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Quote:
Originally Posted by Normk View Post
I don't understand your post. Are you saying that my description is incorrect or ?
On the contrary, Normk! I don't quibble with your pilot screw adjustment procedure. I only wished to clarify; the "1.75 turns out" I posted earlier was only a suggested STARTING POINT for pilot screw adjustment; perhaps an approximation of the final position.

The service manuals publish a pilot screw position in their SPECIFICATIONS, although in my view, the figure may not be optimum. For example, from Clymer (Generation 1):
Quote:
Pilot mixture screw turns out - 1 3/8
I posted the graph to indicate the effective boundaries of the influence of the pilot screw adjustment on the air/fuel mixture, over the throttle range.

Your posts always comprise sound and reasonable information, experience, and ideas.
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post #8 of 14 Old 12-25-2013, 11:54 AM
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I thought I was reading that I was off base which I took seriously, "Oh, oh, maybe I've screwed up....better ask Damocles to confirm."

Thanks for that. I'm always worried that might be leading someone into error.





Quote:
Originally Posted by Damocles View Post
On the contrary, Normk! I don't quibble with your pilot screw adjustment procedure. I only wished to clarify; the "1.75 turns out" I posted earlier was only a suggested STARTING POINT for pilot screw adjustment; perhaps an approximation of the final position.

The service manuals publish a pilot screw position in their SPECIFICATIONS, although in my view, the figure may not be optimum. For example, from Clymer (Generation 1):I posted the graph to indicate the effective boundaries of the influence of the pilot screw adjustment on the air/fuel mixture, over the throttle range.

Your posts always comprise sound and reasonable information, experience, and ideas.
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post #9 of 14 Old 12-26-2013, 08:54 AM
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Quote:
Originally Posted by Normk View Post
There is no way to determine the best adjustment for a particular bike excepting by use of an exhaust gas bench but one can determine a very satisfactory setting quite easily.

An engine runs best with a slightly richer than ideal air/fuel mixture. For normal gasoline blends, this is 14.7 parts of air to one part of fuel, by weight which is a good number to quote to impress the ladies at parties but otherwise just a number.

The most simple way to find an efficient setting is as follows:
1) Engine operating at normal operating temperature and other adjustments correct.
2) Maintain curb idle speed as specified but not necessary to be continually anal about the speed.
3) Adjust idle mixture to lean (turn idle mixture screw in) and out until one finds the idle mixture adjustment which requires the least throttle opening to maintain the correct idle speed. Don't be too picky about this as close is OK.

*NB. for those who use different terms, the idle mixture screw is the one which adjusts the amount of fuel delivery and is located in the screw well in the bottom front of the carb. and adjusted by use of a small slot screwdriver bit. The curb idle screw/idle speed is the one located on the left side lower which is adjusted by finger turning of the knurled knob.

4) Now take care and using ear, tachometer (or better a test tach which gives better indication of differences in speed), adjust the mixture screw slowly inward (clockwise) until the engine begins to stumble.

5) Turn mixture screw out (anticlockwise) slowly until the idle speed stops increasing.

6) Correct idle speed to desired curb idle.

7) Now slowly turn the mixture screw in (clockwise) until the idle speed (RPM) drops about 50 RPM.

8) Readjust idle speed if needed.

Here's the reason for the procedure:
An engine runs better because it produces more power with mixture slightly richer than the ideal air/fuel ratio. We find the ideal air/fuel adjustment by making a rough adjustment by setting idle speed, then adjusting air/fuel mixture to about where the engine runs best. Now we find confirm by leaning out until the engine begins to suffer from lean mixture because it will react more noticeably to an over-lean mix than to an over-rich one. We need to be able to tell that we are about right without going too rich.

Now, we find the real "sweet spot" which will be a bit rich, then lean a bit to find the ideal mixture by going a bit lean. Many people simply look for the idle mixture adjustment which gives the best running which is the shown by the smallest throttle opening.

If you lack an air/fuel or gas bench, this will give a very satisfactory idle mixture adjustment.

This all is much easier with a "thumb knob" type idle mixture screw or a 90 degree adjustment tool but a 1/4" hex drive slot driver bit will do the job. Buy a 1/4" Ratchet wrench as a driver and you're set.

HIH

Norm
Norm, IMHO, this is the best idle mixture adjustment procedure I've ever seen. I'm printing a copy for my Clymer manual. You know with minor changes for the location of the mixture and throttle stop, I believe it will work on any motorcycle.
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post #10 of 14 Old 12-26-2013, 12:59 PM
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Very please that it is useful. Wish I could take credit for developing it but it's a procedure which everyone who does dozens of mixture adjustments in a short time develops. As one will see, it's only a rough setting, followed by finer tuning and goes about as fast as reading.... maybe faster.

Some of the younger guys with better hearing don't need to bother with a tach at all and can crack an adjustment off in a distressingly short time.

I find that the biggest issues for most people wrenching at home is finding a convenient way to turn the mixture screw. Most times the carb mixture adjustment gets done and not touched for months or years which is a disincentive to buy an expensive tool.

Everyone already knows but doesn't hurt to repeat: the mixture screw and carb can be damaged by turning the screw in too tightly. The bottoming point as reference for initial setting, enough that the engine will run, is only by feel for the light touch at bottoming. I recently replaced a carb because the owner cranked the screw in so hard that he wedged the mixture screw and cracked the carb. Nasty!



Quote:
Originally Posted by twinjet View Post
Norm, IMHO, this is the best idle mixture adjustment procedure I've ever seen. I'm printing a copy for my Clymer manual. You know with minor changes for the location of the mixture and throttle stop, I believe it will work on any motorcycle.
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