KDXRIDER.NET

Just trying to improve jetting for my KDX220R. Bought it from a guy who lives at 5900ft. I live at 7400ft, so it's running a little rich...so easiest thing to try is changing clip position on jet needle...which way? Do I move the clip up or down on the needle? Thanks.

Move the clip up one notch to lower the needle and lean the jetting.

Likely need to drop the main jet one size for your altitude change.

Likely also have to open you Air screw some to compensate for the thinner air.

This is assuming it was correctly jetted at 5900 feet.

Hey IdahoCharley,

Thanks for the quick answer.

The 'guy' who lives at 5900 maybe never jetted it for diddle, in which case, making a change relative to that setup is not productive.

The way to 'improve' jetting is to jet it correctly..that based on your bike's performance with you on it in your riding area.

Which is why IC said, 'This is assuming it was correctly jetted at 5900 feet.'


I'm just joining the bandwagon....

High(er) elevation riding on a 220 calls for a head that works...and that comes via RB-Designs.

Have fun!!

Ok, got it...how can you accurately tell whether it was jetted right in the first place (Right now I am going on how the plug reads...it's black down inside it). Doesn't bog or have a delayed throttle response, but it also spooges alot. Thanks.

canyncarvr wrote:The 'guy' who lives at 5900 maybe never jetted it for diddle, in which case, making a change relative to that setup is not productive.

The way to 'improve' jetting is to jet it correctly..that based on your bike's performance with you on it in your riding area.

Which is why IC said, 'This is assuming it was correctly jetted at 5900 feet.'


I'm just joining the bandwagon....

High(er) elevation riding on a 220 calls for a head that works...and that comes via RB-Designs.

Have fun!!

There are two schools of thought on basic jetting: From the top down (WOT to off-idle response) and from the bottom up (opposite..natch). I use the 'top down' approach...main first.

Every circuit in the carb effects every other circuit. If you're main is a good bit out of whack, the bike idle will be goobered, too.

The high speed circuit (main jet) is determined using a WOT plug chop test. With a warmed up bike, put in a NEW plug..run through the gears at wide open throttle. Doesn't have to be a real long haul...20 seconds is sufficient..longer is better. At the end of the run, dump the throttle, kill the engine, pull in the clutch, take out the plug.

What you're looking for is the thickness of 'ring' at the bottom of the insulator. The best way to SEE that ring is to cut the threads off the plug. You will see something like this:



This pic from R. Rohrich

A plug chop test won't leave this dark a ring, or any of the other coloring of the pictured insulator. The point is to show a WELL DEFINED ring, where it is, how thick it should me. 1mm is considered about perfect. No room for any jetting error (cooler temps at the end of the day 'fer instance).

After a plug chop test, you will see a ring more like this:



This one from BJH..it's in his gallery on this forum.

This ring is considerably fatter'n 1mm..which indicates rich..which indicates too large a main jet.


Never...NEVER jet by spooge. That is NOT a telltale of any kind of jetting at all. The following pic is a plug out of a bike (mine) that spoo'ed like a plaster spatter gun:



Don't jet by plug 'color', either. The color of what you see is not a telltale of air/fuel mixture.

There are other specific tuning methods to determine needle, clip position, slide cut, and such. Set the main correctly, set the rest for best seat-o-the-pants response, you'll be fine.

Oh...you can use the same chop method at different throttle positions than WOT. Mark pull positions on your throttle (1/4 1/2 and the like), hold the throttle AT that position with the bike pulling (under load) to find the mix AT that position.

That isn't done too often...I'm just saying that WOT is not the ONLY place a plug chop CAN be used to determine air/fuel mixture.

Also, if you have trouble pulling a load on the engine while performing the run, carvr said 20 seconds, but I prefer a full 30 seconds or even more if you have the room to run, try using the brakes to help apply more load. A slight uphill is great to load a motor as you are doing the run for a plug chop.

Also remember a full throttle run does not need to be in 5th gear. I usually work my way fast up to 3rd gear and pin the throttle at 3rd gear for a full 30 seconds from the time I am in 3rd gear, not counting the seconds from start and accelerating through the gears to 3rd.

Take a spare plug with you; you need to pull that new plug out of the bike and head back to the shop with it after all....

Here is a plug chop on my KDX220 with a 145 main


Here is the same bike with a 150 main


Both runs were as I described earlier. Notice how much fatter the black ring is on the bigger main jet?