All of the info and charts on this page were graciously submitted by Gary,"Pontiac Sr.", our resident rocket scientist on the Yahoo Group. Thanks for your time Gary!

Though Gary has done a lot of the footwork for you, this information is in no way intended to be an instant fix to cure any woes you are having in the jetting department. They may be exactly what you need, but chances are they will only get you ballpark. Different pipes, headers, filters and oiling procedures, backfire screens, and fuel will all make the figures sway a little either direction.

Learn how to read plugs and don't ignore what your piggy is trying to tell you via hp, temps, and feel.

The following information is specific to the XR650R.  While the following jetting charts should be fairly accurate, please use them as a starting point to work from to find your specific baseline jetting.  Your baseline jetting will be the final jet sizes you end up with that gives you the very best performance for a given riding environment.  If your riding environment changes from where you’ve setup your baseline jetting, then simply use the correction factors listed in your service manual to find your new jetting specs.  If your riding environment varies, then try to choose something that fits most of your riding needs, but error on the side of being too rich if you’re in doubt.  Running too lean will lead to premature engine failure.  For optimum performance, plan to rejet if the temperature you’re riding at changes more than 20° F above or below your baseline.  Also plan to rejet if your altitude changes more than 3,000 feet from your baseline.
Mainjet Size

***Note: These specs are for an XR650R Uncorked / HRC tip / B53E Comp Needle***

*** XR650R Uncorked /stock header with HRC tip / B53E Comp Needle***

  • Use a main jet one size smaller when using a drilled stock tip instead of the HRC tip
  • Use a main jet one to two sizes larger when your side panel is cutout
  • If the stock needle is used, then ad +1 to the clip position listed below
  • Use a main jet one size larger with a high flow aftermarket exhaust canister
  • Use a main jet two sizes larger when using a larger diameter aftermarket header & exhaust canister

NOTE: When Gary mentions "Go one size larger", remember that Keihin jets run in sizes of 0, 2, 5, and 8. (170, 172, 175, 178, 180, etc.) A 170 + 2 main jet sizes should equal a 175 if that makes any sense.

When using the formulas above and more than one matches your bike, the increases and decreases in jet size are cumulitive.

Examples:

  • (XR650R uncorked) + (stage 1 cam) + (drilled out stock tip) = no main jet changes The cam increases the main jet size by 1, but the drilled stock tip subtracts 1 so they cancel each other out.
  • (XR650R uncorked) + (stage 1 cam) + (side panel cut out) + (HRC tip) = 2 main jet sizes larger
  • (XR650R uncorked) + (stage 2 cam) + (side panel cutout) + (full exhaust header & canister) = +5 main jet sizes larger

 

TEMPERATURE IN DEGREES °F

ALTITUDE   20    30    40    50    60    70    80    90    100

0          182   180   178   175   175   172   170   168   168

1000       180   180   175   175   172   170   168   168   165

2000       178   178   175   172   170   168   168   165   162

3000       178   175   172   170   168   168   165   165   162

4000       175   175   172   170   168   165   165   162   160

5000       175   172   170   168   165   165   162   160   158

6000       172   170   168   168   165   162   160   158   158

7000       170   168   168   168   165   162   160   158   155

8000       170   168   165   165   162   160   158   158   155

9000       168   168   165   162   160   158   158   155   152

10000      168   165   165   162   160   158   155   155   152

11000      168   165   162   160   158   158   155   152   150

12000      165   165   162   160   158   155   155   152   150
Needle Clip Position

***Clip position counted down from top of needle***

TEMPERATURE IN DEGREES °F

ALTITUDE   20   30   40   50   60   70   80   90   100

0          4    4    4    3    3    3    3    3    2

1000       4    4    4    3    3    3    3    2    2

2000       4    4    3    3    3    3    2    2    2

3000       4    3    3    3    3    2    2    2    2

4000       3    3    3    3    2    2    2    2    2

5000       3    3    3    3    2    2    2    2    1

6000       3    3    3    2    2    2    2    2    1

7000       3    3    3    2    2    2    2    1    1

8000       3    3    2    2    2    2    1    1    1

9000       3    2    2    2    2    1    1    1    1

10000      3    2    2    2    1    1    1    1    1

11000      2    2    2    2    1    1    1    1    1

12000      2    2    2    2    1    1    1    1    1
Pilot Size

TEMPERATURE IN DEGREES °F

ALTITUDE   20   30   40   50   60   70   80   90   100

0          72   72   70   70   68s  68s  68s  68s  65

1000       72   70   70   68s  68s  68s  68s  68s  65

2000       70   70   68s  68s  68s  68s  68s  65   65

3000       70   70   68s  68s  68s  68s  65   65   65

4000       70   68s  68s  68s  68s  65   65   65   65

5000       68s  68s  68s  68s  65   65   65   65   62

6000       68s  68s  68s  65   65   65   65   62   62

7000       68s  68s  65   65   65   65   62   62   62

8000       68s  68s  65   65   65   65   62   62   62

9000       68s  65   65   65   65   62   62   62   60

10000      68s  65   65   65   65   62   62   60   60

11000      65   65   65   65   62   62   62   60   60

12000      65   65   65   65   62   62   60   60   60
Jet Identification

One of the most common questions asked on the subject of jetting is "What's the difference between a 68s and a 68 pilot?".

Only a little, but it makes a big difference in fuel flow! Below are two pictures of pilot jets. The top is a normal 68, while the bottom is of a 68s. Notice that the 68s has smaller emulsification holes. Though hard to see in the pics, the "s" has an "S" in the stamped markings.


Here's what Gary thinks of the different jets:
The 68 pilot jet is similar to the 68s. They both share the same main orifice size for the discharge nozzle, but the 68s has smaller emulsion holes on the sides of its tube as you can see by the picture. The smaller holes on the 68s pilot jet reduces the amount of air that mixes with the fuel before the emulsified mixture is discharged through the jet's nozzle.

Now for a bit of Haymakers free trivia: Websters defines "emulsification" as "To make into an emulsion", where the definition of such is the following:

  1. A suspension of small globules of one liquid in a second liquid with which the first will not mix: an emulsion of oil in vinegar.

Now substitute "oil in vinegar" for "fuel in air", and there you have it!

"What does all this mean?" You ask? The size of globules into which the fuel is made by the jet drastically changes the rate in which it will burn. A Charge of air and fuel will burn much faster and with more force when there are more, though much smaller, droplets of fuel involved. I believe that the different jet has a direct correlation to the fact that the big XR is epa certified for emissions. Maybe I can get Gary to provide a little more insight as to why this change has been made.

Here's a bit of real world theory to help you understand how fuel burns. If you were to light a cup containing gasoline, only the surface of the cup would burn. You'd get a orange, smokey, flame, meaning that it was not burning efficiently.

If you were to take that same cup of fuel, atomize it into a fine mist in an enclosed space with the proper 15:1 ratio of air, then light it, you'd see the difference small globules make. Something I once read stated that "1 cup of gasoline will lift 1 ton, 100 feet, in one second." Maybe it's not correct, but it's something to think about as you are trying to put out your eyebrows because you had to check and see if I know what I'm talking about.

Well, that's about all I have for now. If anyone has any constructive criticism or recommendations, please let me know.