Jetting the 650R by Gary "Pontiac Sr."
 

Special thanks and all credits to Gary for all his hard work making this write-up possible.

 

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 cut-out
  • 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 cumulative.

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 50 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
10,000 168 165 165 162 160 158 155 155 152
11,000 168 165 162 160 158 158 155 152 150
12,000 165 165 162 160 158 155 155 152 150
Needle Clip Position

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

TEMPERATURE IN DEGREES F

Altitude 20 30 40 50 50 70 80 90 100
0 4 4 4 3 3 3 3 3 2
1000 4 4 43 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
10,000 3 2 2 2 1 1 1 1 1
11,000 2 2 2 2 1 1 1 1 1
12,000 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 80 68s 68s 68s 68s 65 65 65
3000 70 70 68s 68s 68s 65 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
10,000 68s 65 65 65 65 62 62 60 60
11,000 65 65 65 65 62 62 62 60 60
12,000 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 us know.

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