From the factory, your DR650SE came equipped with a Mikuni BST40 constant velocity (CV) carburetor. You may begin your journey of enlightenment here:
The BST40 Bible
Airbox mod? If you plan to enhance your DR's ability to breathe, you will need to address the restrctive airbox. If your plans to tweak your carb include stay with the factory jetting, you won't likely need to do more than remove the snorkle at the top of the airbox. If you plan on doing more (bigger jets, Dynojet kit, etc.) you'll likely end up doing the "airbox mod".
Here's a picture, from the Keintech website:
mx_rob has been a man on a mission to improve the performance of the DR. He is a "fixture" over at ThumperTalk, and his early efforts focused on the BST40.
His website www.mxrob.com
, has a BST40 section, including a link to this
thread at ThumperTalk, comparing different needles.
Mikuni / Dynojet Jet Size Comparison
| Mikuni ||Dynojet || |
| 140 || 149.3 || |
|Flow rates based on size markings of Mikuni and DynoJet jets are not comparable. As a guide .. the chart to the left indicates equivalent sizes based on the diameter of hole in the jet. Mikuni jets are chamfered on the inside opening at the threaded end and |
Dynojets have a machined flat surface. Keep this in mind when using this chart. These physical differences will never allow an exact comparison.
This chart is intended to be used as a tool to assist you in making a more
accurate decision when selecting a jet size range or comparing tuning data where an opposing brand is being used.
Actual jet sizes are highlighted with a gray background.
| DynoJet jets are marked according to the diameter of the hole in the |
jet .. ie: DJ150 = 1.5mm and DJ175 = 1.75mm. This is not true for Mikuni or most other OEM equivalent jets. This size
Mikuni jet, (N102.221 Super BN), is marked according to its' flow rate,
ie: 150 = a rating of 150 cc of fuel per minute.
| 142.5 || 152.0|
| 145 || 154.7|
| 147.5 || 157.3|
| 150 || 160|
| 152.5 || 162.7|
| 155 || 165.3|
| 157.5 || 168.0|
| 160 || 170.7|
| 162.5 || 173.3|
| 165 || 176.0|
| 167.5 || 178.7|
| 170 || 181.3|
| 172.5 || 184.0|
| 175 || 186.7|
| 177.5 || 189.3|
| 180 || 192.0|
| 182.5 || 194.7|
| 185 || 197.3|
| 187.5 || 200|
| 190 || 202.7|