On the 96-Present DR650SE's the intake system is the most restrictive part of the motorcycle in overall performance and smooth operation. In stock condition the air box and it's rubber intake "snorkel" (part 14) severely restrict air flow capabilities through the engine. There are a host of mods that have been done to increase air flow like removing the snorkel, removing the air box side cover (6), removing the air boot screen (5), modifying the top of the airbox Other air flow improving techniques involve adding one or multiple filtered vents, velocity stacks or just plain drilling holes in the plastic box. Whatever mod you decide to do requires the proper carb jetting to keep the air/fuel ratio at the proper levels. Standard on the Mikuni BST 40SS carb is; main 140, pilot 42.5, needle 6F19 clip on 3rd slot (Australian, Canadian and UK needle). All US models use a 6F23 fixed clip position jet needle and a Y-5M needle jet. The most popular jetting kit for the stock carb is Dynamometer repair The most complete kit is Procycle's stock carb jet kit. Other options are Factory Pro's kit... This kit is based on leaving the air box in it's stock form. There is a ton of information here on these and other jetting mods for the stock CV carb.
The simplest mod for an otherwise stock system is to simply remove the snorkel, readjust the pilot circuit and raise the needle .5 -1mm. Here is an excellent pictorial write up on this proccess by jessepitt over on ADVrider.
When swapping info with other DR riders do not forget the size difference between Mikuni and DynoJet main jets ie a Mikuni 140 is roughly equivalent to a DJ 150.
Mods to the stock intake system can include:
- Adjusting the very lean factory set pilot circuit. Remove the factory installed pilot screw access plug by increasing the weep hole size so you can screw in a wood screw or sheetmetal screw to remove the plug. Most machines run very well with the pilot screw set between 1 and 1-1/4 turns out from lightly seated.
- Removing the snorkel (More air).
- Raising the stock needle .5-1mm (Best for stock in combination with snorkel removal).
- Drilling 2nd hole in carb slide (Improve slide response speed).
- Shortening the diaphram spring (Decrease the preload - increase slide speed some more).
- New Y-5M needle jet (If the OEM one wears you get a big off idle bog and it will seem to run rich).
- Opening the top of the airbox (Even more air but needs a complete re-jet using the Procycle or Dynojet kits mentioned above).
You need to read the above threads to get a grasp of what (if any) benefit these would be on your bike in your situation.
Of course if you really want to boost performance, throttle response and engine smoothness you have to remove the CV carb and replace it with a direct operated slide carb like Keihin's FCR series carb or Mikuni's TM series carb. For the super industrious there is no doubt a fuel injection system would be the ultimate addition but the work required to fab your own system would be a large undertaking.... starting here.
There is now a fuel injection kit available for the DR from Ecotrons which looks very promising. Over in the DRRider forum there is testing and discussion of the Ecotrons DR650 Fuel Injection Project.
Here's a link to mxrob's DR650 Fuel Injection Project on Thumpertalk.
Here's the condensed version of mxrob's Fuel injection project on msruns.com.
Links that can help Edit
- mxrob's FCR-MX page
- ProCycle's DR650 fuel systems page
- Kientech Engineering's intake page
- Keihin's web site for the FCR-MX
- Factory Pro's CV carb tuning site
- AdvRider BST40 Bible thread
- motolab BST40 carburetor parts store