Jump to content

Mikuni BSR Jetting Tips Requested


Recommended Posts

A search of the forum for DRZ and KLX400 jetting info so far has not turned up much detailed info, at least what was easier to get to.

I just bought a beautiful, low-miles 2003 KLX400SR. My March 2000 DRZ400S shop manual shows it to have a Mikuni BSR36 carb. The application for this bike will be as a commuter/highway machine. I must ride to 1,300 miles to leave in a distant city where I fly in each month to work. I intend to fit it with road tires (120 on the rear), gear it up to 41/16. I will retain the stock muffler because I don;t like loud bikes.

I'm more interested in excellent quality of power than in maximizing power to drag race around.

I don't know this bike intimately, but do know generally what to do to make it run better.

Now the TPS is a curiosity, given no O2 sensor and no fuel injection. I have a KLX330S with a TPS. I left it alone. Anything to know about the TPS on this bike? What the heck does it do?

Likely carb:

BSR36 29FB, USA, non-Californicated:

Main: 142.5

Pilot: 22.5

Air: 135 (versus 165 UK/Euro/Other)

Needle Jet: P-0M

Jet Needle: 5DH37 (versus 5DH32-2nd UK/Euro/Other)

Float Height 13mm +/- 1mm

The bike was tested with the appaling-for-highway-use stock gearing, no snorkle, filter soaked with real foam filter oil, stock muffler (which I will keep). The results? Quite healthy mid to top with very good fuel economy.

The bike runs'EPA lean'in the low-to-mid engine rpm range. It required a significant warm-up period. It's hyper-active when opening and closing the throttle, with excessive engine braking, then surging to life when the throttle is cracked open. Very unsmooth and distracting!

Here's what I normally what I'd do to make the carb work better:

- Remove snorkle but not enlarge the airbox hole

Snokle removal has provided additional CFM and auditory feebback.

- Liberate the fuel screw

Removal of the nanny-government cap so I can adjust low-speed fuel mixture.

- Modify the fuel screw

For while-riding fine adjustment - no tools, no direct sight required. I chuck the tiny screw in my lathe and drill a small hole in the head of the adjuster. Then I take some and solder in a brass wire bent into an L to make an

extender so I can make fine low-speed fuel adjustements by feel and with gloves on.

Here is a pic of a Keihin CVK fuel screw I modified:

090810FuelScrewBentRodMod_.jpg

- Shim the needle up relative to the slide from .050" (1,27mm) to .062" (1,57mm)

...to provide a little more fuel relative to slide position, the shimming amount

depending on ride tests. I've done this on many bikes with good results. I could also buy a needle if you'll please share the number.

- Enrich the pilot jet

...If fuel screw liberation and adjustment range does not entirely smooth out throttle response when cracking open and shutting the butterfly valve.

- Enlarge the main jet to ~145.0 from 142.5

...If after gearing up and testing 80-100 mph against a head wind, excessive softness of power results because fuel supply does not quite meet the load demand

Your feedback please.

Link to comment
Share on other sites

Open the air box to 3"x3" and drop a 155 main jet in there.

That's a significantly bigger jump up in the main than seems to be required by seat of pants, from 142.5, assuming that's what I have. Are you thinking of that main size with respect to any other changes, like to the exhaust system?

The 3" X 3" means just opening up the existing hole more?

Link to comment
Share on other sites

That's a significantly bigger jump up in the main than seems to be required by seat of pants, from 142.5, assuming that's what I have. Are you thinking of that main size with respect to any other changes, like to the exhaust system?

The 3" X 3" means just opening up the existing hole more?

Yeah, open the top of the air box to 3"x3" (no more, no less). With much more air getting in you need a much larger main jet. As it is, it sounds like your 142.5 main is too lean with the factory sized hole (anemic) in the box.

I'm not pulling this stuff out of my arse, this is a common setup on these bikes. Your 25 pilot is good with the factory air screw. If you want to clean up the mid range some more slap in a James Dean Jetting blue needle with the clip in the 4th slot.

Link to comment
Share on other sites

Yeah, open the top of the air box to 3"x3" (no more, no less). With much more air getting in you need a much larger main jet. As it is, it sounds like your 142.5 main is too lean with the factory sized hole (anemic) in the box.

I'm not pulling this stuff out of my arse, this is a common setup on these bikes. Your 25 pilot is good with the factory air screw. If you want to clean up the mid range some more slap in a James Dean Jetting blue needle with the clip in the 4th slot.

Thanks. I have seen mentioned in the forum airbox hole sizes as too big and too small, so experientially you all are probably onto something. Now 9 square inches can sure move a huge amount of CFM, as can the stock, um, what is it, ~5 square inches also can? I just though of what I can do. I've gor some sensitive vacuum gages. I'll take a reading in the box with the stock hole and motor WFO to see if there is any measurable pressure drop, to indirectly look at flow.

Thanks for the James Dean Jetting blue needle. I'll go figure out what that is and where to get it.

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

×
×
  • Create New...