DR650 BST40 mikuni CARB pimping and needle tapering *DYNO Results added*

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Matie spero

Race Dog
Joined
Sep 17, 2012
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Location
Pretoria
Bike
AJS (all models)
Ok so I have done way to much reading regarding the Mikuni BST40 carburettor of the DR650.

Couple of the most predominant and informative sites was:

https://www.advrider.com/forums/showthread.php?t=347184

https://www.thumpertalk.com/topic/396373-dr650se-needle-comparison/page-1

https://drriders.com/topic5201.html

https://www.roadstarmagazine.com/2003/07/principles-of-modern-motorcycle-carburetor-function/

I also took the carb apart and measured everything with a fernier to try and establish what sections of the needle came into play at what throttle position i.e. 1/4, 1/2, 3/4 and full.

from all of this the following mods where made:

Intake side:

1) 50mm hole in airbox
2) Twin Air air filter

Carb:
1) Drilled an extra hole in the slide.
2) Shortened the slide spring by 2,5 turns
3) Pilot jet from 42.5 to 45
4) Main jet from 140 to 152.5
5) Self tapered needle
6) D-shelf mod (grinded off the plastic spacer on the one side)

outlet side:
1) Ground down the weld in the header pipe
2) FMF Q4 muffler




 

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