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

[Matie spero]

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:

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

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

http://drriders.com/topic5201.html

http://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

[Ganjora]

did exactly the same to my DR carb,  except the spring.
then pulled the trigger on a Mikuni TM40 pumper carb.

[Matie spero]

Now here's for the interesting part, THE TAPERING OF THE NEEDLE:

So what I did was I took the fernier and measured the stock needle every 2mm (dark blue line).
Also did this for the factory pro needle (green line) that I have and a stock needle, tapered according to the second website above (Red line)...

I then tried my best to get some measurements for a DJ needle of the web, but could only get ruff stuff but this would serve just as an indication (purple line):

what I ended up with was the below:

I decided that I would like to try a two stage taper myself and that then inserted the light blue line, half way between stock and DJ.

[Matie spero]

Now that I have the plan now came the execution part.

I set up a dremmel in a bench vice

For the grinding i got a thick piece of glass to which I super glued 2000 grid wet sanding paper and 1200 grid to the other side.

The 2000 is way to fine and basically did nothing, ended up using the 1200, but if I had to do it again I would have started with 400, then 1200 and then polished with 2000.

[Matie spero]

after a lot of sanding and measuring and sanding and measuring the results below:

The pinkish line is the actual.

[pielas]

Very interesting, I am running the DJ needle currently, let us know your findings on on the modified needle. You are also welcome to come measure the DJ needle if you want.

[Matie spero]

Very interesting, I am running the DJ needle currently, let us know your findings on on the modified needle. You are also welcome to come measure the DJ needle if you want.

Hi pilas

as I am sure this will not be the risrt and last attempt at tapening, I will take you up on than offer to get the presice DJ measurements.

thanks

[Kykdaar]

Very informative.  The forum needs more threads like this

[pielas]

Very informative.  The forum needs more threads like this

Stem saam.

Matie spero, No problem, welcome anytime.

For whats its worth. here my setup:

Intake side:

1) Snorkel out and Airbox opened up fully on top. I have a spare airbox so decided to gut one.
2) Twin Air air filter
3. Firescreen removed

Carb:
1) Drilled an extra hole in the slide.
2) Slide spring still standard, but I'm thinking about cutting it,
3) Pilot jet from 42.5 to 48 ( from KTM 640, Suzuki only stock the standerd size)
4) Main jet 150 ( would like to try a size bigger sometime)
5) DynaJet 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) GSXR muffler ( Stuart has a gutted GSXR muffler of mine that we will be experimenting with, will see how it goes, The GSXR is still restictive)
3) Header Ceramic coated inside & out ( supposed to improve airflow, not sure, I had it done as I was tired of cleaning the damn thing)

Bike is running very good right now. You must take it for a ride when visiting. Only issue I have is a sort of uneven idle, but plan to play around with that this weekend. Bike was Dyno'd about a month ago. Extended air-screw in so should not be too difficult.

I have a FCR carb, but enjoying the Mikuni right now.

[chopperpilot]

Matie

Is jy met verlof? Jy't baie tyd op hande!

Is Julies se sake in orde?

Sal graag terugvoer oor die werkverrigting van jou DR na al die modifikasies wil sien!


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[Matie spero]

Very informative.  The forum needs more threads like this

Stem saam.

Matie spero, No problem, welcome anytime.

For whats its worth. here my setup:

Intake side:

1) Snorkel out and Airbox opened up fully on top. I have a spare airbox so decided to gut one.
2) Twin Air air filter
3. Firescreen removed

Carb:
1) Drilled an extra hole in the slide.
2) Slide spring still standard, but I'm thinking about cutting it,
3) Pilot jet from 42.5 to 48 ( from KTM 640, Suzuki only stock the standerd size)
4) Main jet 150 ( would like to try a size bigger sometime)
5) DynaJet 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) GSXR muffler ( Stuart has a gutted GSXR muffler of mine that we will be experimenting with, will see how it goes, The GSXR is still restictive)
3) Header Ceramic coated inside & out ( supposed to improve airflow, not sure, I had it done as I was tired of cleaning the damn thing)

Bike is running very good right now. You must take it for a ride when visiting. Only issue I have is a sort of uneven idle, but plan to play around with that this weekend. Bike was Dyno'd about a month ago. Extended air-screw in so should not be too difficult.

I have a FCR carb, but enjoying the Mikuni right now.

Stuart told me to remove the fire screen as well, I think I will do that too.. what could be the possible drawbacks???

I also have the stock spring and factory pro spring as spares. you can take my FP spring if you want and cut it, if it doesn't work you still have your stock spring.

On main jets I will be getting a couple of each sizes 150, 152.5 155, 157.5 will keep every one in the loop and will play around and then anyone can get from me what I don't use... Gave kykdaar my 142.5, 145 and 147.5 as he is not keen on making holes in his airbox for now.

I can also try and get you a 145 pilot if you want (got one in the FP kit I got with the bike).

My thoughts would be the 152.5 or the 155 will be the spot-on's on the mains

Where do you do your dyno's?

[Matie spero]

Matie

Is jy met verlof? Jy't baie tyd op hande!

Is Julies se sake in orde?

Sal graag terugvoer oor die werkverrigting van jou DR na al die modifikasies wil sien!


Sent from my iPad using Tapatalk

hahahaha, nee, ekt nou soveel aangejaag dat ek mamma Durban toe moet vat die naweek om op temaak....

by die werk, werk dit as volg:
Daar is n excel spreadsheet oop so mense dink ek werk, niemand besef ek is eintelik beesig om naalde temeet en te plot nie... hahahaha

[lecap]

...
Stuart told me to remove the fire screen as well, I think I will do that too.. what could be the possible drawbacks???
...

The screen prevents your engine from ingesting stuff. Stuff like the screw which is supposed to hold your air filter in place
A good compromise would be a screen made with fewer wires and larger holes. This would reduce the flow resistance considerably. whilst still serving the purpose.

[Matie spero]

...
Stuart told me to remove the fire screen as well, I think I will do that too.. what could be the possible drawbacks???
...

The screen prevents your engine from ingesting stuff. Stuff like the screw which is supposed to hold your air filter in place
A good compromise would be a screen made with fewer wires and larger holes. This would reduce the flow resistance considerably. whilst still serving the purpose.

I will have a look at this, maby make a screen from that fine wire mesh you place inform of windows?

[pielas]

Where do you do your dyno's?

I went to BB Motorsport, Brian Baragwanath ( the guy competed in the Dakar on a Quad for Team Rhide SA). Just of Botha Ave in Centurion close to the SPCA.

http://www.yellosa.co.za/company/861650/bb-motorsport

Seems like they know what they are doing. Cost was R600.

[Matie spero]

Where do you do your dyno's?

I went to BB Motorsport, Brian Baragwanath ( the guy competed in the Dakar on a Quad for Team Rhide SA). Just of Botha Ave in Centurion close to the SPCA.

http://www.yellosa.co.za/company/861650/bb-motorsport

Seems like they know what they are doing. Cost was R600.

Great stuff, that's 3km from my house!!!

[Matie spero]

kyk ook gou hier, het advise nodig:

http://www.wilddog.za.net/forum/index.php?topic=170654.20

[Matie spero]

ok so I finally got Deedre back from BB motorsport today after some dyno tuning:

below is the results :
light green and purple lines:

Max HP: 44.94
Max Torque: 48.17

pilot jet 45
main jet 152.5
air screw 1.5 turns out
50mm hole in air box
snorkel removed.

Red and turquoise lines:

main jet 146
air fuel screw 2.5 turns out.

Max HP:48.5
Max Torque:51.71

[Matie spero]

an extreamly interesting test though, took the side cover off the air box, and ran it again.

Had a 0.3hp improvement , but look at the torque curves!

a 56% deference at low end!!!!!

Thus more air is not better, on this setup!

[chopperpilot]

Lyk na goeie krag en torque!

Hoe voel en trek jou DR nou?

Watter naald, en posisie het jy gebruik?

Trek joune nou beter as Stuart se DR?

Die 50mm air box gat, is dit slegs met die snorkel uit, of is die oorspronklike gat vergroot na 50mm?

Jy't seker nie nou 'n keuse as om te kom wys by die XR Fest nie! (Het jou 'n PM in hierdie verband gestuur.)


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