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DR650 BST40 Carb Needle / Air Filter Mods

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Hi to all on here. I'm a newbie 1999 DR650 owner. I bought the bike a couple of months ago and it was bog standard. I rode it home put it in the garage and started cutting massive big hole in the top of air filter box... I did this mod on my DR350 and bored it out to 441 and got good results.

I made a new rear section exhaust with can. Got rid of vacuum tap and all associated junk. Drilled out main jet to 1.6mm aprox a 160. Put needle in my lathe and put some more grooves in for adjustment. Lifted the needle about 1.5 mm. Went out to try it out and it was totally gutless. Did lots of messing about with the needle height and the result was still totally gutless.

So I found some info on this forum from mxrob which helped. I took his measurements of the Dynojet needle and worked out the angles and turned my needle as close as i could to match the dj dimensions. Result was it was a lot better, but far from perfect.

To cut a very long story shortish I have machined the ''D Step'' away in the slide as it was getting on my nerves. Got an old Z1000 needle which is much shorter and just stuck it in an electric drill and filed the profile freehand. Went out to try it out and it was awesum and nearly perfect.

So the moral of this story is that the stock needle is no good what so ever with air box top cut out mods, and just about any 3mm needle with a sharper angle on will be an improvement, as long as the straight section is just inside needle jet at idle and needle doesn't come completely out of needle jet at full throttle. I think when throttle is closed stock needle is slightly on the taper at the top of the needle jet which is probably responsible for bog of idle.

1) I now have 4.5mm less preload on the spring, as no D Step and no washers underneath the clip.

2) The straight section of the needle is just sat inside the needle jet with throttle closed.

3) With slide full open the end of the needle is just inside the needle jet... very important.

4) Drilled slide

Also I have put an overflow tube in float bowl just in case float needle decides not to shut off.

Another tip is to pull off the silly resistor spark plug caps off and crimp some car type ends onto the plug leads. I did this mod on DR350 and DR650 and noticed a difference with both. Having resistor plug caps on is like have very corroded copper HT leads.

Edited by fullenglish

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Unfortunately after all of that great work you still have a vacuum operated slide with no accelerator pump. :busted: So it's better but not nearly great. :busted:

Welcome to the forum!

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Unfortunately after all of that great work you still have a vacuum operated slide with no accelerator pump. :busted: So it's better but not nearly great. :busted:

Welcome to the forum!

Hi mx_rob, I was hoping to get a response from you. I have read a lot of your threads and they have been very informative, especially the needle comparisons. Which is what I based my calculations on.

Yeah I know still not a pumper carb, but just being resourceful (tight) with what I have at the moment.

I have a few needles kicking about that i can play with and slides. I have machined different cutaways on the slides to try, but the parameters are getting too great.

One thing is definite and that is stock needle and airbox mod won't work.. i think you may have already said this so just confirming it.

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Yeah I know still not a pumper carb, but just being resourceful (tight) with what I have at the moment.

Completely understandable. That was one of the motivations for me to not just purchase a TM40 for my bike.

I have a few needles kicking about that i can play with and slides. I have machined different cutaways on the slides to try, but the parameters are getting too great.

And that was another one of the motivations.... I like to tinker. :busted:

One thing is definite and that is stock needle and airbox mod won't work.. i think you may have already said this so just confirming it.

Quite true. The air box restriction is performing the same duty as a choke (literally). So down in the 1/4 throttle range there really isn't enough air flow for the restriction to cause any jetting affects.... but as flow increases around 1/3 throttle that all changes and all of the jetting has to be leaner than with a full flow airbox.

In stock form the DR650's performance is literally strangled by the highly restrictive air box opening. :busted:

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Completely understandable. That was one of the motivations for me to not just purchase a TM40 for my bike.

And that was another one of the motivations.... I like to tinker. :busted:

Quite true. The air box restriction is performing the same duty as a choke (literally). So down in the 1/4 throttle range there really isn't enough air flow for the restriction to cause any jetting affects.... but as flow increases around 1/3 throttle that all changes and all of the jetting has to be leaner than with a full flow airbox.

In stock form the DR650's performance is literally strangled by the highly restrictive air box opening. :busted:

I think I have gone as far as I can with BST40 and its probably as good as i'm going to get it and it never going to be as good as a pumper carb or injection. It will have to do for now though. So maybe a pumper carb next year or will see how you get on with fuel injection project.

Too many parameters with vacuum carbs when tuning. Here is a short list of variables.

1) Float bowl fuel height

2) Pilot jet, pilot air jet and mixture screw

3) Needle profile and clip position

4) Main jet and main air jet

5) Throttle slide cutaway

6) Throttle slide spring length

7) Throttle slide air holes drilled in the bottom

Areas of confusion

1) Main jet sizes ie Dynojet vs Mikuni sizes, 150 is 1.5mm or not

2) Jet needle positioning ie circlip on ''D Shelf'' or not. Washer on top or underneath.

If anyone needs any help with these issues reply to this thread and will try to help as i guess mx_rob must be fed up with BST40's and he has now moved on to fuel injection.. good man keep up the good work.

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Could I mod a Mikuni 44mm Carb needle for a 91 DR350?

Its far too big. BST33 is standard and TM33 pumper carb on enduro model. You'd lose bottom end and the cylinder head and cams would probably restrict the flow at the top end and anyway you'd never get it in the inlet rubber.

Try to get hold of a TM33 pumper carb or maybe slightly larger. If you go much bigger you will have to turn down the outer diameter of the carb where it pushes into the inlet rubber, or freeze the inlet rubber and bore it out then match the inlet tract to the rubber. Sorry I'm assuming everybody has a lathe in their workshop.

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My hands are a lathe, lol, jk.

Thanks, I'll start scouting for carbs, lol. :busted:

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Areas of confusion

1) Main jet sizes ie Dynojet vs Mikuni sizes, 150 is 1.5mm or not

https://docs.google.com/a/mxrob.com/document/d/1n1MGH4qSPWAqd4FkmV4PUn8eYp0BL5xJFGzvPta2sf0/edit?hl=en_US

2) Jet needle positioning ie circlip on ''D Shelf'' or not. Washer on top or underneath.

Stock needle length (stock needle or Factory Pro needle): Must use white plastic spacer sitting on edge of D shelf... or whatever modification you want to do the keep the needle at the stock elevated position.

Short DJ style needle: Must base the clip at the bottom of the slide next to the D shelf for the proper height.. 4th clip position is std... or again, whatever modification you might want to do, like using the filed white plastic spacer next to the D shelf or one of Procycle's kits with the proper spacer, and moving the clip the appropriate amount to keep the needle height correct for the short needle.

If anyone needs any help with these issues reply to this thread and will try to help as i guess mx_rob must be fed up with BST40's and he has now moved on to fuel injection.. good man keep up the good work.

LOL! Yeah, it didn't take me too long to become unenamored with the BST.

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Areas of confusion

1) Main jet sizes ie Dynojet vs Mikuni sizes, 150 is 1.5mm or not

I'm sceptical about the size charts as I have loads of Mikuni main jets and the sizes ie 140, 150, 160 are measuring 1.4mm 1.5mm and 1.6mm within a few microns. I know its documented that Mikuni jet sizes are flow rate but i'm not convinced but will do some more tests.

Also Mikuni pilot jets ie 35, 40 are measuring .35mm and .40mm. I can't believe its a coincidence.

The only way I can be sure is to borrow my last employers pin gauge set and check a variety of jets.

Also its been said that Dynojet sizes are size measurements ie 150 = 1.5 mm which would further question the accuracy of the comparison charts.

Edited by fullenglish

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I'm sceptical about the size charts as I have loads of Mikuni main jets and the sizes ie 140, 150, 160 are measuring 1.4mm 1.5mm and 1.6mm within a few microns. I know its documented that Mikuni jet sizes are flow rate but i'm not convinced but will do some more tests.

Also Mikuni pilot jets ie 35, 40 are measuring .35mm and .40mm. I can't believe its a coincidence.

The only way I can be sure is to borrow my last employers pin gauge set and check a variety of jets.

Also its been said that Dynojet sizes are size measurements ie 150 = 1.5 mm which would further question the accuracy of the comparison charts.

It's certainly possible that they measure the same but a Dynojet 160 flows less fuel than a Mikuni 160.

I recall reading somewhere that Dynojet makes their own jets because they wanted different flow characteristics.

Of course it may have been 'cashflow' they wanted to improve.:busted:

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It's certainly possible that they measure the same but a Dynojet 160 flows less fuel than a Mikuni 160.

I recall reading somewhere that Dynojet makes their own jets because they wanted different flow characteristics.

Of course it may have been 'cashflow' they wanted to improve.:busted:

Lol yeah I don't believe everything that's on the net after I found out that the moon isn't actually made of cheese as I was led to believe.

I must stop looking longingly at your 780cc kit !!! I've only just got the DR650 as an impulse buy after my mate bought a CCM R30 as my DR441 is getting a bit tired now.

780 kit next year maybe.

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