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Old 09-15-2009, 09:22 AM   #166
Coho911
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Quote:
Originally Posted by laramie LC4
the stock needle will work fine. i used to ride from 5,00-12,000 ft with it. you notice some issues at the top, but thats just due to lack of O2.

laramie

What 'Groove' is your needle set at (the little C-Clip on the top of the needle) & what jet are you running ? I Picked up a 162 to replace the 152 (for a start - but its not in yet - have to adjust my float first as my bike is running VERY rich - need more air in there).
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Old 09-15-2009, 11:11 AM   #167
laramie LC4 OP
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Quote:
Originally Posted by Coho911
What 'Groove' is your needle set at (the little C-Clip on the top of the needle) & what jet are you running ? I Picked up a 162 to replace the 152 (for a start - but its not in yet - have to adjust my float first as my bike is running VERY rich - need more air in there).
i would start with it at the 3rd clip. you can move the needle clip really easy, you dont even have to pull the carb so adjusting it is never a problem.

162 main jet is about as rich as you should need. depending upon what mods you have done and the elevation it may be too much. i always ran a 157.5 but i have an older, non-high flow head. the main once again though, can be changed fairly easily. if i was you, id be focused on my float hiegth and my idle jet settings. those are the ones that take removing the carb to work on, and will cause most of your issues.

laramie
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Old 09-15-2009, 12:01 PM   #168
wrk2surf
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got some mikuni pn's from my file

Main N100.604

Pilot non bleed type N151.067

air jet B/S 30/97

needle and seat ..no info..HS 40?? 990-786-044 viton

the little viton tip is all what wears out...

My high flow 03 runs like a champ.. but debating on the fcrmx after 6 yrs
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Old 09-15-2009, 01:25 PM   #169
motolab
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Quote:
Originally Posted by wrk2surf
needle and seat ..no info..HS 40?? 990-786-044 viton
needle & seat, 786-35009-2.8 (HS40 is different)
seat o-ring, KV/10
Quote:
the little viton tip is all what wears out...
Indeed. That's why I stock the needles by themselves. Typically, by the time the tip has a witness line (inspect under magnification) and is therefore in need of replacement, the o-ring should be replaced as well (I stock those as well). Of course, if the needle seat o-ring is shot, the rest of the o-rings and gaskets are probably as well (I'll have those rebuild kits ready soon enough).

Regards,

Derek
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Old 09-15-2009, 03:20 PM   #170
laramie LC4 OP
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Quote:
Originally Posted by wrk2surf
My high flow 03 runs like a champ.. but debating on the fcrmx after 6 yrs
you wont regret it. i love my 640E with the full akrapovic and FCR-39 (with a powernow of course). the BST-40 is a GREAT carb but it does have its limitations. if you want pure performance you have to go with the FCR.

laramie
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Old 09-16-2009, 11:06 PM   #171
bmwktmbill
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The needle wears verylittle, the needle jet wears, the needle wears at the top because the slide hole wears.

The flat needle tip wears, the seat doesn't, the O-ring on the seat does wear but it's the only one. the gasline wears.

I wish I had a rebuild kit for the fuel pump.
i think it's a good idea to carry the small parts that hold and space the main jet needle, I could drop them and be all F'd up. Once lost the spring on the low speed fuel needle but I had a spare. The bowl screws should be replaced with Allen screws.

If someone makes up a spares kit I WILL BUY ONE.
bill
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Old 09-24-2009, 06:04 PM   #172
motolab
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I now have rebuild kits for the KTM BST40 carb in stock. They include a float needle, needle seat o-ring, cold start enrichment feed pipe o-ring, float bowl gasket, drain plug o-ring, pilot screw o-ring, slide guide o-ring, two throttle shaft seals and two stainless steel socket throttle blade screws. Price is $33.90/kit. Thanks to wrk2surf, who provided a used carb for me to use to spec out the components.

Regards,

Derek
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Old 09-24-2009, 06:38 PM   #173
Teriyaki
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Question BST34 Problems

First of all, this is my first post to your forums. Second, don't beat me up, since I don't own a dual sport bike. My son races KTM's but they are all dirt bikes. That is how I found your forum, since we have some racing friends who own a Duke and Adventure. We bleed orange.
Now my problem is a BST34 Mikuni Carb. This is my first time to work on a CV carb, and it is different from the carbs on the 2-stroke KTM's I am used to working on. The carb is on a Polaris ATV. The ATV forums don't seem as knowledgeable as you guys on these type of carbs.
About 2 months ago the quad wouldn't start. But, it would usually start if I keep trying through the day over a period of several hours. Once it starts, it runs great and will start just fine. But leave it overnight and it will do the same thing.
I have replaced the needle and seat in the carb, and set the float level to 15mm. The float was pretty high. I cleaned the pilot jet and removed the pilot screw. The rubber diaphragm on the slide looks good, and the slide moves smoothly. I feel it is something in the pilot circuit, but I cleaned everything I think.
The fuel pump works good, I have disconnected the fuel line, cranked it, and the fuel pumps good. Plenty of vacuum to the pump. The choke (enricher) seems good. I was concerned about the enricher, because a few times when I removed the air filter from the air-box, some gas would splatter into the air box when it was cranking. That seemed strange to me, but I understand the enricher pumps raw fuel into the engine.
I am planning to go buy a new pilot jet this weekend, even though the one I have seems clear, or at least my eyes seem to see thorough it.

Any help is appreciated. Maybe I have missed something or should just start over. I wasn't drinking any beer, so maybe that was my problem.


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Old 09-25-2009, 11:54 AM   #174
guytan
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In an attempt to preserve the spring

I want to precise one thing about BST-40 tuning :
I agree with motolab when he says :
"Clipping the spring increases the spring rate, but reduces the preload."
But...
It's another way you can get the job done.
Mikuni has already thinked about it.
I don't know about your foreign carbs, but in France (I'm a poor frenchy guy, you know), the black cap is stamped either "1A" or "1B".
If you put the black spring guide off the cap, the "1A" show you 3 pods which are 5mm high.
The "1B" pods are only 3mm high.
Why not cutting theses pods to get the "1B" configuration ?
This is an easy modification, and surely much more reverseable than the spring clipping (you can put a 2 mm disc to get back to genuine configuration).
Hope this help...

Sorry for my poor language...

A BST-40 evangelist

EDIT : Afterwards, I can see the cap is "1D" stamped at the beginning of the thread (I don't have any closed pipe in the top of my BST-40 carbs), but I don't know how high are the pods inside this one...

guytan screwed with this post 09-25-2009 at 12:33 PM
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Old 09-25-2009, 01:40 PM   #175
motolab
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Quote:
Originally Posted by guytan
I want to precise one thing about BST-40 tuning :
I agree with motolab when he says :
"Clipping the spring increases the spring rate, but reduces the preload."
But...
It's another way you can get the job done.
Mikuni has already thinked about it.
I don't know about your foreign carbs, but in France (I'm a poor frenchy guy, you know), the black cap is stamped either "1A" or "1B".
If you put the black spring guide off the cap, the "1A" show you 3 pods which are 5mm high.
The "1B" pods are only 3mm high.
Why not cutting theses pods to get the "1B" configuration ?
This is an easy modification, and surely much more reverseable than the spring clipping (you can put a 2 mm disc to get back to genuine configuration).
The only problem is that this approach will also change the maximum allowable slide excursion, which I don't think you really want.

Regards,

Derek

motolab screwed with this post 09-26-2009 at 08:27 AM
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Old 09-26-2009, 02:11 AM   #176
guytan
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Quote:
Originally Posted by motolab
The only problem is that this approach will also change the maximum allowable slide excursion, which i don't think you really want.
... Problem which can be easily solved by putting a plastic washer on the other side of the spring guide to limit the slide course, I guess.
I don't really know about it, but it seems to be quite obvious.
If not please tell me, I always learn from your posts.

Regards,
Tanguy
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Old 09-26-2009, 07:30 AM   #177
guytan
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Here are some pics of the caps :

The 1A (left) and the 1B (right)


On the top of them :



The pics are crappy, but I use a phone to get pictures and this magnifier too for the two ugly caps specs ones...
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Old 09-26-2009, 08:30 AM   #178
motolab
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Quote:
Originally Posted by guytan
I want to precise one thing about BST-40 tuning :
I agree with motolab when he says :
"Clipping the spring increases the spring rate, but reduces the preload."
But...
It's another way you can get the job done.
Mikuni has already thinked about it.
I don't know about your foreign carbs, but in France (I'm a poor frenchy guy, you know), the black cap is stamped either "1A" or "1B".
If you put the black spring guide off the cap, the "1A" show you 3 pods which are 5mm high.
The "1B" pods are only 3mm high.
Why not cutting theses pods to get the "1B" configuration ?
This is an easy modification, and surely much more reverseable than the spring clipping (you can put a 2 mm disc to get back to genuine configuration).
It also must be noted that this approach is not the same as clipping the spring in that it affects preload only, whereas clipping the spring affects rate as well.

Regards,

Derek
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Old 09-26-2009, 08:34 AM   #179
motolab
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Quote:
Originally Posted by guytan
... Problem which can be easily solved by putting a plastic washer on the other side of the spring guide to limit the slide course, I guess.
I don't really know about it, but it seems to be quite obvious.
If not please tell me, I always learn from your posts.
Right, but if you put a washer on the spring guide in order to restore the stock slide limit, will the spring preload not also be right back to where it started?

Regards,

Derek
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Old 09-26-2009, 09:35 AM   #180
guytan
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Quote:
Originally Posted by motolab
Right, but if you put a washer on the spring guide in order to restore the stock slide limit, will the spring preload not also be right back to where it started?
Not if the washer let the spring go to the base of the guide.
It's quite difficult for me to explain... Perhaps you can imagine an o-ring glued to the base of the guide -stopping the slide course- but letting only the spring go to the base.

Pfew. The language barrier is sometimes difficult to pass.
I hope you can understand me the way I want...

Best regards
Tanguy
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