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Old 12-05-2007, 12:02 PM   #1
joekanter OP
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Caborater Q for the 640A-please treat the noob gently-

-high folks. I'm in the process of getting a 2007 KTM 640A. This question may not be KTM specific. Living out here in North West Nevada, my base altitude is 4500ft. The trails go up from there. Top out to 7400ft, or even higher. I know very little about jetting and carb adjustments. Yes-I have gone through the index and have read up on all the techniques. So, 1st question-is a carb adjust and jetting the same thing? If not, what is the difference and why? Also-if the carbs need to be adjusted for altitude-how difficult is that to do and what is the range (IE, alt range) that can be adjusted for. I thank you guys in advance for the information and being gentle. LOL.
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joekanter screwed with this post 12-05-2007 at 12:06 PM Reason: spelling error
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Old 12-05-2007, 12:45 PM   #2
ChrisC
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First step... go take the weekend and re-read the "Index Thread". More there on the BST40 than you'll likely ever need.

The BST has only one externally adjustable circuit, the idle fuel screw. It's located under the the carb on the front side. It's a bitch to get to unless you have popped for one of Colin's special screws http://www.stenhouseracing.com. Typically it is set at 2.5 turns out from seated, but your numbers may vary.

My '03 is still running OE jets (152.5 main - 42.5 idle air - 1.2 pilot) and since I run from 5,000 feet and higher (11K+). I have never felt the need to fatten it up. Some folks have used mains as large as 175+, but I think that's waaay overdoing it.

As an aside, being a CV (constant velocity) carb, the BST tends to compensate for altitude as part of it's design...

So, at your altitude I wouldn't be in a big hurry to add additional fuel via up-jetting it...

p.s. it's "carburetor"
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Old 12-05-2007, 02:18 PM   #3
meat popsicle
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don't worry joe, Chris was being gentle; rough is not answering...

So you walked thru the index and read up on the carb, and particularly in creeper's setup thread right? To me jetting is one of the modifications you can do to your carb to set it up; adjusting is something like turning the air/fuel mixture screw that ChrisC referred to above. But it doesn't really matter does it? You may change jetting and adjust settings (such as the valve needle) in your quest to improve engine performance as best you can.

Since the BST is supposed to be fairly lean from the factory then perhaps your experience will be similar to ChrisC's since he also lives at altitude and the higher you go the less lean it will be because there is less air (pressure) to thin out the mixture.

From the factory the stock exhaust can is fairly restrictive (you read this in the index right?) and the supertrapp disc adjustment cannot compensate for restrictions in the internal baffles so some modify the stock can and some get an aftermarket can. Probably many do not... but if you do change this part of the system you will likely have to setup your carb to suit the less-restrictive exhaust.

ChrisC is also giving you good info on altitude tolerance for the BST, but the best thing to do is to try it in your rides and if it misbehaves then come back and tell us what you experienced (details are important) and we'll try to help.
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Old 12-05-2007, 02:18 PM   #4
dhally
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I ride my 640ADV with the BST carb from near sea level to 4-5000 feet routinely. Once I got the correct main jet installed, all I ever need to do is turn the idle mixture adjustment screw about 1/4 turn for large elevation and/or temperature changes.

When I went to the top of Steens Mtn (9500 ft) I did feel a drop in power.
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Old 12-05-2007, 04:12 PM   #5
joekanter OP
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Thanks guys---

---for responding. I am such a noob, I fart in my own general direction. With that being said, I, like Sgt. Schultz-know nothing. I am starting from scratch. I have gone through the thread index and to be honest, I am overwhelmed. Now that I have read the responses (thanks Chris for being gentle and correcting my spelling error), I'll go over the index again and see what I learn. Master Popsicle-your articles are brilliant. They tax my small brain but I am learning. Thanks again guys.
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Old 12-06-2007, 01:10 AM   #6
bmwktmbill
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Hiya Joe and welcome,
I am riding an '02 Adventure with the stock BST carb, stock jetting even though I modified the air intake and the exhaust.
I would go into my modifications because your cylinder head is different than mine.
I have what's called the low flow head.
Yours, like ChrisC's, has the high flow head.
The change came in '03.

Read what Chris had to say, he is giving you the truth about the BST carb.

My travels took me to the KTM factory in Austria this summer(at Mattighofen).
The factory front door is about 40 miles from the Austrian Alps so I had the opportunity to ride in them both on and off road.

It isn't said much but the 640 engine is designed to haul ass up, down and around in any mountains.
The Alps and KTM's test riding are two of the reasons why KTM makes such good motorcycles for world travel.

Yes they win races in the desert but in the mountains on tight roads the Adventure and the Supermoto are a match for anything because the engine is always in the power.

Keep the revs above 4000 rpm and hang on.
I think you will be surprised.
A couple a simple modifications and you will be amazed.
bill
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