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Old 08-25-2007, 11:57 AM   #1
metaljockey OP
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640 overflow problem

I have fuel dripping from the carb overflow. It never drips while stationary though. Only when I'm riding. It turns into quite a little stream when I'm riding hard.

My first question is: is this indicative of too high a float level?

My second question is: if I want to adjust the float level with the carb in situ, by blowing on the fuel inlet line, should the fuel cutoff still be where the float is parallel to the carb body?
I know that it should be parallel when touching the needle when the rotation method is used. I'm just not sure whether cut-off and touching is close enough together to give the same result?
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Old 08-25-2007, 12:16 PM   #2
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MJ...

Get thee to the INDEX THREAD:

http://www.advrider.com/forums/showthread.php?t=88680
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Old 08-25-2007, 12:53 PM   #3
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I did check out that thread. It didn't answer my question re the difference in shut-off and touching the needle though.
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Old 08-26-2007, 12:51 AM   #4
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Hey metaljockey, (spoken in my best South Afrikaan) if you let the rest of us young springboks in on what model carby you have, we will gladly throw our opinions in your direction otherwise you will lose your diplomatic immunity and be forced to wear a rather large shipping container on your head.
Is that alright with you kaffer?
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Old 08-27-2007, 03:11 AM   #5
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Sorry, that will be the venerable BST40
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Old 08-27-2007, 03:20 PM   #6
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Quote:
Originally Posted by metaljockey
Sorry, that will be the venerable BST40
Does it bog when hitting jumps or large woops?
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Old 08-27-2007, 10:12 PM   #7
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Nope.
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Old 08-28-2007, 12:36 PM   #8
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Ok, I pulled the carb and the float level was way high. I adjusted it as per manual and now I have a bog off of closed throttle and from idle. And the leaky overflow problem is still there. I fkn hate it when this happens, you try to meddle in shit you dont understand to sort out one problem and suddenly there are new problems lining up to piss you off.

I'm gonna have to pull it again to try and set it as it was.

I was surprised to find that the needle appears slightly bent?
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Old 08-28-2007, 04:29 PM   #9
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How many KMs on the bike? I've had to replace my float needle and seat twice after leaking issues. I seem to get 15,000 - 20,000 KM out of a set and then they begin weeping. Once this happens your fuel level in the float bowl will be erratic causing the engine to run rich and making it near impossible to tune properly. Also check to see if the o-ring in the float seat is doing its job.
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Old 08-28-2007, 10:13 PM   #10
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It's at 11000k's. Thanks, I'll check the needle and seat too.
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Old 08-29-2007, 02:58 AM   #11
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Patience grasshopper........

I bent the needle on my BST when frigging around with jetting and adjusting things. If you are careful, try straightening it in some 'soft jaws' in a vice. Thats what I did with no probs. The bog of idle may be a number of things but I would start with making sure the float adjustment is right first. I never tinkered with my floats from new and I always had a flat spot on big bumps and jumps, regardless of jetting changes/adjustments.
This is what I had as far as jetting and other settings were concerned, when I was running the BST 40. This is for sea level to 4000ft elevation and 50 to 99% humidity; my riding area basically:
Main jet # 157.5
Pilot jet # 45
Stock needle; clip set on 4th from top
Fuel mixture 2 and 1/4 turns out
Here are some piccys; firstly, a bottom view of the BST and the fuel mixture screw indicated by the screw driver tip. Thanks to my son Zach for holding things while I snapped away
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Old 08-29-2007, 03:04 AM   #12
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BST from 2005 model 640

Here's an exploded view of the needle 'arrangement'. Make note of the small white synthetic washer mounted on the needle itself. It is a very tight fit so be careful sliding it down to adjust the clip position as you can bend the needle in doing so !
Also, note the small metal washer that fits between the needle and inside the slide.
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Old 08-29-2007, 03:06 AM   #13
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Hereis a piccy of that synthetic washer that is incorrectly mounted too far down the needle...........
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Old 08-29-2007, 03:07 AM   #14
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Here is the way it should be............pressed hard up against the clip.
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Old 08-29-2007, 03:13 AM   #15
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I am not a big fan of cv carbs so I opted for the Keihin FCR 41mm MX carb, wacked a sexy Akro pipe and muffler, opened up the airbox some more and turned the 640 from a mild mannered cruiser into a fire breathing arm wrencher!
Hope this helps some.
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