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Old 06-08-2006, 10:43 AM   #1
rob748 OP
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Talking 640 Carb Parts And Bogging "UPDATE" !!!!!!!

BIKE
05 640 120000 miles.smog off, stock exhaust cored, re jetted, slide drilled, larger main jet, ran great For 6000 miles after that.
PROBLEM
So l am having the bogging problem on jumps/hills and l am trying to sort it out ( read all the threads in the index)
l understand some people have this problem and some don't which l do not under stand, the only thing l can think of is the dirt that has gotten into the top (worn slide) as l never had the problem when new.
took the carb apart to inpsect, the slide is grooved alot so thinking about replacing it with the diaphram,spring and needle
dealer quotes
$38 for the spring
$33 for the diaphram
$9 for the needle
$38 for the slide
so $118 and it might fix the problem
any body have a cheap sorce for these parts ?
will check the bowl level tonight and t vent the carb and reroute the filters so they do do let all the dirt in like before.
Reading all the posts l have not seen anyone that has fixed this problem without going to the FCR $$ unless l missed it but at this stage l am on the fence, do l replace the parts $118 or drop the $$ and get the FCR
l like to ride the bike in all terrain and treat it like a 250 on the trails, the bike is great but this problem is killing me, last weekend l tried to climb a big hill and when l got near the top it got rough and the bike cut out so l had to lay it down rather than watch it tumble down the hill. then l had to slide it down the hill on its side over rocks and other crap (tank looks very pretty at this stage). next time it happens l may not get so lucky so l need to find a fix for this problem
HELP
so after finding the parts at cost (thanks pete ) and T venting the carb the problem is fixed.
First l T vented the vent line that went to the canister at the back and this made a huge difference, also l moved the little filter to under the tank to stop the crap getting inside the top and then new carb spring ,slide and diaphram it is back to what it felt like new and then adjusting it a little it runs great.
LONG LIVE THE 640

rob748 screwed with this post 08-21-2006 at 12:48 PM
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Old 06-08-2006, 10:57 AM   #2
Drif10
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That avatar of yours precludes me helping you at all.
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Old 06-08-2006, 11:16 AM   #3
ChrisC
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Quote:
Originally Posted by rob748
BIKE
05 640 120000 miles.
My guess is you need a new engine....

But seriously folks, the "Mystery of the Bog" has yet to be fully explained. Some contend it's a g-out condition suffered by the vacuum controlled slide, others are in the float level camp. Some have fled to the dark side and installed Beelzebub's carburetor, the much acclaimed Keihin FCR and no longer have an issue.

As for me, I ride slower than Creeper so luckily enough, I have yet to experience the dreaded "bog".

CC
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Old 06-08-2006, 11:45 AM   #4
rob748 OP
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G out

its not like lam doing big double jumps, it is happens on small stuff and even climbing hills with little rock jumps on them
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Old 06-08-2006, 10:17 PM   #5
Tholund
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I think you are on the right track...it's probably just time to go thru and do a good cleaning and install a rebuild kit on the carb. If the bike ran fine for a long time...then obviously it's something that degraded...rather than a functional issue. Restoring the conditions should restore the performance.

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Old 06-09-2006, 08:11 AM   #6
humdinger
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My '05 did the same thing since new but gearing down to 15 tooth front and a little tuning (valves, air box, exhaust and jetting) the 'bog' is reduced down to approx. 2800 rpm.
Drop her back a gear or keep revs above this and problem 'bog' gone. Had an XR 600 with a flat slide Mikuni that was the same. Good revs and progressive feeding of the throttle over jumps or rough stuff helps alot. They're not a motorcross carby, not designed that way. I have found weight reduction is helpful also.
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Old 06-09-2006, 08:19 AM   #7
rob748 OP
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bogging

when l say bogging l don't mean it is a little slow down below, it bogs enough to send you over the bars and it last for a few+ seconds and even than it is not great for a minute or two.
done the tooth down ,airbox etc and it was ok for 12000 miles, it did it a couples time s before but nothing like it is now
how many miles on your 05 ?
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Old 06-11-2006, 12:33 AM   #8
humdinger
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I've only got 3000kms on mine. Whats it like when idling and you quickly blip the throttle? Sounds like fuel delivery at a guess. Have you checked the fuel pump in the tank and all fuel lines,taps etc? When mine bogs, its only very mild but enough to drop the front end over jumps and annoy hell out of me!
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Old 06-11-2006, 06:56 AM   #9
gunnerbuck
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Rob, perhaps it's an electrical problem, when you hit some rough stuff it activates a short and cuts the spark. Wiggle some wires when it's running and see what happens. My bike had a break inside the keyswitch that caused me all kinds of grief. Good luck.
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Old 06-12-2006, 05:51 AM   #10
rob748 OP
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bogging

went out saturday and it was fine until l started jetting airborne then it just got worse, the rider behind me said he could smell fuel
ordering the carb parts this week and l will see what happens
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Old 06-12-2006, 06:44 AM   #11
holycaveman
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Quote:
its not like lam doing big double jumps, it is happens on small stuff and even climbing hills with little rock jumps on them


Obviously you guys havn't ridden with rob748!
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Old 08-21-2006, 12:47 PM   #12
rob748 OP
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update

see top
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Old 08-21-2006, 10:50 PM   #13
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Cool2

I think I have solved the bogging problem......at least for the most part. My bike bogged really bad on rocky trails and got worse the steeper the trail got. It would bog on jump faces. Not big jumps just water bars and such. Enough to make me kiss the bars a few times. After a couple of rides I said fuck that and started looking for a fix. So I read all of the stuff here and thought about it for a while. It was my float height. It was about 1/4 of an inch off. How can the float be 1/4 inch off you say????? It is easy with the BST carb. The float seems extra finicky while trying to get it to just touch the needle. If you tip the carb back a little to far you pull the needle out of the seat and trick yourself into thinking you are just touching the little plunger. When I did this the float seam looked parallel to the carb. Make sure the needle is in the seat and.......the float prong is just touching the top of the springloaded plunger but not depressing it. Make sure at this point the float seam is parallel to the carb body. I am 90% sure this is the problem. Maybe they come from the factory hosed up? Maybe it is bad adjustment by someone at some point? After I fixed this my bike doesn't bog on rocky trails at all anymore. And almost never does on the jumps but it still does a little once in a while. I think I am going to drop my float another mm to see if it helps more. But I think I still want an FCR because of the instant throttle. It will act more like my dirtbike then. And hopefully wheelie better on the street.

This could also mean that you need a new needle and seat . It might not be shutting off. Does you bike smell gassy when just sitting around with the gas on?

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Old 08-21-2006, 11:10 PM   #14
rjf
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Mine was bogging as you described. It's much better once I adjusted the float height. Does it slightly every so often, but its much, much better.

Actually what zero dog said goes for my experience as well.
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Old 09-21-2006, 10:13 AM   #15
meat popsicle
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I am going to rebuild my BST

fuel economy is gone to hell, which might be partially due to the puddle that I get from the overflow when the bike is off - sometimes even when the petcock is off

Anyways, Sudco says no rebuild kit for the BST40, waiting to hear from CarbParts... but I am thinking I will have to pull the carb and then see what she needs, order - wait

Perhaps with the rebuild I will sneak in a smaller pilot fuel jet to see if off idle throttle response improves... well, not that it is bad for me but I was always curious if this trick of Dellorto's works.
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