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Old 09-08-2006, 12:39 PM   #16
Speaker
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hrm, DR650s use a BST40 carb as well, i wonder if we could do this mod
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Old 09-08-2006, 01:31 PM   #17
creeper
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Quote:
Originally Posted by Zerodog
Nice explanation creep! I will clip 2 and see what it does. It might help my power wheelie quest or it could hinder it. I will find out and post the results. If it sucks I guess I will have to get a new spring. I just like the instant response of my dirtbike. The BST seems like it lags just enough to not get the pop needed for power wheelies....

Creeper.
On the SU carbs did you notice an improvement or a problem by clipping the springs? And have you tried a shorter spring on your 640?
There are several springs available for SU carbs... they are rated in ounces and tenths of an ounce. Understand that an SU does not have a diaphragm, but the piston is a close tolerance fit in a "dome" in which is slides up and down.
We never clipped springs.

Because of the metal slide, a much stiffer spring was used to slow it's upward momentum... that or in vehicles where rapid acceleration was less an issue, an oil filled damper, much like a very rudimentary shock absorber.

SUs required a great deal of maintenance keep working right... mostly because everything was metal to metal.
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Old 09-08-2006, 03:30 PM   #18
ChrisC
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Quote:
Originally Posted by creeper
There are several springs available for SU carbs...
Creeper is (almost) the only guy here old enough to have a working history with Skinner's Union...
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Old 09-08-2006, 05:19 PM   #19
creeper
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Quote:
Originally Posted by ChrisC
Creeper is (almost) the only guy here old enough to have a working history with Skinner's Union...
Almost...
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Old 09-08-2006, 07:58 PM   #20
Greg Podlaski
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spring

Clipping two coils off the spring will increase its spring rate causing the slide to open even slower.Get a Fcr
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Old 09-08-2006, 10:31 PM   #21
rjf
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Sommer KTM sell a different spring and a slide with bigger "transfer ports".

They only recomend it for the DUke IIRC.

I'll try and find a reference, but my german Suck's, don't hold ya breath for a part # from me.

I'm thinking I'll leave the spring alone and drill the ports on mine.

After I lower the float height a smidgen, as I thought I cured the "BST bog" when I did it last time, but a few laps of an mx track made it show up again
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Old 02-11-2007, 07:31 PM   #22
Flyin Brian
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Ok, so I've been thinking of this bog situation and I had this thought.... I might be way off here as I'm not an expert by any means when it comes to carbs....
I've read through most of the bogging threads and it seems that the float height adjustment improves the bogs situation, but not completely. This baffles me. I started thinking along the lines that the float gets to bouncing up and down when going through whoops or rocks (as mine does when it bogs). Based on the oscillation thought it got me thinking of airplanes (my work). When we change any of the flight controls we have to go through a lot of flutter analysis in order to find a balance of correct feeling at the control stick and not having an aileron or elevator start to shake it's self to pieces. The aileron and elevator have counter weights to help with this.

So my thought is what if one was to add weight to the tip of the floats? Wouldn't that slow the float movement down or help reduce oscillation? Of course you'd have to find a new sweet spot as far adjustment of the float goes.

I don't know if this will work or not, or even if I'm on the right track, but I've been thinking about this for a couple of days and I had to get it out for discussion. My bike is totally taken apart right now for maintenance so I haven't tried this. Any feedback would be appreciated.
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Old 02-22-2007, 09:20 PM   #23
potatoho
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My recent thoughts regarding bog is that the main jet isn't shrouded properly. One carb I have has some strategically placed fuel cell foam which shrouds the main jet area. The idea is that it dampens any splashing going on and prevents the main jet cavity from going dry.

So probably what I would try, is take some of the yellow fuel cell foam and make a sort of shroud which fits over the main jet and seals against the bowl when you assemble it. I would probably make a washer shaped piece with a 1" hole and thick enough so that when you tighten down the bowl it stays put. That's what they did on my PSI carb and that was the explanation of its purpose.
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