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Old 09-08-2006, 04:00 AM   #1
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Clipping BST40 Spring?

Other than bogging in woops, what are the drawbacks, if any, of clipping x coils of the BST40 needle spring? Index chatter implies two polarized camps of riders on whether to do or not, but no real reasons given other than bogging as why it shouldn't be done. Just curious.
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Old 09-08-2006, 04:24 AM   #2
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What is the benifit of clipping a few winds from the spring? Having a slide that opens earlier?
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Old 09-08-2006, 04:26 AM   #3
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That

and faster throttle response are the benefits the 'pro' side state.
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Old 09-08-2006, 06:44 AM   #4
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One side says "don't clip the spring 'cause then it will bog even worse over the whoops" and the other group of miscreants shouts "clip the beejezzuz out of it and it will pop a wheelie like an FCR".

A clipped spring gives the slide a little head start over the OEM spring on throttle opening. So the reality is that both camps are slightly right. It's how "slightly" that's at issue. Clip away.... you can always get a new spring.
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Old 09-08-2006, 08:34 AM   #5
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I think the bogging is mostly a float height thing. If it is too high it floods it making the engine bog. I had this problem, reset the float and now the problem is almost completely gone. I think I will try trimming the spring and see what it does for me. I don't believe that it will make it bog.
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Old 09-08-2006, 10:10 AM   #6
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Clipping a spring...

When you clip a spring, you change it's length and rate.

My lengthy history with altering spring rates on SU CV carbs tells me this. A weaker spring allows the slide to rise faster (not to a higher final position, as the air pressure overrides the spring pressure) while a firmer spring allows the piston to rise more slowly... all relative to throttle position.
Again, these are transient movements under acceleration... steady throttle piston position would remain the same as positive and negative air pressures easily overcome the spring, once stabilized.

When a piston rises faster, for a given throttle position, the needle goes with it and you have a larger opening for fuel thru the needle jet. Does the fuel delivery keep up? Initially, no, it does not, and you have a lean condition... a flat spot if you will.
Until negative pressure rises to match venturi area, fuel delivery "stumbles".

The opposite occurs when a springs pressure is increased; air volume is reduced as the slide is held in a lower position for a given throttle opening... but pressure is high and you get excellent fuel delivery... perhaps more fuel than needed for the volume of air being delivered to the engine, and you have a rich condition.

So tell me... what is the net effect of clipping two coils off a spring?

The design of a CV carb permits the omission of an accelerator pump because in theory, you almost can't deliver too much air to the engine... the vacuum piston prevents it by maintaining high pressure in the venturi, "in balance" with the throttle position.
If you whack the throttle open, the piston will rise at a rate slow enough to allow adequate pressure to be maintained with the resultant steady and hopefully matching fuel delivery.
Allowing the slide to rise too quickly would be the rough equivalent of turning off the accelerator pump on an FCR carb... a terrible lean bog, and perhaps even a flame out.

Your wrist is connected to a throttle plate that follows your directions to the letter, even if they're wrong. The vacuum piston is there to dampen the wrong directions and smooth over your mistakes relative to air volume and pressure.
On an FCR, or similar carb, instead of a vacuum piston to limit excess air volume as on a BST, you have an accelerator pump squirting lumps of fuel to offset that excess volume.

So... if you didn't know this stuff before, now you do... so go ahead and whack away on them springs.

C
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Old 09-08-2006, 11:52 AM   #7
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Old 09-08-2006, 12:03 PM   #8
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Wow nice work, but what does it do, and how well does it do it?
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Old 09-08-2006, 12:34 PM   #9
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Quote:
Originally Posted by Max Kool
Wow nice work, but what does it do, and how well does it do it?
[Regarding bore divider] On the two stroke carb it doesn't do crap. On a flat-slide carb it alters the air velocities, which kinda feels like having a quasi-reduced bore; i.e. it can make a 42mm feel similar to a 40mm at part throttle. On the BST40 it made the off-idle very torquey and responsive, otherwise not much.

I don't know really what it is doing on the CV carb. Maybe it is similar to a weakened spring, in that it allows the slide to move a bit quicker. It got rid of a lazy feeling to the motor at low rpm and gave it more chug. I didn't do a whole lot of jetting changes on that carb. Now that there are some needles available, that's probably what I would try.

Increasing the atomization seems to work well on this motor too. I installed a yost power tube on my 42mm flat-slide and it was noticeable right away. Really cool product. Made it run better. Not available for BST40 though.

Darn, you are making me want to hack up my LC4 again LOL. I was thinking about selling/donating it to make room for a '07 TE510
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Old 09-08-2006, 12:39 PM   #10
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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, 12:13 PM   #11
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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?
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Old 09-08-2006, 01:31 PM   #12
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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   #13
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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   #14
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Creeper is (almost) the only guy here old enough to have a working history with Skinner's Union...
Almost...
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