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Reformed DucFlogger 09-08-2006 04:00 AM

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.

Max Kool 09-08-2006 04:24 AM

What is the benifit of clipping a few winds from the spring? Having a slide that opens earlier?

Reformed DucFlogger 09-08-2006 04:26 AM

and faster throttle response are the benefits the 'pro' side state.

ChrisC 09-08-2006 06:44 AM

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. :deal

Zerodog 09-08-2006 08:34 AM

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.

creeper 09-08-2006 10:10 AM

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.


Prairie Dog 09-08-2006 10:57 AM


Jeepers, nice explaination!

If you can hear it or feel it - the fuel delivery change that is - it is slower!

potatoho 09-08-2006 11:00 AM

I never got around to clipping springs when I ran a BST40. I did like the idea someone mentioned of using dental floss to temporarily tie a couple of coils together.

One thing which worked out well for me was to use an intake divider on the BST40. It made it quite a bit more responsive off-idle. I liked it a lot, but my install was careless and it ate itself. Hated the carb without it, so instead of building the divider again I started trying different carbs; HSR42, TM40 etc. To be honest, after all this time and effort, I kinda liked my modded BST40 the best.

Max Kool 09-08-2006 11:12 AM

Tell us more about that intake devider, is that the same thing Sommer is offering?

Does it work? Does it work well?

potatoho 09-08-2006 11:42 AM

Nowadays I just use a needle file and I slot the bore. Then I press fit in a thick brass plate. Similar to how I did this on the motor-side of a carb:

It's a lot of work to do by hand. On the BST40 you'd want it on the intake side. A lot of filing and shaping of the part. There's a lot of pressure there so you don't want to do something weak, like simply glueing it. BTDT. It will get sucked in.

potatoho 09-08-2006 11:49 AM

Here was the first one I did on the BST40, which FAILED!

If I do another, I will slot the bore and not just glue it in there like that LOL. Plus the thicker brass. That particular vane actually warped before it failed because it was too thin. But I'll probably use the same overall shape. Be careful!

meat popsicle 09-08-2006 11:52 AM

Jeepers Creepers :rofl

Max Kool 09-08-2006 12:03 PM

Wow nice work, but what does it do, and how well does it do it?

Zerodog 09-08-2006 12:13 PM

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....

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?

potatoho 09-08-2006 12:34 PM


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 :lol3

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