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Old 09-08-2006, 11:10 AM   #6
creeper
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Joined: Dec 2003
Location: Puget Sound
Oddometer: 10,718
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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