Yeah - what Brian said. Constant velocity of air as in v=w/(rho*a), where
w is mass flow, rho is density (or, v=q/a, where w/rho = q, volumetric
flow) and a is the area of air passage. As the q (or w/rho) increases with
demand, the piston varies "a" to keep the velocity constant.
Ron Soave
'61 Bugeye
----------
> From: Brian Evans <brian@uunet.ca>
> To: jphender@inav.net
> Cc: spridgets@autox.team.net
> Subject: Re: SU carb dashpot function
> Date: Wednesday, November 05, 1997 11:01 AM
>
> See comments interspersed below...Brian.
>
>
> At 09:47 AM 05/11/97 -0500, you wrote:
> >
> >Hello all,
> >
> >I just re-assembled the first of my two SU2s last night and am a bit
> >confused as to how it functions. This is what I THINK I understand:
> >
> >There are two methods for getting more fuel into the air channel. One
is
> >to pull down on the jet (ie choking) and the other is to pull the needle
> >out of the jet (ie acceleration).
>
> Yes to both points. The SU is a 'variable jet', 'constant velocity'
> carbatooter. That means that the carb tries to keep the velocity of the
air
> in the carb throat the same, hence the vacuum signal to the jet the same,
> and varies the size of the jet with the needle to meter the fuel to match
> the air flow volume.
>
> >
> >Questions:
> >
> >1. How does the needle get pulled out of the jet?
>
> A vacuum is created as the air rushes through the bore of the carb into
the
> engine. This vacuum is applied to the upper side of the dashpot, causing
it
> to rise in response to engine air volume demand (not throttle plate
> position). You can have the throttle plate wide open, at low rpm, and
the
> piston/dashpot will only rise a little, since the engine isn't using that
> much volume of air. Conversely, the engine can be at high rpm but with
the
> throttle plate closed, and the dashpot will still be almost closed
because
> the throttle plate doesn't let the engine have much volume of air.
> >
> >2. If you fill the dashpot "to the threads", won't the oil drain into
the
> >engine?
>
> Yes, but who cares ;) This is what I think of as an auto-level-setting
feature.
> >
> >3. What engages the piston lift-pin?
>
> The pin pushes up on the piston/dashpot. The idea here is that the idle
> mixture is correct when a little more fuel doesn't either bog the engine
> (idle is already too rich) or cause the engine to speed up (idle is lean,
> and wants more fuel). the idea is that the lift pin acts only on the
piston
> and needle to give more fuel, while air is kept constant because the
> throttle plate hasn't moved.
> >
> >4. Shouldn't there be a gasket on the dashpot cover?
>
> No. The machining is to a high quality of fit, so no gasket is needed.
> Also the fit of the dashpot within the cover is critical, as the
tolerance
> must help to create a vacuum bleed that matches the spring and the mass
of
> the dashpot to govern how high the dashpot lifts in response to the
engine
> vacuum.
> >
> >5. WHAT MAKES IT GO?
>
> MAGIC!
> >
> >Thanks in advance for what I'm sure will be another great lesson!
> >
> >Jim
> >
> >
>
> You tell me if it was a great lesson!
>
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