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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