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Dashpot springs

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MGretired

MGretired

Senior Member
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Anyone ever hear this before? At our last club meeting, a member who has a few TR2's & Tr3's said that the Triumph red book for the TR2 & 3 with SU carbs recommends you take the dashpot spring out if you are operating above 4000 foot altitude. Never saw this in any MG book...
 
no, me neither - they usually recommend a different (leaner?) needle for high altitude. I suppose the piston would rise much higher with no spring but what would be the effect on the mixture? Leaner because the air valve is much more open? The acceleration enrichment when you snap the throttle open would probably be lost totally.
 
Hello EJN,
the mixture should remain the same as the piston rises it lets in more fuel. Also it is not the spring that gives the enrichment but the damper, as it slows the pistons rise when the throttle is opened.

Alec
 
Many SUs made during the Classic-era(1925-1948) don't use springs at all (actually, some don't use oil either! They just use heavy pistons.)

Alec is right, the whole theory of constant velocity venturi carbs is when the piston rises, the venturi area is increased, which as you physics fans know, causes the fuel to be sucked in...since the needle is tapered, the amount of fuel allowed in meets the measurement of air in the venturi area to maintain a constant air-to-fuel mix.

I can understand that removing the spring would make sense, as it simply helps with the return action. In higher altitudes, this hypothetically would be redundant. I'm not sure it would hurt to leave it in - at least in practicality, but hypothetically, I can see that the theory might be the piston would drop too quickly after throttle close leaving just a momentary lean condition. (We're talking a small fraction of a fraction of a second duration here.)

Just noodling out loud. /ubbthreads/images/graemlins/wink.gif
 
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