what does the dash pot do?

[anaxemurderer]

What exactly does the dash pot do on my su carbs? As i understand it operates as a damper for the needle. Why did unscrewing the dash pot top whiles the engine was running lower the revs and subsequently upset running? Topping up dash pot and unscrewing whilst engine stopped has sorted it but i am curious!

Nick
3.5 V8 bobtail rrc

[thermidorthelobster]

IIRC when you unscrew and withdraw the plastic cap it pulls the whole piston assembly upwards and hence pulls the needle right open.  So it'll overfuel like mad.

[Rossko]

The damping also produces a accelerator pump effect. Other carbs have a physical pump to add a dollop of extra fuel when you floor the pedal. SU & Stromberg have the dashpot to slow down the piston movement and give an equivalent temporary fuel richening. Elegant solution :)

cheers, Ross K

[greasemonkey]

You just need to keep the oil topped up otherwise they do stop working right though
Cheers
Steve

[anaxemurderer]

Yeah thats roughly what i thought and makes a lot of sense when i think about the symptoms. They don't normally cause problems it was just because they were unscrewed whilst it was running, must have displaced some oil.

Cheers

Nick

[Range Rover Blues]

Apparently SUs should have very little oil in them, they work ok with almost none, they work great when full, but half full they run rough :?

Used to tinker with A series a lot when I was younger.

And yes, basically the damper (or damper rate) controls fuel enrichment during throttle opening (transience) to prevent missfiring as you clout the loud pedal.

[Rossko]

Used to tinker with A series a lot when I was younger.

Y'know I almost mentioned the old "tuning" tweak of putting in thicker oil for more accel pump on A series!!  I think the advice is not to do that on V8 though, you shouldn't want to if the original design was right.

Someone knowledgable will remind us of the correct oil for V8 SU & Strombergs in a bit ...

cheers, Ross K

[DEANO3528]

Over the years I think I've heard most of them, from 3 in 1 through straight 30 and ATF to 90 weight!
Truth is, they are supposed to run 20/50 engine oil and they run well on it.
You'll soon know when the oil level is dropping in the dashpots because the motor starts spitting back when you floor it as the resistance to the vacuum lowers.

[anaxemurderer]

Always thought they wnated thinner oil than 20/50, will know for the future though. She is back to running very nice now!

Nick

[Range Rover Blues]

SUs run fine on whatever the engine drinks, stroms are fussy and I know of a couple of applications where a thinner monograde was preferable.