Rick Morrison wrote:
>
>
> O-
>
> The dashpot oil in the carbs controls the RATE at which the piston
> rises
> as the throttle is opened. A lighter oil (ATF, 5W motor oil, sewing
> machine oil, etc) will allow the piston to rise at a faster rate than
> a
> heavier oil (50W motor oil, etc).
> The piston spring controls the ultimate height of the piston at a
> given
> throttle opening.
> In fact the whole system is not unlike the suspension system on your
> car. The spring controls ride height, and the oil (in the shocks)
> control
> the rate of movement.
> Using a heavier oil in the dashpots will give a greater accelerator
> pump
> effect. When the throttle is opened suddenly, the flow of air across
> the
> jet begins to increase immediatly, but by slowing the rise of the
> piston,
> the effective venturi is held smaller for a longer period of time.
> This
> increases the speed of the air, and lowers its pressure across the
> jet.
> Because the increase of fuel flow at the reduced pressure is not
> linear
> with the air flow, a slightly richer mixture is obtained. This is the
>
> same effect as an accelerator pump on other carbs. The mixture in the
> SU
> will return to "normal" as soon as the piston finds equilibrium (ie,
> the
> vacuum above the piston is equal to the sping pressure).
> You can use this trait to help tune the carbs. If you find that your
>
> car tends to bog when the throttle is snapped open, and other causes
> are
> eliminated, it may be that it needs a bit less accelerator pump. Go to
> a
> slightly lighter oil. Conversly, if it spits back like a lean mixture,
> a
> bit more pump action may be called for, hence a heavier oil
>
> Sorry, I rambled on, but wanted to get my 3 cents worth in
> Rick Morrison
> 72 MGBGT
> 74 Midget
Well said, Rick. That was a very good 3 cents worth.
Bud Krueger
52TD (20 weight oil)
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