On Thu, 24 Jul 1997 11:13:41 -0400 "Jason F. Dutt"
<simjason@ix.netcom.com> writes:
>Yup, it's true. You really can put a lot of different things in there
>for
>different applications. I know some folks use Marvel Mystery Oil.
>Some
>use Motor oil of varying weights. Some folks, including myself, use
>automatic transmission fluid. If you think about it, the lighter the
>weight, the faster the pistons will rise/fall. Honestly, I couldn't
>tell
>you exactly how this affects how the car runs. I'm sure someone on
>the
>list could enlighten us. You just have to find out what works best
>for
>you.
>
>> From: John Steczkowski <stecz@pswtech.com>
>> To: MG Mailing List <mgs@autox.team.net>
>> Subject: dash pot oil
>> Date: Thursday, July 24, 1997 10:38 AM
>>
>> I was reading in the haynes manual that I just got that motor oil is
>ok
>> to use in the carb. ('75 MGB). Is that true? Any drawbacks to
>regular
>> motor oil over official dashpot oil?
>>
>> --
>> John Steczkowski
>> PSW Technologies, Inc.
>> 512-343-6666
>> stecz@pswtech.com
>> http://www.pswtech.com/~stecz
>> http://www.pswtech.com 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
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