I have always (30 years) used engine oil as recommened in the manuals and
never had any problems, winter or summer. The grade of oil should be the
same as for the engine i.e. taking local climatic conditions into account.
The workshop manual specifically warns against using light oils as they
cause fluttering of the piston.
The damper may well stop the piston flapping up and down when going over
potholes, but that is not its main function. Its main function is to supply
the correct (rather than enriched) mixture when the throttle is opened. You
see, when the throttle is opened both air and fuel have to accelerate their
flow in order to get more fuel/air mixture into the engine. But air is much
better at accellerating than fuel as it has a lower mass. Therefore there
is a time when the engine is getting increased air but not increased fuel,
which results in a weak mixture, causing hesitation. The damper causes the
causes the air and fuel to accellerate more gradually, therefore the rate of
accelleration of the fuel can keep up with the air and the mixture remains
'correct'.
The viscosity of the oil controls the rate at which the piston rises when
the throttle is opened. A thick oil will cause the piston to rise slowly,
possibly delaying accelleration, a thin oil will cause it to rise more
rapidly, possibly causing hesitation.
Because the piston and needle are connected together, the piston controls
how much air gets in to the engine just as the needle controls how much fuel
gets in. The shape of the taper on the needle controls how much fuel gets
in, and hence the mixture, at various positions of the piston.
PaulH.
-----Original Message-----
From: Bill Schooler <schooler@erols.com>
To: Csooch1@aol.com <Csooch1@aol.com>
Cc: mgrick@ptd.net <mgrick@ptd.net>; mgs@autox.team.net <mgs@autox.team.net>
Date: 17 December 1998 12:43
Subject: Re: [Fwd: MG Q&A]
>Csooch1@aol.com wrote:
>>
>> Listers,
>> I personally would not use anything that thick(20w50,10w30) in the
dashpots.
>> Throttle response will be horrible until the carbs come up to temp. and
the
>> oil thins out. I have used Marvel Mystery oil in every SU, stromberg,
dashpot
>> type carb, and have never had any problems since it is pretty thin. You
can
>> get this red oil at most hardware or auto stores.
>> Cheers,
>> Chris
>> BJ8...many,many pieces
>
>Chris,
>
>Could you explain a bit more about how and why the viscosity of the oil
>used in the dampers has anything whatever to do with "throttle
>response"? My understanding (perhaps mistaken?) is that the function of
>the oil in the carb dampers is to provide for mixture enrichment upon
>throttle opening (an accelerator pump function) and that the the height
>of the piston in the SU carb affects only the mixture, not the amount of
>air/fuel getting into the engine.
>
>(I did switch to 5W-30 when operating an MGB in Montana and North Dakota
>winters, but have never sensed the need to use anything of lesser
>viscosity in the dampers.)
>
>--
>*Bill Schooler *Check the MGCC Wash DC Centre Web Page
>*Woodbridge, VA *http://members.aol.com/mgccwdcc/
>*schooler@erols.com
>*53 TD
>*60 MGA
>*69 MGB/GT
>
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