Rather than inserting another opinion, I think I'll cite F.E. (Chip) Old
from his "Back to Basics" article on carburetion as found in The T
Series Handbook (an excellent reference work that not only covers
specifics of the T Series MGs, but provides very clear explanations of
the mechanical/electrical functions applicable to all our cars.)
Quote
> To provide the richer mixture needed for rapid accelleration, most SU
>carburetors have an arrangement sort of like a crude tubular shock absorber
>which slows down the rise of the piston when the throttle is opened quickly.
>On all SUs, whether or not they have this "shock absorber", the piston is kept
>centered in the suction chamber by a tubular extension called the piston rod,
>which sticks up from the top of the piston, The piston rod slides up and down
>inside a corresponding tubular bore inside the suction chamber. The hollow
>piston rod is filled with oil, which lubricates the bearing surface between
>the rod and its bore. When the engine manufacturer determines that a richer
>mixture is needed for good accelleration, a plunger is attached to the cap on
>top of the suction chamber, extending down into the oil-filled piston rod.
>When the piston rises, the resistance of the plunger against the oil prevents
>it from rising too quickly. This makes the venturi area temporarily!
small in relation to the air flow, so vacuum is temporarily higher than usual
in the venturi. This higher vacuum sucks extra fuel from the jet, providing a
rich mixture until the piston is able to rise to its normal height. The
plunger offers very little resistance to small, gradual movements of the
piston, and offers almost no resistance to downwards movements of the piston.
On some cars only the plunger is needed to provide the right amount of
enrichment. On others, [a] piston spring... is enough to do the job. Some
cars need neither, while others need both. It all depends upon the
requirements of each engine design.
Unquote
Bill
Paul Hunt wrote:
>
> 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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