Same here !!
This is a good list !
-----Original Message-----
From: John Suchak [mailto:suchak@mediaone.net]
Sent: Tuesday, April 20, 1999 11:25 AM
To: DANMAS@aol.com; spitfires@autox.team.net
Subject: Re: ZS carb question! - long-winded response, part 2
Dan, you RULE!
Thanks for FINALLY making me understand CD carbs!
John
DANMAS@aol.com wrote:
>
> The dashpots in a CD carburetor are probably the most misunderstood part
of
> our Triumphs, even more so than the Lucas electricals. There is almost a
> mysticism attached to them, and to what kind of oil to use.
>
> To get a understanding of the dashpot function, pull the cap and plunger
from
> one of your carburetor dahspots and examine it closely. You'll notice it
has
> a slender rod, with a "gadget" on the end. You'll see that the gadget is a
> small cylinder, held in place between two washer-like items. You'll also
> notice that the cylinder diameter is only very slightly smaller than the
> internal diameter of the guide tube on the piston that it fits into. If
you
> look real close, you'll notice that the cylinder, acting in conjunction
with
> the two washer-like devices, forms a check valve. When the cylinder is
down,
> oil flows freely throuigh the center of the cylinder. When the cylinder is
> pushed up, oil flow is blocked from the inside of the cylinder, and the
only
> way for the dashpot oil to flow is in the very slight clearance between
the
> cylinder and the guide tube walls. Oil, being a very viscous liquid, will
not
> flow freely through the limited clearance around the cylinder, but will
flow
> quickly through the cylinder.
>
> You can demonstrate this to yourself by putting the plunger back, with the
> dashpot filled with oil, and make this simple test: With your finger, lift
> the piston in the carb, and notice how much force it takes to move it, and
> how slowly it rises. Now release the piston, and see how quickly it
returns
> to the bottom of the carburetor. The design of the dashpot and the check
> valve is such that it is only effective in keeping the piston from rising
> quickly, and has no effect at all on the piston's falling.
>
> (BTW, this tells you exactly how much oil to use, if not what kind -- As
long
> as you have enough oil to cover the check valve when the piston is in the
> lowest postion, you have enough oil. Any oil that goes over the top of the
> guide tube has no effect on the operation of the dashpot, although you may
> want to add a little over the guide tube to make up for losses so you
don't
> have to top off as often)
>
> Now that we know HOW the dashpot works, the next question is WHY. Very
> basically, the dashpot serves EXACTLY the same function as the accerator
pump
> in an American type carburetor, ie, it gives an extra squirt of gas when
you
> stomp on the accelerator pedal. That is the ONLY purpose of the dashpot.
> During steady speed driving, or during gradual acceleration, the dashpot
> serves no function. Anytime the required rate of rise of the piston, from
an
> increase in speed, is slower by nature than the limits imposed by the
> oil/check valve, the dashpot has no effect. During de-acceleration, when
the
> piston is falling, the dashpot not only serves no function, it doesn't
even
> work.
>
> As the dashpot is only used during quick acceleration, we need to analyse
> what happens when we put our right foot down hard. When you stomp on the
gas
> pedal, two things happen - the throttle plates snap open, and the engine
> requires a richer mixture. When the throttle plates snap open, the top of
the
> piston is immediately exposed to the full vacumm of the engine intake, and
it
> would immediately go to the top of its travel if the dashpot were not
there.
> However, the engine speed cannot change instantaneously, so the air flow
> doesn't immediately increase sufficiently to offset the rise of the
piston.
> We have the piston at its high speed position, but the airflow doesn't
create
> the requisite depression, so we end up with a lean mixture, just when we
need
> a rich mixture. By limiting the rate of rise of the piston, the needle/jet
> assembly still sees nearly the same depression as before, plus it is now
> exposed to the larger vacuum from the engine, so more fuel is drawn from
the
> jet than before, giving the required richer mixture. As the air flow
begins
> to catch up, the piston will have had time to rise to match.
>
> Now, then, what type of oil to use? For most of us, within reasonable
limits
> I doubt that we would notice any "seat of the pants" difference at all.
With
> no oil, or extremely low viscosity oil, the engine will stumble upon
> acceleration, very noticeably. With too high a viscosity, we would most
> likely notice sluggish acceleration, and would also probably notice a lot
of
> black smoke from the exhaust from the rich mixture, although I'm not sure
of
> this. Also, I wouldn't be surprised if you could get by with out any oil
in a
> race car, as the throttle would be wide open, and the engine revs up,
almost
> all the time, the only exception would be when braking or shifting gears.
> Even when braking, I would think the revs would stay up, as the engine is
> used for braking as well as the brakes. If your grandmother were to be
> driving your car, no oil at all would be required most likely, as she
would
> probably never stomp hard on the gas.
>
> If you want to be sure, the only way to test is to try different
viscosities
> and see. Just remember, the ONLY time it matters is during hard
acceleration
> -- there is no need to monitor ease of starting, smooth running, gas
mileage,
> top speed, or any other performance aspect.
>
> I may well be full of crap here, but at least that's the way I understand
it.
> I know I'm not 100% correct, but I think I'm close enough for a layman's
> purpose. I stand ready to be corrected, and to be banished back to the
world
> of Lucas!
>
> Dan Masters,
> Alcoa, TN
>
> '71 TR6---------3000mile/year driver, fully restored
> '71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion -
see:
> http://members.aol.com/danmas/
> '74 MGBGT---3000mile/year driver, original condition - slated for a V8
soon
> '68 MGBGT---organ donor for the '74
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