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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