Dan, thanks for the explanation. This weekend I will be sitting down and
going through this with your explanation to see if I can make sense out of
it. Patrick
At 12:22 PM 4/20/99 -0400, Bud Rolofson wrote:
>Dan
>
>Great explanations as usual from you. And I always thought constant
depression
>referred to the feeling you get from messing with the ZS carbs.
>
>The only thing I'd add is that apparently the designers built the dampers to
>operate with a "design" viscosity of 20wt. oil (recommended in some
manuals)l
>so you may want to try to achieve that viscosity so the dampers perform as
>intended. The topic of hot debate on the list has been HOW to achieve that
>"design" viscosity given the various climates and seasons/temperatures we
run
>these cars in. I'm probably leaving some out but here's what I've heard
>suggested for use as dashpot oil: ATF, Marvel Mystery Oil, 3 in 1 oil (blue
>can), motorcycle fork fluid, 5wt oil, 20wt oil, 5w-50 synthetic (my choice),
>10w-40 oil, 5w-30 oil, Moss dashpot oil, 20w-40 oil, 20wt in the winter
30wt in
>the summer, use whatever you use in the engine, don't use anything-run
them dry,
>....What it boils down to is try them and use what works best for you.
>
>I think your point is well taken that the ONLY time it matters is during hard
>acceleration and I don't find myself putting the pedal to the medal very
often.
>I'm more apt to be cruising too fast than accelerating fast.
>
>Bud 71TR6 CC57365
>
>
>____________________Reply Separator____________________
>Subject: Re: ZS carb question! - long-winded response, part 2
>Author: DANMAS@aol.com
>Date: 04/20/1999 10:34 AM
>
>
>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
>
Patrick Bowen
'79 Spitfire
Jacksonville FL
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