Trying to understand. Are you saying the E clips effectively prevented the pistons from moving on their shafts — eliminating any up and down play?
The free movement on the damper is an interesting situation. I had a somewhat lively discussion about it on the UK forum (I think) a couple of months ago - the question is - why is it there? What we’re speaking about is the 1/16 to 1/8 inch free movement of the big piston before encountering resistance from the oil. It’s caused by the free movement of the brass piston that is the damper. On my '68 I’ve also restricted it’s movement, but not eliminated it, and it has helped with the stumble I’ve been experiencing. I’ve never seen any discussion about this, but I believe the free moment is to facilitate quick piston drop when you get off the throttle. If you want differential movement up and down you need to facilitate movement of the oil around the piston in the down direction. An examination of the damper shows a) the damper is hollow facilitating oil movement around it by flowing through the inside, and b) as the big piston rises the force of the oil pushes the damper up where it seals itself against a washer on the damper shaft above it, and hinders oil flow, providing resistance to a quick rise. When the big piston drops as the throttle comes off it unseats the damper and the oil flows through it’s center.
I modified mine by placing a second washer off some spare dampers between the damper piston and the top washer. It doesn’t stop all free movement but it does restrict it, and went some distance to reducing my stumble.
Almost exactly what I did except I wanted to maintain the oil flow as much as possible…or…it was just easier to install my “fix” without disassembly, so I went with the E clip. The free movement of the the damper piston is ALMOST eliminated, but not quite. From memory, the distance the carb piston moves is a consequential(in my mind) 1/8" BEFORE encountering any damping from the damper piston assembly. And 1/8" represents a rather large increase in the throat size(vacuum drop) from idle conditions. As I say, this slight mod did, indeed, show a marked improvement in off-idle smoothness for the automatic-equipped car I worked on.
I am going to visit Southern Carburettors today and take my dampers, to check they are the correct ones for the HD8 and also check the free movement of the brass piston is the correct amount. Also, when I bought the rebuild kit the little rubber washers for the volume screw are of an inferior plastic type rubber and after one installation have started to split and disintegrate. But on the website, buying individually they look far better shape, so will buy some more. Also rechecked my jet position and found them both to be 0.047 down, so going to have another go with them at 0.060.
As an experiment, I am going to try the heavier oil in the dashpot. Also separately I will try the stronger springs as I have a pair of Red/Greens
Interestingly, the late Healey 3,000 which has two HD8’s uses those springs ! The standard Blue/blacks in the Jaguar E are 4.5oz whereas the Red/Greens are 11.25oz so quite different.
The real problem being my engine/carb/cam is so non standard that I guess any experiments are worth trying ??
An interesting discussion. The effect of oil viscosity is to damp (slow) the piston’s upward movement once it has begun moving, which adds enrichment. It contribute nothing to the static position of the piston whilst cruising with constant throttle input. But it also doesn’t affect the ~1/8 inch “slack” in beginning of upward movement before the damper engages.
Early on, pistons were heavier (brass) but when aluminum alloy was substituted the springs were added. This was because, in steady state, the upward force caused by the venturi vacuum within the suction chamber above the piston was now opposed by a smaller downward force (less piston weight). The solution was to add “weight” via the compression spring. Although the spring force does oppose piston rise during the “slack” period, stiffening it would also affect the steady state piston height, which is presumably optimal with the original spring.
A factor not considered AFAIK is the inertia of the piston. This opposes any acceleration, including onset of rise. Adding weight to the piston, and removing the spring to compensate, should result in the same steady state piston height as before. But the added inertia would slow the rise, including during the “slack” period, enriching the mixture. That’s how it presumably was during the brass piston days.
To take this concept farther (adding even more inertial mass than the amount needed to replace the spring force) you could increase the suction chamber diameter (and that of the piston top) but not the diameter of the piston itself in the venturi. That would add upward force because the vacuum in the chamber is integrated over a larger area. The force balance would remain (more upward vacuum force, more downward weight) with proper design. The added mass would have no effect except to slow acceleration of the piston, including during the “slack” period, which should enhance the “accelerator pump” effect. Speculation and IMHO as always.
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Just one comment, based on busloads of examples: a properly-set up SU, with a good-running engine, suffers no tip-in throttle stumble.
At all
If it does, something is fundamentally incorrect… and, it’s not a design fault.
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Regarding oil viscosity, a bumpy road causes the pistons to “bounce” with the oil acting much like a shock absorber. The thinner the oil the less absorption. Many I know use ATF in their carb dampers.
Use/d ATF in Margaret, and Tweety, and drive/drove both on bumpy dirt roads: if there’semphasized texta difference, it’s not sensible.
I really appreciate the thoughts about my stumble problem, the depth of knowledge on this forum is amazing.
I went to Southern Carburettors today and compared my dampers and they are the correct AUC8115HB’s for the HD8. The movement of the brass washer is also the same and correct. Also bought some new little rubber rings for the bottom of the volume screw under the cup washer. They are totally different to the ones in the rebuild kit which have disintegrated in a very short time.
But, just to emphasise again, my engine as described with its modifications is not standard, thus the stumble may not to be as easily diagnosed as a standard XK with H6’s and standard 15/57 cam.
Thanks for that clarification: that dies indeed make a difference.
Have you tried really heavy oil, say 90 weight, to see if that cures the stumble, ceteris paribus?
One way is to set carbs progressivly .005" lower as a pair, then drive.
For HD8, (with standard tune), I found the range of .060 to .070 … (1.25 to 1.75mm) jet recess to be good
Hammill (once again) suggests this is the best layman way to tune.
adjust, straight road, stopwatch, time between 2 points, adjust, do it again etc
According to the Haynes SU Carburettor Owners Workshop Manual, pages 30 and 54, “ the damper is only effective for upward movement of the piston, the piston falls immediately the throttle is closed, preventing an over-weak mixture being produced”.
Well, unfortunately for us, they don’t explain exactly how this is accomplished.
I suspect that somehow, the small brass bushing and washer mechanism at the bottom of the damper rod are supposed to act as a one way valve, allowing quick descent of the piston, while slowing upward rise of the piston.
Even the XK120 service manual on page C.10 refers to the ‘small valve” at the bottom end of the damper rod.
I still can’t figure out why there is supposed to be a little free play in that valve mechanism, or even how it is supposed to act as a valve.
The dampers in my H6s consist of a washer fixed to the lower portion of the damper rod, below that another washer floating freely on the rod, the larger brass bushing below that, and the E clip below that.
Hopefully this is the correct orientation of components, is this the same as everyone else’s dampers? Luckily they seem to work perfectly, no stumble at all.
Wait a minute, here is a possible explanation of how the valve might work:
consider…
when the piston tends to rise, the brass bushing and floating washer move upward on the damper rod until they come up against the bottom surface of the fixed washer, preventing any upward fluid flow through the annular orifice between the damper rod and the Eclip and the bores of the bushing bushing and floating washer, thereby increasing fluid resistance to piston upward movement.
When the piston falls, the brass bushing and floating washer tend to go to their lowermost position, allowing oil flow downward through the annular opening between the damper rod and the bores of the floating washer and brass bushing and E- clip, minimizing fluid resistance to piston downward movement.
So, perhaps the tiny amount of free play occurring in the damper valve mechanism, which can cause tip in stumble (elegantly corrected by Lee and others by adding an Eclip), might be an unintended consequence of the SU damper ‘valve mechanism’ needed to enrich mixture upon acceleration?
Perhaps the poster experiencing the stumble could check to see how the washers and bushings are oriented on the damper rods?
Annular passageway between damper rod and the bore in the brass bushing is blocked by fixed washer when piston rises.
Wiggles I agree with you that a stumble is not standard on these carbs. I have the fortune (?) to own two of these cars. One is a Ser 1 coupe 80K miles with a stock engine. No stumble, but with light oil in the damper, I’m met with a lot of resistance when I try to manually raise the pistons. On the second car, with 250k miles plus, I have a heavily modified engine, valves, cams stroked to 4.7l, and it has the stumble. With light oil I can raise the dampers with ease. Switching the dampers (only) between the cars makes no difference to the modified engine. As with every engine I’ve modified with higher lift and greater duration cams (not much more duration allbeit) idle is profoundly affected with SU’s. On Weber’s not so but they have other unpleasant issues.I believe that the difference is low vacuum caused by the cams, and the SU can’t handle it at idle. I’ve got much better control of the idle by using an EDIS system and using a initial ignition curve that control’s idle speed, but the SU’s still stumble - although not as much with gear oil and reduced damper free play. Even with this heavy oil I still don’t have the upward resistance I have with the first stock car.
John I don’t know what you are trying to achieve by lowering your jets. I think that you need to regulate mixture by the method in the manual - that is lift each piston and see if the idle speed increases or decreases and modify the setting as needed. If you lower your jets simply to get rid of the stumble (and it may work) you may have grossly rich mixture at all other speeds. Your issue relates to a condition that is specific to low engine speed, and partial throttle conditions (A very difficult time for most carbs - not just SU’s) that is probably not related to a final solution of making it richer everywhere.
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Maybe we now know why race Jags swapped to Webers.
Ok, so today I first of all I replaced the very small rubber O ring under the small metal cup washers on the HD8 volume screw. See my previous comment about these. I tried the thicker oil instead of the SU SAE 30. I could not see any noticeable difference in the running.
So next I thought I would replace the UO’s with very slightly richer UVO’s - the car was terrible and what ever position I put the Jets from 0.04 down to 0.07, it just would not run properly. Sure it would rev up from 1,000 to 4,000. I took the needle out and compared it in the book with the UO, then I realised that although mostly richer, it is actually leaner at needle stages near the top, where the car would idle.
So, next experiment was to try the UVR, which is richer than both the UO and UVO, but all the way down. NOW the car runs far better and the stumble has all but disappeared. So, I guess with my combination engine it needed these needles on only two HD8’s with a 3.4 engine.
Drove the car hard around neighbor roads, pulls well, no black smoke.
The trip to Goodwood next week and Silverstone the week after will be interesting to see the fuel consumption. It really does amaze me that I very minor change in needle makes such a difference and also how just a quarter of a turn on the mixture screw has such an effect. This saga all started because I had new bushes, shafts and butterfly’s. But I do also think the little rubber O ring in the volume screw have helped.
So, thank you to all the advice and thoughts from everybody on the wonderful forum.
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