There is never any discussion on the spring in the HD8…they must deteriorate over time…what is the standard…new ones are not expensive…and what about the little retaining “cups”
I assume you mean the piston springs not the ones on the mixture screws. I replaced mine, but didn’t notice a bit of difference. However I think Burlen offers them in different colors so you can color coordinate them with the car. I’m not sure what cups you’re referring to.
Yes, but when the paint wears of the spring it’s time to get a new car.
I always wondered why there are so many specs for different piston spring rates, depending on application of the HD8. Particularly after I read (recently, perhaps on this list) that original pistons were brass. When SU replaced them with Aluminium they added the springs, which hadn’t been present before at all.
Odd that. I wonder if it was causal or coincidental. Either piston would fall at the same speed, but the brass would have more inertia and a bigger clunk. I guess if the jet isn’t centered perfectly a bigger clunk would help, but it should obviously be aligned. Maybe they just realized the spring should have been there all along.
The spring is there as resistance to the needle opening too quickly, colour coded to the car would be nice but its more to do with the different requirements of differing engines.
Agreed when “clunk testing” in your garage. But I think it’s more complicated than that because the piston isn’t in free fall when the motor’s running. It’s weight is opposed by the upward force that derives from the venturi vacuum, which is applied to the vacuum chamber above the piston. A heavier piston, and/or stiffer spring, means that more venturi vacuum is required to raise it, so the piston sits lower. The greater venturi vacuum draws more fuel, which is only partially compensated by the needle sitting lower in the jet.
Swiped what is below from an MG forum. If I understand it, it suggests that one can shift the range of needle adjustment by swapping out the spring. Stronger spring = overall richer range and vice versa. Interesting…
There are two purposes to the variable venturi in an SU carb. One is to maintain a constant air speed (and constant vacuum near the jet) and the other is to move the tapered needle so as to allow a greater volume of fuel to mix with the air when the volume of air passing through the carburetor increases.
In an SU carb, the depression (or vacuum) in the venturi is applied to the top of the piston assembly. Atmospheric pressure is applied to bottom. This lifts the piston until the weight of the piston (a downward force) equals the upward force (which is the difference between the air pressure on the bottom vs the top multiplied by the area of the piston). In theory, the designers could have eliminated the spring entirely and used only the weight and area of the piston to control the venturi position (and therefore the airspeed through the carb). Alternatively, they could have allowed the addition of weights to the venturi to trim the setting. Unfortunately, adding weight (mass) to the venturi would increase inertia and make it slower to react - particularly to drop when the throttle shut quickly.
So, it appears they deliberately made the piston area overly large (which would cause the piston to operate too high) and then allowed the technician to select an appropriate spring to dial the position (and airspeed through the carb) exactly.
In theory, a spring is not as correct as the weight of the piston. Weight is a constant force but in the case of a spring the resistive force varies with the degree of compression of the spring. This will tend to make the piston run lower than it should at open throttle (when the spring is most compressed). However, the designers compensated for this by choosing a spring with a lot of coils and a low “k” (spring constant) value. Over the relatively small travel of the venturi (~2 cm) the force due to the spring is very nearly constant. Plus the spring has lower inertia than the weights would.
So, the various strength springs actually set the the height of the venturi at a given throttle position. More importantly, they set the airspeed (and vacuum) in the carburetor (which is constant regardless of throttle position). So, a stronger spring will force the venturi to operate lower which increases the airspeed (and vacuum) and will suck more fuel from the jet (although this is partly compensated for by the needle being more closed). In other words, a stronger spring should make the engine run richer. A weaker spring will make the engine run leaner. It would be interesting to have someone with an O2 meter verify this by comparing O2 measurements with different springs (and no other changes).
As I understand it, the very fine tuning is done by choosing needles with different tapers. However, if you were unable to get the desired mixture by tweaking the needles, the next step is to change the spring so that the mixture is within the range that can be tuned by needles. But this is rarely necessary unless you are moving the carb from one application (engine) to a completely different application (different size or type of engine).
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The springs are, for all practical purposes, constant force springs, and changing them will have EXACTLY the same effect as adding, or removing, weight from the pistons. They provide a means of adjusting how far the piston moves up for a given air flow rate, to control where on the needle the jet is operating. A heavier piston, or stronger spring, will provide less rise of the piston for any given airflow, hence a correspondingly richer mixture. It is an additional means of tuning mixture as a function of RPM and load, much like changing the taper of the needle, and is one of the many ways SUs must be matched to the engines mixture needs.
Suppose, as is not un-common with UM needles, the idle mixture is perfect, but the at-speed mixture is lean. One way of tuning that out is to change needles for ones that are richer at the higher “stations”. Another is to put in a stiffer spring. Trouble is, there are only a handful of different springs readily available, and no specs to work from, so getting it right is a matter of either trial and error, or making custom springs, which few of us are equipped to do.
As for springs “deteriorating over time”, forget about it. Unless they’ve been abused, they are virtually guaranteed to be just fine, even after 50 years of use. They see exceedingly little stress in use.
Regards,
Ray L.
Makes sense…piston lower for given airflow, needle deeper in jet, less fuel, leaner.
But also…piston lower for given airflow Q, smaller venturi cross-sectional area A, greater velocity v since v = Q/A, greater Bernoulli suction, more fuel, richer.
So if I understand, Ray, you are saying that the needle’s restriction as it is lowered into the jet dominates the greater Bernoulli suction (lower piston = leaner), whereas the MG forum guy is saying the opposite (lower piston = richer)?
Bob,
Since you were foolish enough to ask:
Regards,
Ray L.
I call the damper spring in the SU the “Weber Factor”. SU looked at Weber’s and saw that they were full of parts whose use in theory was easily explained, but whose use in actual practice was utterly incomprehensible and said: “Our carbs have no legitimacy without some parts like that”, and hence the damper spring for fine tuning the needle, that you never realized you needed to do. How very Weberish.
Except the function of that spring is totally comprehensible, and very logical, once you understand exactly HOW SU/ZS carbs work…
Regards,
Ray L.
Priceless!!! …stealing!
There is a method to check if the compression of each spring is the same as the others, which could be worth checking on a tri-carb setup, if they might have been changed out etc
Detailed in Des Hammills book
a plastic tube to contain the spring, and weights on top.
did homemake one using a section cut from a clear plastic drink bottle & clear tape
I tested mine by a method in some book I have. They are specified as having a certain force at a certain length–different forces and lengths for different colors. I pushed down onto a postage scale or similar until the specified length was reached, then read the force. You can figure out what color spring you have, plus see if all 3 springs are similar.
Yeh I know in theory how they work, but as you said:[quote=“Ray_Livingston, post:7, topic:370052”]
“and no specs to work from, so getting it right is a matter of either trial and error, or making custom springs, which few of us are equipped to do”.
[/quote]
I’m still confused about whether a stronger spring makes things richer or leaner. Ray, above you say,
Thanks–I’d forgotten about the informative old thread that you cite, even though I participated at the time. In it you said,
"So, you can see that for any given initial airflow, forcing
the piston to stay further down than it normally would, will
enrich the mixture dramatically,"
And also,
"At any given airflow, if the mixture is correct, lifting the piston
will lean the mixture, lowering it will richen the mixture (seems
backwards compared to the conventional wisdom, doesn’t it?)"
[sorry, can’t use the quote feature because it wants me to reopen the ancient thread]
You go on to say that of course, if a correct mixture is changed by changing the spring’s stiffness, you would go ahead and readjust the jet height until the mixture was restored. But then different stations on the needle would be in use–the final result would be a different range of mixture over the piston travel than before…maybe richer, maybe leaner at some particular engine speed, load etc.
All of this (from the old thread) is consistent, and agrees with the MG guys I quote above. But I can’t figure out how your comment in this thread relates to what you said in 2004 and 2009?
Bob,
I assume you’re referring to this:
A heavier piston, or stronger spring, will provide less rise of the piston for any given airflow, hence a correspondingly leaner mixture.
Which is wrong. That was a typo (I’m fighting the flu today, so my brain is only partially functioning). It should (and now does) read:
A heavier piston, or stronger spring, will provide less rise of the piston for any given airflow, hence a correspondingly richer mixture.
Regards,
Ray L.
Yay! I never thought it might be a typo, only that I was missing something. It’s very confusing to me, sorta a chicken or egg first thing. For example, I think there’s another sentence further down (“softer spring”) in your post that you might want to edit, but maybe it’s correct as is?
Bottom line, I don’t think I’ll be trying to tune by changing springs anytime soon. 
Correct. I had problems with my MGA leaning off under acceleration which was not fixed with a richer needle so after reading Des Hammills book I put in a stronger spring which fixed the problem.
Right you are. That is also now fixed. I better stop trying to use my brain today…
Regards,
Ray L.
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