I have noticed after topping up the damper tubes of my SUs to the
prescribed level with 20 wt oil that the effort required to
manually lift the pistons up in their bells is variable. Observing
the rise and fall of the pistons while reving the engine indicates
as expected that the piston requiring the most effort to lift
manually rises and falls more slowly than the piston that requires
the least effort to lift manually. As a consequence the fuel/air
mixtures entering the cylinders on acceleration and deceleration is
variable. With the damper tubes dry the pistons rise and fall in
unison, but more abruptly.
I’ve been able to approximately balance the dampers by shaving off
a few thou from two of the damper plungers. I now want to fine tune
them by using a digital scale to measure the effort required to
lift each piston and adjust the diameters of the damper plungers
accordingly.
Is there a spec for the ounces or grams of effort required to lift
the carb pistons in their suction chambres?–
Nick Saltarelli '68 Cdn mkt E-type S1� OTS, '54 XK120SE OTS
Niagara, Ontario, Canada
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from N�ck sent Sat 13 Sep 2014:
Nick,
There is a spec for the spring which also influences
the rate. Did you check that all 3 springs were the same
when installing, and are you sure you have the correct
springs?
Andrew–
The original message included these comments:
Is there a spec for the ounces or grams of effort required to lift
the carb pistons in their suction chambres?
–
1968 3.8S
Zurich, Switzerland
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from N�ck sent Sat 13 Sep 2014:
I have spent several hours on this this weekend.
There is no spec that I know of although that does not mean one
doesn’t exist. The ‘‘drop test’’ is about checking that the
piston fits the bell. The springs add very little to the
damping; most if not all of it is due to the hydraulic effort
of forcing the oil past the damper piston.
To get a number for lift effort you could use a spring scale or
similar. As you say what’s important is not the absolute but
rather that they’re all the same. If you can achieve this then
you can always fiddle with different weights of fluid.
What I did was swap the damper plungers around, trying
empirically to get the lift effort approximately the same. All
my dampers are old and I did fettle one with sandpaper a bit to
achieve this. des Hamill’s book on race tuning SU’s is always
worth a read.–
The original message included these comments:
I have noticed after topping up the damper tubes of my SUs to the
prescribed level with 20 wt oil that the effort required to
manually lift the pistons up in their bells is variable. Observing
In reply to a message from N�ck sent Sat 13 Sep 2014:
Nick while balancing the pistons may not be a fruitless
effort I would have thought they were close enough as they
were new. The spring rate is important but the most
important factor in my experience is always the fit of the
pistons to the covers. As a child I remember a vacuum
salesman demo. to my grandmother of his powerful vac. He put
a steel ball on the carpet and his vac picked it right
up–later I realized that the adapter he had placed on the
hose was just the right fit for the ball so that little or
no depression was lost. So is the case on your carbs on
accel. The better the fit the more action. I read your post
rec. that you had a carb piston hanging up and that you
sanded it. While that worked the further the piston and
carbs move from new spec the more issues one will have. I
have seen issues in this area more than one time that were a
result of over zealous polishing which resulted in
deformation of the bell. So my point is that while weight is
a factor it is far from the most important one. Good luck.–
The original message included these comments:
I’ve been able to approximately balance the dampers by shaving off
a few thou from two of the damper plungers. I now want to fine tune
them by using a digital scale to measure the effort required to
lift each piston and adjust the diameters of the damper plungers
–
George Camp
Columbia SC, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from George Camp sent Sun 14 Sep 2014:
Yes, all three springs are the same. I tried different weight oils
to equalise the dampers and although acceleration was smooth it was
also more sluggish.
The damper rods are new. Only one of the bell/piston sets is NOS,
another I bought used at a flea market, and the third is original.
I have two original spares. The three in use all pass the drop test
almost equally, the other two are a second or so faster. The flange
of the bell that was scored by grit was more polished than sanded
smooth and there was no perceptible affect on the drop test time.
No doubt it’s the dampers making the difference.
If one removed the damper rods entirely, lifted the pistons and
then pushed the damper rods down through the oil there was also a
noticeable difference in the effort required, the least effort on
the new set. Switching the damper rods around makes no difference
because the plungers on all three measure the same, 0.336’‘. Or
rather measured the same. I took them off the rods, secured them on
a mandrel, chucked them in a drill and spun them against 400 grit
paper, reducing their diameters to 0.332’'. Now the damping effect
on all three is empirically close.
Absent an actual specification I’ll use a pull type digital scale
to measure each piston lift and will equalise the effort across all
three to match the lowest reading.
Thanks for the responses!–
The original message included these comments:
Nick while balancing the pistons may not be a fruitless
effort I would have thought they were close enough as they
were new. The spring rate is important but the most
accel. The better the fit the more action. I read your post
rec. that you had a carb piston hanging up and that you
sanded it. While that worked the further the piston and
carbs move from new spec the more issues one will have. I
have seen issues in this area more than one time that were a
result of over zealous polishing which resulted in
deformation of the bell. So my point is that while weight is
a factor it is far from the most important one. Good luck.
–
Nick Saltarelli '68 Cdn mkt E-type S1� OTS, '54 XK120SE OTS
Niagara, Ontario, Canada
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from N�ck sent Sun 14 Sep 2014:
Should clarify two things. Smoothed the scored piston
flange, not the inside of the bell. Reduced the diameters
of two damper plungers and left one unchanged.–
The original message included these comments:
almost equally, the other two are a second or so faster. The flange
of the bell that was scored by grit was more polished than sanded
rather measured the same. I took them off the rods, secured them on
–
Nick Saltarelli '68 Cdn mkt E-type S1� OTS, '54 XK120SE OTS
Niagara, Ontario, Canada
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from N�ck sent Sun 14 Sep 2014:
Are you sure the dampers are assembled correctly? Each has
two small brass rings, and they are not only different from
each other, but each has a definite ‘‘top’’ and ‘‘bottom’’. If
assembled incorrectly, the damper will not work as it’s
supposed to. They should allow the piston to fall (more-or-
less) freely, but rise slowly.–
Ray Livingston - '64 OTS Santa Cruz, CA
Santa Cruz, CA, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from Ray Livingston sent Sun 14 Sep 2014:
They’re all assembled as received, Ray, but the new dampers (from
SNGB) are not the same as the originals. The new ones lack the
second ring, have a pronounced taper and the retaining circlip is
smaller:
This is the digital scale I used to measure the resistance of each
damper:
It’s a luggage scale with the strap replaced with a piece of �’’
aluminum flat stock angled 90� to hook under the carb piston. As I
discovered, the reading varies depending on how quickly one raises
the piston so an absolute spec would be hard to pinpoint. However,
repeatedly lifting each of the pistons slowly and steadily gave me
an average low of 0.8 pounds and an average high of 1.4 pounds. A
significant difference I should think. I equalised them all to 0.8
pounds by reducing the diameter of the two damper pistons (or
plungers) with the higher readings. Fwiw their diameters are now
0.336’‘, 0.332’’ and 0.329’'.–
The original message included these comments:
Are you sure the dampers are assembled correctly? Each has
two small brass rings, and they are not only different from
each other, but each has a definite ‘‘top’’ and ‘‘bottom’’. If
assembled incorrectly, the damper will not work as it’s
supposed to. They should allow the piston to fall (more-or-
less) freely, but rise slowly.
–
Nick Saltarelli '68 Cdn mkt E-type S1� OTS, '54 XK120SE OTS
Niagara, Ontario, Canada
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from N�ck sent Sat 13 Sep 2014:
Nick, if I can remember this correctly but I did read an
article somewhere specific to SU’s on E’s. The measurement
used on the dampners was one of time. Again as I recall
the air vents on the bell & dampner was plugged & a time
measurement was used to get the closest balance to all
three. I think it may have been 10 seconds. Switching
dampners/bells to the closest times may achieve what you
are looking for.
As an aside is the balance that far out requiring what you
are doing. I usually find to my dismay once started down a
path of modifying a part on my E does not achieve the
results I wanted & a new part is required (again). Best
example to me was trying to achieve TDC & cam degreeing
which for me achieved no visible gain & I just resorted to
the original Jag marks on the flywheel & left it at that.
However to close on the positive, Joe Curto may be a good
source here.
Stephen. 1965 OTS #10275.–
stephenc
winnipeg/manitoba, Canada
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from stephenc sent Sun 14 Sep 2014:
You’re referring to drop times, Stephen. You plug the two vents at
the bottom of the piston, push the piston to the top of the bell
with damper in place and then time how long it takes to drop out. 6-
7 seconds is good. As Ray says above the dampers affect the speed
with which the pistons drop but not much. Most of the affect they
have is in the upstroke, which is what I have successfully
adjusted. I’m just back from a 70 km run to visit a fellow
enthusiast with a newly acquired S3 2+2 and the car ran quite well.
Best yet. Plus I got to drive another E-type, and that always makes
my day. :o)–
The original message included these comments:
Nick, if I can remember this correctly but I did read an
article somewhere specific to SU’s on E’s. The measurement
used on the dampners was one of time. Again as I recall
the air vents on the bell & dampner was plugged & a time
measurement was used to get the closest balance to all
three. I think it may have been 10 seconds. Switching
dampners/bells to the closest times may achieve what you
are looking for.
As an aside is the balance that far out requiring what you
are doing. I usually find to my dismay once started down a
path of modifying a part on my E does not achieve the
–
Nick Saltarelli '68 Cdn mkt E-type S1� OTS, '54 XK120SE OTS
Niagara, Ontario, Canada
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from N�ck sent Sat 13 Sep 2014:
Nick,
Nothing to add to the carburetor discussion but thought I would
let you know that your Moody Blues reference was noticed and
appreciated!–
Jim Horvath, '67 OTS, 1E13653
San Jose/CA, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
