I read in an old manual that the dampener pistion to rod axial clearance ought to be .004"-.007" for my SU carbs. Mine is about .050" and I have a dead spot off idle bigtime. Does anyone have a reccomended method of shimming the piston clearance so the rod does not slide so far before moving the piston or visa versa? Or....does anyone know if new ones are available in some place I haveent found yet?

Thanks
Chris

Chris;

Do I understand this correctly?...do you mean the rod connected to to the threaded cap which has the brass ferrule and hardware, and extends down into the dashpot oil? If that has .050" (how did you measure this) of axial clearance to the bore, it sounds as though it has either worn from running dry (inspect brass ferrule for wear)...and will allow oil to bypass too easily and not give much dampening or maybe someone machined them smaller and then tried to use a more viscous oil(?)....a used but unworn replacement should be fine.

To check dashpot dampening, remove air cleaner and lift dashpot with fingertip...if there's oil (ATF) in the bore, there should be no more than about .25" of free travel before you feel dampening of motion...any more than that, could be responsible for your off-idle stumble.

Cheers

In this case remove a damping piston from the carb. Hold the screw in cap in your left hand with the rod and piston extending to the right horizontaly. Hold the piston on the rod to the left and use a feeler gage to measure clearance between right end of piston and retaining clip. The published dimension is 0.04" to 0.07". My mistake....I read .004" to .007".

The question still stands. If this clearance is excessive can it be repaired with a shim or some other method? I did find a carb kit on the Swedish Classics pages that has new butterflys, jets, and what looks like it might have new pistons to install on the old rods too. $135.00 usd.

I will try your measuring/inspection method too.

Thanks
Chris

You may be overthinking this one. In my 35 years of playing with SU's I have never gotten to that level of diagnosing a stumble.
Strongly suspect that if throttle shafts are in good condition and needles and jets aren't worn, careful adjustment should correct any stumble.
As noted above, if the dampener has all its parts and looks good visually, its unlikely to be cause of your problem. It would be a shame to invest $135 until you have exhausted the other possibilities.
There is lots of good advice in older postings here.
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123GT 1967

exactly svend!

I am not interested in spening any money at this point...I am just looking for the most likly culpret and devising a remidy as cheep as possible. This car has been through two sets of grosly errant hands in the last few years and it has been a few years since i worked on SU carbs. It is taking me some time to get my thinking straight. Biggest problem I have today is I cant find my UNISYN to measure the carbs air flow. I will have to resort to visual tuning for now I guess. Thanks for your input. Even still not in top running condition this car is a blast to drive!

Chris

Very rarely ever used a Unisyn.
Get a piece of garden hose and listen to the music.
By matching the sound from each carb, you achieve what the Unisyn does and it really looks trick if you do it with an audience.
Takes a little practice. Make sure you are holding the hose at the same position and angle for each carb.
Loosen off the levers that engage throttle levers on the carbs until you are done, to be sure there is no pressure.
Work back and forth between mixture, idle speed and air flow until you get the 3 of them to where it "feels right".
With 3 carb Jags and Healys, it was really the only way.
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123GT 1967