Hi Morris
Thank you very much for your advice!
Strange enough the pistons have holes, but not the dashpots and not the damper caps…
Best Regards Lukas
Something doesn’t sound right. Maybe you would do better to post some pictures.
There are definitely no side holes in the dashpots, thus they seem to be the original ones (May 1950).
Are you sure the dashpot in picture 83714 has no side hole. How about the other dashpot.
If you dont have a hole in the dashpot, then you need to drill a small hole in the damper.
The dashpot must have been changes sometime in the cars life.
On the picture it just looks like because of some scratches. But no, there is definitely no side hole in both of them.
Anyway, thank you very much for your advice with the 1/16 hole 
Best Regards Lukas
In picture 083719 I see a side gusset. I’m surprised that there is no hole drilled through it there. Are these the tall or the not so tall dashpots (i.e. handbells). Can we see a side view?
Hi Rob, this are the not so tall ones, obviously the intermediate dashpots as Roger described earlier in this thread (March 1950 until 1951).
Side view picture will follow shortly.
I tripple checked the pots, there is absolut no hole inside 
.
Best Regards Lukas
A side view and two show the inside (each sector ~180 degrees)
…and the inside, again each sector shows ~180 degrees
Lukas
I would assemble all back together except air filter, fire up the engine and view the piston rise through the venturi. and see how high it rises while increasing the revs.
Then drill one damper, and re do the test and compare front carb with rear.
So drill a small hole in both dampers, smaller 1/16 or smaller. then do a road test and see what the difference is.
Only fill with light oil up to the top of the piston shaft, NOT the dashpot.
Also check to see if both springs are the same height.
Morris.
Man, I would be VERY reluctant to start drilling holes in stock parts until I knew exactly what I had. I appears that some parts were swapped out sometime in the past
Pat
I fully agree. But a 1/16 hole in one damper can very easily be filled again with a brass pin.
Lukas only has to do one damper and compare the pistons rising.
On my modified carb set l found that l had a hole in the damper and a hole in the dashpot, the piston only rose 1/4 inch at 3000rpm. After plugging the dashpot the piston rose all the way up, my other carb had no hole in the dashpot thats why l plugged the dashpot
M
Hi Pat and Morris
Thank you very much for your thoughts.
Of course I will examine very careful what is going on in the piston chambers before I modify something.
When I moved the gas pedal to accelerate from standing, the engine allways had a short but distinctive hesitation and it felt like one or both pistons where not moving instantly.
And as Roger Payne pointed out earlier in this thread, there where 3 types of dashpots : the tall necks, the shorter necks without the side holes but with a small vent hole in the damper caps and finally the same shorter necks with the side holes but without the vent hole in the damper caps…
Regarding this it’s thoroughly possible that the hesitation is because of the missing vent holes in the damper caps.
But as said, careful investigation first
Best Regards
Lukas
I had similar issues with a hesitation when applying throttle after engine braking at low revs. It was running perfect otherwise. My xk120 has the head and inlet manifold from a xk140 and was running with xk120 WO needles. I could not get rid of the hesitation no matter what I tried with either mixture, damper oil or ignition settings. I bought new standard needles (SJ) for xk 140 and the hesitation problem disappeared completely. Worth trying if you are running other needles in a standard xk140 engine.
Regards /Svante
1 Like
GOOD NEWS,… Finally the hesitation problem on our XK120 disappeared completely after installing a pair of damper caps with 1/16" holes👍
Now the car drives really beautyful - thank you all so much for your advice 
Best Regards Lukas
2 Likes
Could we be over-thinking this, guys? As I understand it, the purpose of the hole in the damper cap is to relieve pressure build up as the piston rises. If there is no hole in the piston itself or in the damper cap I can see this being a problem. BUT, if the piston has the hole in it (ie: is vented) there will be no pressure build up as it rises, even without a hole in the damper cap, as the pressure will equalise through the hole in the piston.
I’m assuming there were two systems:
1 - vented piston and no hole in damper cap;
2 - unvented piston with drilled gusset and hole in damper cap
My December 1950 car has the intermediate type of dashpot with short neck, unvented damper cap, and no drilled gusset, and I’m pretty sure I have noticed a vent hole in the pistons, but I would have to check to be certain.
To sum up, you just need to make sure that you have the correct setup to prevent the piston trying to compress the air in the space above it in the dashpot as it lifts.
Just looked up the subject in my Burlen Services SU catalogue. It lists two types of damper cap: DP = dust proof (no vent hole) and NDP = non dust proof (with vent hole)
It also lists 2 types of piston for the H6 carb - AUD9190 DP with side keyway, and AUD9187 NDP with front keyway.
I can’t remember which keyway our cars have, and do they mean to the front of the car or towards the air cleaner when they say front keyway??
Thinking about it some more, I can’t see what harm drilling a small hole in the damper cap could possibly do!