Jet centring method

[PhilBell]

Having replaced the jet/diaphragms on my SU HD8s I decided to check the jet centring. The Jaguar manual says remove the piston damper, push the piston fully up then release, listening for a metallic click when it drops onto the bridge. This would indicate that it has fallen freely without friction between needle and jet. Unfortunately, mine lands with a dull thud. Removing the diaphgragm/jet gives the same result.
Anyone know what's going on here?

[Mich7920]

Hi Phil,

You say that without the diaphragm/jet you don't get any metallic noise when the piston falls ? That's surprising.
Is the piston stroke free, without obstacles?
Do you put the piston upright as it is into the engine?
Mich

[abowie]

Dull thud is fine. That's how I'd describe the noise.

The piston needs to fall unimpeded and hit the bridge.

I do it with the spring out as well.

[mgcjag]

assuming the damper is out when you do it..as you lift the piston you should be able to tell if the needle is lifting straight out of the jet and not scraping on the sides....Steve

[Mich7920]

Sorry, I don't agree.
When the carburettor is removed from the car, the piston makes a metallic sound when it hits the "bridge" which is normal for 2 metal parts hitting each other. .
It all depends on what you mean by thud and metallic sound.
The sound is not that of a hammer on a good anvil, which is a metallic sound par excellence, but certainly not that of pieces of wood hitting each other, which is a thud.
Mich

[PhilBell]

The sound is certainly metallic, but isn't the sharp click that I was expecting. Supporting the carb perfectly upright (bolted to a piece of ply clamped in my bench vice), the piston falls smoothly but progressively, rather than being in total freefall. I wouldn't expect it to drop totally unimpeded because the piston surely has to displace the air beneath it. Correct me if I'm wrong on that.
I haven't tried it without the piston return spring in place.

[Mich7920]

Without spring, the piston fall down slowly and progressively, that's correct!
From the factory, each piston are paired with his " bell"
Don't sand the piston...
As the piston are close to the bell, even if you have a spring the sound can't be very hard but metallique anyway
Mich

[PhilBell]

Progress! Curious why the piston was dropping progressively and landing with a more dull sound than the click specified in the manual, I took the dashpot off and discovered oil on the edge of the piston and inner surface of the dashpot. Clearly the cause of some drag on the piston movement. After cleaning everything and carb reassembly, the piston now drops faster and lands with a gratifying click. All three carbs were the same.
How did the oil get there from the damper chamber? Confession time - I left the carbs on their sides in the boot of the car while I removed the camshafts (another story).