SU damper rods

[monty]

I have brass damper rod screw caps rather than the newer plastic variety on my SU HS carbs. At an event today I noticed a Mini had a similar setup with brass tops but each one had a small hole drilled in it. I remember reading something about these holes some while ago but have forgotten what they were for. Can somebody remind or enlighten me on their purpose & should I have them in my tops! 
I have no running problems with no holes in the caps & I always ensure the chambers are topped up with oil. Just curious to know!

[ianc]

As the piston rises inside the SU, the air above it has to get out somewhere.

Ian Cornish

[RobH]

As Ian says they are breather holes for early carbs to stop pressure build-up under the cap as the piston rises.  Later SUs have a breather channel in the body and don't need holes.  It is in the diagonal fillet at the top of the body going up to the cap which early carbs do not have. 

 

Edited August 14, 2022 by RobH

[monty]

28 minutes ago, RobH said:

As Ian says they are breather holes for early carbs to stop pressure build-up under the cap as the piston rises.  Later SUs have a breather channel in the body and don't need holes.  It is in the diagonal fillet at the top of the body going up to the cap which early carbs do not have. 

 

Thanks for that Rob, thought there was a reason somewhere previously. I appear to therefore have fillets in my carb bodies & plain brass (no holes) caps instead of black plastic. (Brass does look better than plastic).

[RogerH]

Hi Folks,

here is an explanation that  has been sent to me.  I will not change from how it was sent -

 

The brass damper caps with a hole in them were for the home market, or countries with little or no dusty conditions.

The 'cast web' on these did not have a breather drilling in them. 

For those countries with 'dusty' conditions the brass caps did not have a vent hole in them

and were therefore sealed. Ventilation for these carbs was via the breather drilling in the web casting.

 

Roger

[Tr4aJim]

In the “Tuning SU Carburetters” book by Speed Sport, it mentions the following on page 30:

   
Finally top up the piston damper with the recommended engine oil until the level is 1/2 in. (13 mm.) above the top of the hollow piston rod.
Note
On dust-proofed carburetters, Identified by a transverse hole drilled in the neck of the suction chambers and no vent hole in the damper cap, the oil level should be 1/2 in. (13 mm.) below the top of the hollow piston rod.

30
 

Notice that the oil fill level is different based on whether the chamber is vented or not.

Oh, also check sucarb.co.uk. They appear to offer solid or vented brass caps 

Jim

Edited August 14, 2022 by Tr4aJim

[monty]

Thanks to all. Now sorted & understood. Took the carbs apart to check exactly what I had (drilled holes in the fillets so no need for holes in the caps) & cleaned the insides of the bodies & the pistons while I had them apart.