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SU Damper Cap Vented vs Non-Vented

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This discussion is right on time for me. I just bought some carbs and they came with brass dampers, no hole in the dome. Good thing is that I have a pair of carbs from a 4.2 Jag that I can steal the dampers and exchange for the brass type.
Jerry
 
Jerry said:
This discussion is right on time for me. I just bought some carbs and they came with brass dampers, no hole in the dome. Good thing is that I have a pair of carbs from a 4.2 Jag that I can steal the dampers and exchange for the brass type.
Jerry
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I bought my HD8s off a guy online and they came with brass dampers and neither type of hole. I drilled them (1/16") per the pictures above.

Not knowing any better at the time, I drove the car without the holes for several years. It ran OK but wouldn't idle. Didn't find out about the holes until I read some SU books by way of boning up.
 
So, to continue this conversation: we know that you either want internally vented dashpots with solid, non-vented dampers or you want the reverse of that. However, I am curious, what is the effect if both the dashpot and the damper are vented? Similarly, what is the effect if neither the dashpot nor the damper is vented? Does anyone really know? Has anyone tested the effect?
Lin
 
Interesting to read all the above AH comments with HD8 carbs. I have a 3-litre Rover P5 with a single HD8 carb and have blown the stem on my plastic capped damper out by backfiring also. I've also had a problem maintaining a consistently slow idle without having to frequently blip the throttle as mentioned at > HD8 high idle aggravation < on this forum.
 
AUSMHLY said:
My 1964 3000 MkIII has SU HD8 carburetors. Mine have the black plastic damper cap. It has a vent hole in the cap.
My friend has a 1960 3000 with SU HD6 carburetors. His has the brass damper cap. It is not vented.

Why are some vent caps brass vs plastic?
Why are some vent caps vented vs non-vented?
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Likely the vent is more discrete , on the lateral and underside of the plastic cap, as I recall. A square open channel on the underside.
 
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