Triumph Repair: TR6 1969 Carbs lean, lean mixture, brass cap

Question
Hi;  I have a 1969 TR6 with dual Stromberg 175 CD-2 carbs.  It looks like they are running lean, however there does not appear to be an adjustment in the dampers and the trim screws don't seem to help.
Backgroud:
After 5min of driving engine hesitates and eventually stalls.  Checked Timing and it is OK.
Spark plugs are white;  after lifting carb pistons the engine starts to stall.  => I think these both indicate Lean mixture.
I removed the piston dampers and tried to adjust, but there doesn't look like anything to turn there.  
Is this an old version with a cap preventing adjustment?  What do I do to make the misture more rich?

Answer
I'm not that familiar with the earlier TRs and carbs; take a look on the bottom side of the carb; is there a brass cap there, or a longer fitting with hex flats?  If the longer fitting, you have a set of adjustable jet and needle carbs, which I'm not familiar with tuning; but that would explain why you can't adjust the needle from the top; you adjust the jet from the bottom instead.  The Haynes guide on ZS should help if that's the case.