[alfa] slow engine deceleration in off-throttle conditions -- ignition problem?

[White Anthony <anthony.white@xxxxxxxxxxxxxx>]

Before Christmas, I dropped a rebuilt head on my 116.  At the same time, I
took the Dellortos off my 105 (under restoration), and put them on the 116,
as the Dellortos on the latter need rebuilding.  
 
The 116 Dellortos have an integrated vacuum take-off for the vacuum advance
on the ignition.  The 105 carbs do not.  In the interim I have blocked the
vacuum advance mechanism on the distributor.  For the most part, the car is
running really well.  However, when lifting, particularly to shift gears,
there is a pause of around half to three quarters of a second in which the
engine speed will stay the same before the revs will drop, and even then the
drop is quite slow.  This is longer than it takes to change gears, and so is
providing a rather unpleasant sensation.  
 
Throttle return spring strength is good.  The carburettors didn't cause the
same problem on the 105, so I doubt that the problem is with them.  
 
I'm guessing it is an ignition problem -- specifically, that the centrifugal
advance mechanism on the vacuum advance distributor is weighted differently
from that in a mechanical advance only distributor, and so, without an
appropriate vacuum take-off, isn't retarding the ignition when I lift the
throttle.  However, it's been some time since I swotted up on ignition
operation, so this is fairly uninformed speculation.  I do have a mechanical
advance distributor sitting in my workshop, so I'll try swapping that later
in the week.  
 
Until then, is anyone able to offer any advice or suggestions?    
 
Kind regards,
Anthony White
Wellington, NZ
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