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

[Mark Battley <M.Battley@xxxxxxxxxx>]

Anthony asked:
----------------------
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?    
----------------------
Hi Anthony,

I'm not familiar with the 116 vacuum dist, but based on how they normally
work I'd be surprised if that was the cause. More likely is that it could be
caused by incorrect balance between the two carbs (one is closing before the
other), this can cause slow return to idle. Presumably you would have had to
adjust the balance setting screw between the carbs after refitting them?
Perhaps check this again, or at least change it and see if it makes a
difference.

Good luck,

Mark Battley
Auckland, New Zealand

1973 Alfa Romeo 2000 GT Veloce (with two stainless resleeved brake boosters,
and brakes that feel better than they ever have). Shame it had a flat tire
this morning... now fixed.
1989 Fiat Uno Turbo
1999 Fiat Marea Wagon, 2.0 20V
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