Date: Tue, 06 Jan 2004 11:28:39 +1300
From: White Anthony <anthony.white@domain.elided>
Subject: [alfa] slow engine deceleration in off-throttle conditions --
ignition problem?
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