Re: Slow return to idle (alfa-digest V7 #1023)

[David Rubie <drubie@domain.elided>]

> 
> Date: Mon, 6 Sep 1999 14:19:35 +0200
> From: CN=Per-Olof_Lindroos/O=Tillquist_Process_AB@domain.elided
> Subject: Ang: alfa-digest V7 #1017
> 
> Slow return to idle.
> 
> I have a GTV 2.0 1994. When I lift my foot from thev accellerator the r=
> ev.
> of the motor drops to approx 2000 rpm. It stays there and decrease very=
> 
> slowly. I have adjusted the Dell=D3rtos according the boook and even ha=
> ve
> them checked by an Alfa Romeo Garage. Mechanicly the carburatos are bac=
> k in
> the idle possition but the it idles 2000 rpm. After 1 minute the revs
> slowly drops to the right idle, 750-800 rpm. I drive this car very seld=
> om.
> I have read all I have come over about Dell=D2rtos but never seen this =
> fault
> described.
> 

Here are some things to check:

On the standard factory Dellorto setup there is a central solenoid
nestled between the carburettors.  This solenoid controls the
idle speed (it acts as the throttle stop when the throttle plates
are closed, holding them open fractionally to wherever it's
adjusted to).  It is also controlled via answitch on your clutch
pedal, so that when the car is on "overrun" (i.e. the engine is
slowing down while you change gears or come to an intersection) it
holds the throttles open a little more to assist  in cleaning up
the emissions somewhat (stops the engine just "dumping" loads
of fuel when you back off by making it gradually slow down rather than
just instantly backing off).

Anyway, if this solenoid is stuck open, the engine will idle high
and slow down very slowly.

Another problem you might face is if the carburettors are poorly
synchronised (i.e. one set of throttle plates is shut but the others
are open) you can get much the same effect, only the car will probably
hesitate on acceleration and idle roughly.

Also, the "choke" might be stuck open - there is a metal cable
that runs to the top of both of the carburettors, if the cable
is stuck in it's sheath you'd get similar symptoms to what you've
described but you'd also get a lot of plug fouling, backfiring
and oil thinning problems too.

Further problems might be caused by a vacuum leak, usually from
the brake booster pipe (it comes from #4 inlet right at the back
of the block).  Either the pipe is old and cracked, or the vacuum
valve is operating poorly (or worse, the brake booster is starting
to leak).

There is another, much smaller vacuum line from #1 inlet which feeds
into the crankase ventilation system but it can be unhooked
without you noticing.  (this is a small black hose that goes to
the valve cover - there is an intersection of pipes on the distributor
side of the engine, one big black one going to the air box, one small
one heading back to the fuel tank via the charcoal canister, one small
one going down to #1 cylinder.  

My guess is that (a) your carbs are poorly synchronised (there is
an adjustment screw between them - you need a set of vacuum gauges,
a set of "mercury" carb adjusters normally found in motorcycle shops
or a "CarbTune" to set them up properly)  or (b) the solenoid
between the carbs that controls the throttle stop is poorly
adjusted or stuck open.

dave.

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