Alfa Romeo Forum banner

1 - 11 of 11 Posts

·
Registered
Joined
·
57 Posts
Discussion Starter #1
I'm changing the carbs on my 1989 Spider. The old set are weber dcoM and I'm changing them to weber dcoe's.

On the old set there is a connection for a hose which comes from just below the distributor (see pics below).

On the new carbs, there is no corresponding connection for this hose to fit to.

Does anyone know what it does, and more importantly what might happen if I just remove the hose when the new carbs are fitted?

Thanks for any help

alfa hose 1.jpg

alfa hose 2.jpg

Jon
 

·
Registered
Joined
·
1,553 Posts
Yep, looks like vacuum advance to me too.
Without it connected your ignition timing will be out in the higher rev ranges.
 

·
AO Detailing COTY Winner 2018
Joined
·
547 Posts
Yep, looks like vacuum advance to me too.
Without it connected your ignition timing will be out in the higher rev ranges.
Vacuum advance is actually a fuel economy/efficiency device.

When you're bowling along at a steady cruising speed on a fairly light throttle, there's a partial vacuum in the inlet manifold. This is used to mechanically advance the ignition timing so that the (relatively lean and slower-burning) air/fuel mixture is ignited sooner, resulting in more complete combustion. When you floor the throttle the vacuum is destroyed, so the ignition retards back to it's 'normal' setting, the carbs add more fuel (creating a faster-burning mixture), and off we go....!
 

·
Registered
Joined
·
3,532 Posts
Earlier 105's didn't have vacuum advance but just relied on mechanical, apparently the vacuum advance helps with economy and low end running, I have to assume the non vacuum advance dizzy's were configured differently from the vacuum ones as mine (without) runs absolutely fine - options are to tap a new vacuum take off from no.1 inlet or to fit an older dizzy, it would probably be best to tap it as I don't know what else was different on a 89
 

·
Registered
Joined
·
57 Posts
Discussion Starter #7
Thanks for the advice and information.

Is tapping into No1 inlet going to involve major surgery, or is it straightforward?
 

·
Registered
Joined
·
135 Posts
Check to see if the scope have a connection you can use. Early Dollorto and I suspect Weber's had a vacuum connection for tuning. My son had the same issue and we simply screwed in a connector to one of these and connected the dizzy to this.
 

·
AO Detailing COTY Winner 2018
Joined
·
547 Posts
I know (on a series 3 2000) at a steady 3500 rpm disconnecting it significantly retards the ignition (according to my strobe)...
Yes, the amount of vacuum advance is usually quite a bit. A typical vacuum advance under light throttle/moderate engine speed of say 3500 rpm is around 12-15 degrees, creating a theoretical maximum advance of maybe 36 (centrifugal) + 14 (vacuum) = 50 degrees BTDC....
 

·
Registered
Joined
·
1,553 Posts
Early Dollorto and I suspect Weber's had a vacuum connection for tuning. My son had the same issue and we simply screwed in a connector to one of these and connected the dizzy to this.
I think this is the easiest route
You can use the vacuum gauge take off on no 1 (if it has one, and it will) for the vacuum advance.
No drilling and the minor expense of a connector.
 

·
Registered
Joined
·
57 Posts
Discussion Starter #11
Found it. Thanks again, I wasn't sure my bank balance was up to a new dizzie after forking out for the carbs.
 
1 - 11 of 11 Posts
Top