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Difficult Dellorto's?
Having difficulty getting an Alfa 105 series Giulia with a later 2.0
transplant to idle smoothly. The engine is from a European Alfetta (as
reported by the prior owner, not verified) with Dellorto carbs. The
engine was rebuilt within the past couple thousand miles before it was
transplanted into this '73 Giulia Super 1.3 in Europe.
When I first looked at it, all mixture screws were five turns out except for cylinder four - it was six turns out. The prior owner said he was unable to
adjust the mixture on cylinder four.
I removed the carbs from vehicle. Opened them up. The Dellorto DHLA
carbs are in really good shape. Found nothing unusual. Float bowls
were spotless, passageways clear. Float level OK. All jets are
numbered the same. Reassembled carbs and back on the car they went.
After reinstalling, I set up the balance between carbs visually by the
progression holes underneath the plugs. Can anyone tell me where the
throttle plates should be approximately at idle when viewed through the
progression slots? If I turn the idle screw all the way out until it no longer
acts on the throttle lever, the throttle plates contact the carb bores
(plates fully closed) and cover the first hole (cylinder head side) and half of
the second. If I turn the idle screw out to where it just begins to
contact the lever, the first hole is half open and the second is covered
- so I assume that is about where the base idle should be.
The engine will not idle at this setting though, it dies, even with the
mixture screws out five turns. If I increase the idle speed, then the
engine has a tendency to overrev (3000 rpm or so) and not return to idle
on deceleration.
I did a power balance test at idle by shorting plug wires. All cylinders have
fairly even rpm drop as they are killed except for number four - no noticeable change in rpm.
Number four has good compression and spark (new NGK B7ES plugs, have
swapped the plug and wire and still have miss. It does not miss off
idle. Running a new Centerline ID405 electronic ignition with Bosch
Blue coil and plugs gapped at .030. Ignition timing is set at 35
degrees total advance, 11 degrees advance at idle. Runs well outside of
idle circuits - no problems accelerating.)
Checked for vacuum leaks with smoke machine. No leaks present at intake
manifold. Was hoping to see one around cylinder number four explaining
idle miss but it is sealed up tight. Brake booster is not leaking either
(still had lots of vacuum present with engine off when I removed the vac
hose to put the smoke in there.) Also went around intake with propane
while the engine idled - no change in rpm. If I add propane at the air
filter intake - the idle smooths out as cylinder number four comes up. Back carb does not seem to deliver enough fuel on idle circuit even with the mixture screw all the way out.
At this point, I'm not sure what to do next. Any suggestions? Thought
I might swap the front carb with the back carb expecting the miss to move
from cylinder four to two. That will at least confirm what I already know.
I put a vacuum gauge to the brake booster line while it was off - get
15-16 inches at idle. I was expecting closer to 18. I am thinking the cams
are timed properly because the power off idle is good.
Thanks in advance for any thoughts.
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