What happens (to the engine and Hg reading) if you if you unplug the carb
ports while the engine is idling? And when you unplug the ports and rapidly
(hopefully before it stalls, you could rev it up first) connect the
ventilation pipes? Do both carb ports have an equal effect? Or one a large
effect and one little?
The crankcase must have a restricted and filtered intake somewhere, or the
ventilation will suck dirty air in and too much air will flow through the carb
ports weakening the mixture. However in my experience (UK spec) this should
not be enough to cause a correctly running engine to stall, in fact it will
idle a little faster due to the vacuum leak. A non-emissions engine should
have a vented oil filler cap, which both restricts and filters. An emissions
equipped engine should have a non-vented oil filler cap. In that case the
filtering is done by the charcoal canister, and the restriction by the port on
the back of the rocker cover. If the charcoal canister is removed (no good
reason to do this by the way) you should seal the rocker cover port and use a
vented oil filler cap. If only one carb causes the problem then it would seem
its port has suddenly become a much larger diameter, like if a brass 'jet' has
fallen out (never heard of this).
Other points to be aware of. Your era of car originally had the distributor
vacuum advance pipe connected to the inlet manifold, in earlier years it was
connected to the rear carb. On the inlet manifold it gives maximum vacuum,
and maximum advance, at idle (actually on the overrun, but that is neither
here nor there). To do dynamic timing on these cars you must remove the pipe
from the manifold and plug the port. Failure to do this will give a very high
advance reading.
If you blow gently in the float chamber vent port you should be able to see
fuel bubbling up the jet (air filter off and piston raided with a finger) if
the float chamber is full and the ports are clear.
PaulH.
----- Original Message -----
I had deduced, after sending the message, that the hoses were for crankcase
ventilation. I concur that my symptoms make little sense. Let me give you the
lengthy chronology.
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