Thanks for all tips regarding my vacuum problem. I have now
repacked the inlet manifolds and found that the gaskets situated
between the manifold and the carb housing were wet with oil. This
could be a leaking point but still uncertain. I also checked the
carburettors witch seems OK. The only thing suspicious was one of
the piston dampers that had a bad thread. When blocking
inlet/outlet on the carb and blowing into the balance pipe inlet it
leaked quite heavily through the piston damper top mounting. This
was on the RH front carb. The one with the high vacuum is the RH
rear. Can the front carb affect the rear in this manner?
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In reply to a message from Tellan sent Tue 23 Sep 2008:
Hi Peter,
The carb throat/venturi is ‘connected’ to the void space under the
top cover of the carb by 2 holes in the piston. This is the
operation of the ‘Constant Depression’ principle. There is a small
~1mm drilling connecting this void space to the threaded area where
the plastic damper screws-in. I don’t understand any way that you
could have a ‘heavy leak’ of air out through the top of the
damper…as the only airpath is thru this tiny drilling, even if
the carb diaphragm was torn the air would still need to pass thru
the drilling to get to the outside world. Only scenario I can think
of that would allow a big leak there would be if the Mixture Needle
adjuster sealing ‘O’ Ring was missing.
Anyway, there are balance connections Front-Rear and Left-Right, so
in extreme cases one carb can ‘feed’ the whole engine… if three
carbs are blocked the remaining one is probably running/supplying
the engine on one bank since the bank crossover/balance pipe is
quite restrictive.–
The original message included these comments:
the piston dampers that had a bad thread. When blocking
inlet/outlet on the carb and blowing into the balance pipe inlet it
leaked quite heavily through the piston damper top mounting. This
was on the RH front carb. The one with the high vacuum is the RH
Visit the Jag Lovers homepage at http://www.jag-lovers.org for exciting services and resources including Photo Albums, Event Diary / Calendar, On Line Books and more !
Thanks again for all input regarding my vacuum problem. The
solution was really quite simple but the fault still difficult to
find. During engine rebuilt I used a well known gasket compound to
seal the carb housing/inlet manifold. It now seems that this was
not suitable for use regarding these gaskets. After repacking I now
have a really sweet engine for the first time! The carb restoration
probably increased the level of vacuum problems, due to tighter
throttle axels etc.
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