Have installed a rebuilt 2.8L (in 86 AMC Jeep) which doesnt develop
enough vacuum to adequately control the carb/emissions stuff, etc.
At high idle it only pulls 14-15" Hg vac. instead of a normal 18-20".
the vac advance to the distrib. is therefore weak and I have had to
manually advance to about 20" BTDC for best (?) HP output and idle rpm.
Due to the low vacuum control, the carburator (Rochester VariJet 2SE) is
corrrespondingly running rich (probably due to bad control of the EGR).
The rich mix plus the inability to advance the dist. correctly has
yielded some 'driveability' probs and poor fuel economy. Have 2 carbs,
... both carbs perform EXACT same. Current fuel mpg is 16-17.... on
most 2.8s I can usually 'tweak' to 20-22 mpg. Distributer has been
advanced to get maximum vacuum at high idle (about 20 deg BTDC with
centrifugal advance engaged).
Due to rich mix and low vacuum (to the dist advance), I CANT get this
engine to a 'preignition/knock' state (under lugging load) .... can
advance all the way to high speed stumble - all without ANY pre-ignition.
Ive run propane all over the edges of the intake manifold and carb base
without seeing any rpm increase (gasket leakage). Engine idles VERY
smooth and an applied vac gauge on the intake manifold is VERY steady so
Im sure the valve train is properly adjusted (not tight). Dry cylinder
compression is all normal and equal. All vacuum connections/hoses are
ANY vac hose (either connected to intake manifold or lower throat of
carb - high vac zone of venturi) that is 'pulled' while idling results
in INCREASE or rpm .... instead of 'stumble'.