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 integral. 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'.
Any thoughts?