Engine Mechanical problem
1969 Volkswagen Van 4 cyl Two Wheel Drive Manual 89000 miles

Engine is newer probably 71-73. seems to fire on left bank and can adjust distributor to run "smooth". Can pull plugs wires off cylinders 1 or 2 and there is no change in running. pull plug wires off 3 or 4 and vehicle dies. Has compression and spark on right bank. Could there be a "plug" in the "feed" from the carburator to the right bank? Thank you for your help.
LLoyd