My 1968 4.2l engine has been rebuilt with new rings and valves and has good compression on all cylinders. It has a new Petronic distributor and coil and all new plugs. Timing about 10 btdc. It runs well with good power. Two zs carbs which have equal air flow and correct mix. Exhaust header temperatures suggest all cylinders are firing. Plugs all clean.
All good but I get strange results when I remove spark plug wires one by one (and engine running at about 1000 rpm)
Only removal of plug 2 or 5 significantly reduces rpm (to about 700 rpm). Removal of all other wires (one at a time) does not change engine speed or sound to any noticable extent.
This seems very strange and never seen this on any other engine.
Would anyone like to suggest what is happening, including possible defect in my testing procedure?
what was your compression test procedure & results per cyl?.. i have seen/heard these oddities in other motors…any particular reason for removing plug leads while running - what’s it like on a road test?
…ummm, what? Crank pins #2 & #5 are indeed in the same radial position. But so are #1 & #6, and #3 & #4. Each pair is 120 degrees apart on the crank. So what has this to do with Phillip’s observation that when he disconnects either #2 or #5 plug the revs drop, but when he disconnects any of the other 4 it makes no difference, or significantly less?