Strange engine behavior

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?

Runs well: check.
Cools well: check.
EGT all good: check.

Answer: quit taking the plug wires off…:smirk:


I took off plugs to see if all engines firing as I.have done with many other engines. Part test and part just curiosity.

In spite of your comments, I still find it puzzling that only two cylinders are apparently making the engine turn. Am I wrong?

No idea if you’re, “wrong”, but I really don’t know why it’s happening.

can you give a more accurate reading of what all rpm response is on all the cyl’s one by one
study this motion and you may find the answer…


1 Like

Double check the routing of the plug wires, make sure they are all going to the correct cylinders.

1 Like

As this is just when stationary i would not worry about is. Cyl 2 and 5 are closes to the cabs, with the shortest route. This could explain this.

Had a jeep cj5 with a straight 6 …dealer could not explain this either.

If you look at # 5 & # 2 cyl’s on the gif animation i posted long enough, you’ll see the answer

I may be dense, and I may not have looked at nearly enough six-cylinder Jaguar engines… What am I supposed to see that explains this?

#5 & #2 are opposing cyl’s to 134&6…but still waiting on Phillip to say how much RPM do the other cyl’s drop by if 5 & 2 drop 300 RPM

…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?


If you haven’t, try a digital RPM readout, drop can be as low as 50 rpm

take it for a hard thrash with all wires connected, then try the same with any suspect off, seat of the pants will be instructive

The more I look at the graphic, it looks like the crank doesn’t quite go over centre. :thinking: Mind you, that’s probably not the OP’s problem.