tach' question

More or less out of curiousity I put an inductive tach (Fluke) on
friend's '82 CB750 today. #3 cylinder has been warming up slowly lately
even after thorough carb clean and sync last year and recent re-sync.
coil resistance seems okay, at least for a cold engine - both coils
around 2.4 and 12K ohms.


What I can't figure out are odd readings on #3 and #4 cylinders. #3 is
consistently reading about 3 times what the bike's mechanical analog
tach shows and #4 about double the bike's tach. Whereas #1 and #2 agree
more or less with the bike's tach! Re-tried about half a dozen times on
all four plug wires with the same result. Any ideas as to why?


(just in case anybody's wondering, from left-to-right, the coil wires
got to plugs 1,4,2,3).

 

forgot to mention that I didn't take the tank off, don't have much
experience with these inductive tach's, perhaps because of less
clearance with the tank on, I was seeing the effect of some kind of
interference. but if so, why only on #'s 3 and 4?

 

Dual lead coils have been known to do strange things when one of the
spark plug pairs is fouled.

 

As background info, each spark plug fires twice as often as it needs
to. Once at the top of the compression stroke, once at the top of the
exhaust stroke.

Dave

 

Thanks for that clue, now I have an idea of other tests to try.

 

The latest is that today I inter-changed the plugs, moved #1 to #4 and
vice-versa, #2 to #3 and vice-versa. Waddayaknow, now #2 reads triple
and #1 reads half! (#3 and #4 look close to the bike's tach and #3
warms up faster than before, but I already knew it had other problems -
long story, i swapped coils from pal's parts bike which I suspect had no
effect, even though the parts coil wires seemed to have firmer
connections). so, as suggested, the mis-reading has certainly moved
with the plugs.

(all four plugs were somewhat black and only #1 had a light nose.
doesn't surprise me since bike hasn't had any serious exercise for a
year. just for laughs, I'll clean plugs and try again before telling
pal to buy some new ones - he gets a discount because he's a welder,
don't ask me why. just for my own edification, i think i'll make a
standalone coil tester and keep a fouled plug around for future tests on
other coils.)
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