Posted by paul c on August 28, 2008, 9:55 pm
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).
Posted by paul c on August 28, 2008, 9:59 pm
paul c wrote:
> 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?
Posted by . on August 29, 2008, 1:51 am
> 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?
Dual lead coils have been known to do strange things when one of the
spark plug pairs is fouled.
Posted by paul c on August 29, 2008, 3:23 pm
. wrote:
>
>> 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?
>
> Dual lead coils have been known to do strange things when one of the
> spark plug pairs is fouled.
>
>
>
>
Thanks for that clue, now I have an idea of other tests to try.
Posted by paul c on September 1, 2008, 7:24 pm
. wrote:
>
>> 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?
>
> Dual lead coils have been known to do strange things when one of the
> spark plug pairs is fouled.
>
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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> 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).