79 GS750L tach reads very low, is slow to respond also. ideas? ?

[standswithabeer]

79 GS750L
can't figure this one out! on rev'ing, the tach climbs very slowly to approx 1500 rpms' max.

the tach cable has been removed from the sheath, cleaned, lightly oiled, and reinstalled. the slotted end and opposite, squared end looked fine and the cable itself looks as-new.

if i disconnect the tach cable at the engine and connect the slotted cable end to my drill, the tach reacts perfectly. my drill is rated to 1300 rpm's and the tach runs smoothly up to 5000/5500 rpm's. the needle is steady, no wavering, very nice. as drill speed is varied, the tach needle rises and falls nicely. i think this is telling me the cable and tach are fine.

if i look at the engine output tab where the tach is connected, it's spinning pretty fast when rev'ing the engine. i think this is telling me then engine output is fine.

but when the tach is reconnected, it's back to reacting very slowly, and reads nowhere near the rpm the engine is turning. thought my diagnostic efforts were logical, but am missing something. when the valve clearances were checked, the drive and driven gears for the tach looked excellent.

any ideas on what i'm missing or how to next proceed? many thx,
bp

 

[Steve]

How about cleaning the inside of the sheath? :-k

I'll bet that it has all kinds of crud in there and just cleaning the core isn't enough.

.

 

[Redman]

Operating the cable with your drill motor was a good test of the cable and the tack itself.

With cable disconnected from the engine, and engine not running, look at the the drive unit. See if you can turn it.
ANd/or, with engine running, touch it with something and see if you can slow it down.


79 750L ...... that have the tach drive unit in the cam cover....? Or in the head....?

 

[Pop-up kat.]

Could be worth checking the cable run for a tight bend.

 

[standswithabeer]

just now back from checking a few things, and many thx for the quick replies!

the routing has no tight bends, and i loosened the "collars" at each end of the cable w/no luck. tach is still slow to respond. i disconnected the cable at the tach and tried to stop the spinning inner w/my fingers, and then with a short section of small dia hose wrapped around it and tightening a bit with pliers. no way to stop the cable from spinning. all appears solidly connected between the engine output to the tach input.

i did rev the engine until nearly WFO and held it there, the tach rose to 5K rpm's. hadn't tried this before.

on the tachs i have seen opened up ( japanese and my bmw) the cable spins a disc that is adjacent to, but not connected to, another disc. this design gives a smoother action than earlier mechanical/gear tachs. am thinking that a drop of light oil on the shaft of the second disc would cause my tach to respond quicker. i think the disc/shaft/tach needle assy needs to be cleaned and lightly lubed.

make sense?

 

[Steve]

Definitely worth a try. :encouragement:

.

 

[Redman]

You said the tach worked fine when spun the cable with drill motor. So don’t suspect problem with tach or cable.

By yet you say can’t stop the cable spinning .... so don’t suspect the cable or the drive unit.

i am out of ideas.

 

[standswithabeer]

have tried to spin the cable drive (at the engine) with the engine stopped, and tried to stop the cable from spinning when the engine is running - no go both methods. the symptoms are pointing me at least, to a sticky tach needle shaft. everything else is functioning / tests fine. but how to lube the shaft w/out disassembling the tach? maybe i could drill a small hole at the correct location, apply a drop of two of fine lube and plug the hole. think i'll try to find a pic of a tach interior, and take mine off to see if there's any access. thx,
bp

 

[Pop-up kat.]

Something I just remembered (little grey cells not as fast as they used be).Had a similar issue with my GSX. Turned out that a spider had set up home in the tacho and was using the needle as an anchor for its web.would rise to 3k then no more. Like you said there are 2 Adjacent discs .The web was restricting the top disc spinning while the lower carried on.not being mechanically linked,. Careful wipe out with a cotton bud and good to go. Worth a look.
NB. No spiders were hurt during the making of this repair.

 

[standswithabeer]

fixed !

i removed the tach and looked into the interior via the access holes available (the two bulb holders) and determined that the best location for drilling was at 12:00 and at the largest O.D. i secured the tach such that the drilling would take place horizontally, with more control than drilling straight down. i started w/a 1/8 inch drill, gradually increasing until a 1/2 inch bit was used. i wrapped the end of each bit with sticky tape, to prevent over-drilling and catch any plastic bits.

once done and looking down, i could see the needle shaft and it's support bearings, and the driven side bearing. i put a few drops of light oil on all bearings i could see. if there is such as thing as a "minor catastrophe", this then occurred. there apparently was a little plug in the oil dropper i was using, and it suddenly let go. more oil than expected is in there now.

i tested the tach w/my drill and it was more responsive. i sealed the drilled hole w/ a bit of scotch 33 tape and installed the plastic shroud. it's now waterproof again.

installed the tach and everything functions as it should, including the fuel level gauge and both instrument lights.