Dodgy speedo.

Can anyone think of a fault that would cause intermittent operation of a
speedo head? Its on a 76 Honda CB200 - magnet spins in close proximity to an
aluminium disk - definitely nothing electronic about it

Its not the cable, that had very slight damage to the outer so it was
slightly longer relative to the inner so I replaced it.

My problem is to eliminate any cause other than the drive gear, its not
mashed as I've seen happen on other bikes (Norton Commando) but it may be
worn, the surface of the teeth is slightly concave but I don't know if
that's normal, and the cable drive turns when the wheel is rotated.

Any help?

TIA.

 

The only time I've had an intermittent speedo problem[1] it was a worn
speedo drive - traced it by swapping out the cable then the drive from a
parts bike I had handy

If you don't have a parts bike you could try using a drill to turn the
cable while it's connected to the speedo, that would let you see if it's
the speedo drive that's playing up or if the problem is at the head.

Thinking about it, I'd use a cordless drill which had decent low speed
control - I've no idea how many rpm a speedo cable turns at and you
wouldn't want to ruin your speedo while trying to establish if the
speedo drive was the problem.

If it is the head I've no idea what your next step would be.

Bob

[1] seemed okay in town but would drop from 60 to 20ish before climbing
back up when holding a steady 70 on the motorway

 

Perhaps an itsy-bitsy spider has crawled up your waterspout?

Oh, and I might as well say something nasty to Piggy before he turns
this into a 100 post thread.

Sod off, Piggy.

 

Does the odometer work perfectly? If it does, then the speedo is faulty.
If not, then it'll almost certainly be the drive gear in the wheel. But
if the cable inner is turning fine, then my money would be on a dying
speedo.

 

Unfortunatly that is very common on that era of honda bikes. I had
many of them and they all were in the speedo head. KB

 

<Shrug>

I had many and never had a problem....

 

I probably have a drive gear or two in the mixed odds & ends box but there's
no telling which if any are the right pitch, secondly both driving and
driven gear are probably worn, so I'll be relying on full tooth height of a
new driving gear to compensate for wear on the driven gear if that's what
the problem is.

Last time I tried that it soon became obvious that the drill was turning the
speedo backwards (handy for "clocking" the odo) a bit easier nowadays as I
have a reversible cordless. So its knock the handle off an electricians
screwdriver to turn the slotted end of the cable or find a scrap cable with
a snapped inner I can cut to convenient length to drive the speedo head.

 

There aren't any milestones around here I can check the odo by, the speedo
can work ok for a few miles at a time (as estimated by RPM in top gear) most
often it takes a few yards to pick up from zero and sometimes doesn't get
above about 60% of true speed.

It never reads more than actual speed so I'm pretty sure its not sticking.

 

That reply probably comes closest to answering my question, just checking
whether anyone else knew of a failure mode I hadn't anticipated, I've
checked the cable - that only leaves the drive gear.

Thanks.

 

Seems like it could be the drive gear
or a problem at either end of the cable
where it engages to another component
or in the speedo head itself.

The best question asked so far was whether
the odometer keeps on working when the
speedo gets flakey. If the odometer also
gets flakey, then the problem would be in
the cable engagement, cable or drive gear.

If the odometer keeps working OK, then you
would probably look for a problem in the
actual speedo.

My understanding is that the speedo itself
is just a needle on precision bearings
with a spring to pull it towards zero and
a spinning magnet driven by the cable
which creates a magnetic field which pulls
the needle away from zero. Speedo and
odometer operate more or less independantly.

 

Here's maybe an oddball one: I had a Honda ch250 scoot with a
mechanical speedo' that worked intermittently. I finally came to the
conclusion that the set screw was a bit narrow at the tip, perhaps
from somebody trying to remove the cable without loosening the screw
or maybe from somebody tightening it too much. When I replaced the
set screw with one that had a nice thread at the tip, (along with some
loctite), the problem went away. One theory is that the old screw was
letting the groove on the gear slide up just enough for the tang on
the cable to disengage. Sometimes mere vibration seemed enough to
make it slide down. On the other hand, maybe it was as simple as the
tang having worn down.

 

Well, no, it leaves the speedo unit itself. Failure here is rare but not
unknown.

 

An old speedo I found at the back of the shed temporarily taped to the fork
leg does the same, so since I've eliminated the cable it must be the drive
gear, a replacement gear is now on order - hopefully the driven gear is
still in useable condition as its not available as a spare - you have to buy
the whole brake plate.

 

It's very easy on those front wheels to get the little tangs that
drive the speedo gear misaligned with the slots for them in the hub.

I'd also double check that the cable is OK and that it's correctly
fastened at either end. And pull out the inner, insert it into the
speedo head, and twiddle it to see if the needle moves as it should.

Whenever I've had any speedo grief, it's always been the cable. Apart
from a couple of occasions where I misaligned the drive tangs (see
above) and once where a speedo unit just died.

Oh, and then a couple of times I've had a spider set up home in the
speedo and jam the needle with cobweb

 

(The Older Gentleman) wrote in

So what models did you have? I had at least one new bike a year in the
70s and I would say 30 percent had the speedo replaced under warrenty,
and another 30 percent had to be replaced before 10,000 miles. All had
the exact problem described. KB

 

I've never had that problem with this particular design of hub end drive.

I'd already taken the inner out, inspected, cleaned and greased it. As I
mentioned in my original post, there was slight damage to the outer so it
was stretched a tiny bit, as the square bit at the top didn't protrude very
far with the slotted drive at the bottom flush, I rummaged in the shed and
found another one - this was also inspected, cleaned and greased before
fitting.

 

Space doesn't permit, but maybe 300 bikes of all types in the last 30+
years. Of small-bore Honda twins in particular, um, three or four
CB200s (can't be specific as to the number), four or five CB175s (same
bike, basically), a couple of dozen CD175s (the basic commuter
version), three or four CD200s, a CJ250, CB360.... Never had any
speedo problems with any of them.

The best of that lot was the CB175 and I wish I hadn't sold the last
one I had, maybe 12 years ago. The CB200 was nowhere near as good a
bike. The CD200 was/is the toughest: unbelievably strong engine in
those things. The 250 Rebel was basically just a development of the
CD200 lump.

 

I agree that 175 was a heck of a bike for what it was. The rebels were
ok but not in the same class as the 100-125-175s were. they had the
quality of the big bikes in a small package. the rebel class was a cheep
mass seller of the lower end. KB