Is it the correct tacho for the bike ?

Mark got right down to my input...simple no way to mess up. Unhook the tach cable at the motor. Connect the drill to that end (motor end) and switch drill to reverse. Turn on drill and tach will work. Don't get hung up on how your looking at what. Do my simple 3 steps and then you'll know. Post the outcome.

Thanks guys, I'll have another go at it this weekend. In the mean time I'll try and figure out if there's some sort of slow-mo recording I can do on my phone.

BTW I have two cables: the brand new one on the bike and the original worn and gunky one both rotate freely. I did wonder if magical pixies put a 1:1 hidden gear in my new one but as the inner cable slides in and out as it should probably not (I noticed a brass collar at the engine end of the cable.)

IIRC the drill was in reverse 'cos the first time I spun it would have been in normal drill mode (reverse rarely gets used) and didn't do anything that way.

I'm assuming it's the correct tachometer only 'cos I've looked at all the DT clocks on eBay and this looks the same. The only variation seems to be where the red line is and how many lights there are (like older DT1 clocks might not have hi/lo beam.) Anyhoo, the only reason I can see to have a "reverse tachometer" is for some of the old Triumph/Norton/BSA/Enfield stuff. I did contemplate getting an angle drive intended for one of those but it makes no sense that I should need one!

So here's the plan:

1. Connect tachometer to existing cable, start engine and see if anything happens
2. Disconnect engine end of tacho cable and try with drill in reverse
3. Do the above with the old cable
4. Check over both cables for play, damage, length etc.
5. Film tachometer working (whether it be by drill or engine)
6. Film tacho cable with engine connected
7. Pre-book bed in psychiatric ward
8. If all else fails, pull off crank case cover and examine tacho gears!

Completely dumb question but I have to ask: is it possible to somehow accidentally have the engine go in reverse? (For a points ignition bike.)

Yes, it is possible for it to run in reverse of what it's supposed to. I've seen it more than once. My DT400C that I converted to points ignition ran reverse once.

Wow! Really? How does it happen?!

Thing is: every time I've tested the tachometer the bike's been static and every time I've test ridden it the tachometer was disconnected (and I've not paid attention to the tacho cable!) If there was say a 50/50 chance I might have just got unlucky, I've only looked at this a couple of times.

<addendum> NVM, I now feel like a total n00b: loads of YouTube videos on the subject. Scratch that list, the first thing I'll do is take the plug out and slowly kick it over and see which way the tacho cable rotates. I have the relevant magneto puller in my toolbox if it is running backwards

If you have been riding your bike , ( and going forwards ) , then it isn't running backwards .
I had a T 350 Suzuki , with a electronic ignition fitted . The klutz who installed the ignition ,
( me ) , left a magnetic blanking plate out , and the alternator magnets could trigger a stray
spark at inappropriate times . It used to cough briefly and then run backwards .
It did it once at a set of traffic lights , and when I let the clutch out , off she went in reverse .
Luckily no one was behind me .

First, I think you solved your problem... or it will be solved if you install the old cable... It would not be the first time I saw a new cable that had a inner core that was a smidge too short... funniest one was the "turn and play"... turn bars to the right, tach would work. Turn bars to left, tach would stop.

(And maybe the new cable will work too... could just maybe be the core didn't engage right or something as it sounds like most of your running has been with Tach disconnected)

As far as running backwards, there's nothing you can "fix" under the flywheel. If you give it a weak kick or more often a "bump start", the engine can start backwards with a points magneto. The 175's and 100's must have been more likely to do this because Yamaha started putting a diode in the wire from the points to the ignition coil to prevent it from starting backwards. Yours is new enough to have the diode... should be in the harness right at the connector where the engine harness plugs in to the main harness.

(Diode works because if mag is spinning backwards, pulse to coil when points open is reversed polarity which diode blocks from getting to coil so no spark)

Looking forward to good news!

No diode that I've seen. Unless it's hidden under the flywheel with the points and condenser I haven't seen it.

Interesting though: if it did turn out to be the problem (running backwards that is) at least I'd have a permanent fix available after sorting the timing out. Many thanx!!!!

<addendum> Mark, you legend, I searched to see what this diode looks like and the first thing I found is one of your posts talking about it Anyhoo, having seen pictures I can 100% say I don't have that diode fitted.

Diode we had on the points model AG175's was the last thing inline before the plug coil under the tank & was added to stop that very thing happening because severe damage is more than likely to occur if you accidentaly touched the kick start while running--like split from ear to ear. By memory it was encapsed in something black, only a short looking thingy. Sometimes they failed but then it wouldn't run at all. Was a bit of a learning curve back then. Mind you, for that to happen while kick starting it'd need the timing very retarded, usually caused by points barely opening if it has a fixed stator plate or very incorrect timing if the stator late is movable. Be very unlikely that that is your problem [running backwards] even though i sort of jokingly said 'that' [well it had to be said lol]. I was riding at a motocross in the late "60's where a guy on a Husky 360 was eyeballing the starter bungy cord operator hoping to be first to the first corner, & full song in reverse was what happened lol. Bike was running a bit hairy so was fanging it heaps to clear its throat. Man i miss those days. This was international stars coming to NZ for the MX series. JN Roberts was his name & was a lot of fun for us young guys. Well he wasn't much older but ad a trail of fans [women lol] wherever he went [& we were jealous haha]. Sorry--not sure how a tacho cable story morphed into this. I'll try harder!!.

oscarbyalpinestars.blogspot.com/2010/11/jn-roberts.html

one more relevant thing is i've found tacho's & speedo's that might've sat for years can stick at zero & need a good spin up [in the right direction] to get them active. Not sure why but maybe whatever lubricant they use must dry out till they got moving again.