Out of the blue I'm riding to work this morning, doing about 70MPH on the interstate, I look down at the tachometer and see that the needle looks like it broke off of the tachometer. As long as the revs are above 4000 RPM, the needle flutters about 150 RPM in either direction.

Bike seems to ride fine, battery isn't loose, electronics all work fine.

What the heck is going on? I'm not home yet, but I plan on pulling the spark plugs to make sure they're not fouled up, but this problem just popped up out of the blue.

This bike has the electronic tach. It is NOT the mechanical cable that threads into the cylinder head.

Muffed wiring? If you can't hear or feel any surging along with the fluctuations on the tach, I'd say grab a volt meter and hop to it. It could be something as simple as moisture in a connector. My car horn sounds absolutely pitiful after I splash through a puddle, then it's fine 2 days later once the leads dry out.

Well I did some more checking today, but no luck so far.

- Wires behind tach are just fine. Connections are snug, no moisture present.

- Spark plugs were a bit loose, or at least they didn't require much force to unscrew. Both cylinders have very clean and optimal burn with a brown/gray ash on the tip.

- Battery voltage increases with RPMs, up to 14.10 volts. Battery voltage with engine off is right at 12.8 volts.

- No wires seem to be frayed or damaged.

- The needle flutter only occurs above 4500 RPM and I'm starting to think it's related to combustion rather than electronics. In other words, the RPM is actually fluttering a few hundred in each direction.

- Unrelated, but I didn't have my battery tender SAE lead secured and the tip ended up getting chewed away by the chain over probably 2 weeks of riding. Luckily they only cost $6 shipped prime on Amazon.

Next step is to pull the tank, clean the carbs, and I might as well check the valve clearances.

Assuming your clutch is not slipping and everything is tight, at 4500rpm in the 6th gear you should be doing ~55mph. If your rpms were actually oscillating +/- 150rpm, that means a total amplitude of 300rpm, which is 1/15th of your total rpm. That would mean that your bike's speed oscillates between 53 and 57 mph. If that is happening, then your ride must be hella rough and your tachometer is showing the right thing (check whether or not your speedometer does the same thing?).

If this is not what you have experienced, then either something's wrong with the tacho itself, or with the sensor inside the engine?

Quote from: jsyzdek on June 23, 2014, 03:45:57 PM
Assuming your clutch is not slipping and everything is tight, at 4500rpm in the 6th gear you should be doing ~55mph. If your rpms were actually oscillating +/- 150rpm, that means a total amplitude of 300rpm, which is 1/15th of your total rpm. That would mean that your bike's speed oscillates between 53 and 57 mph. If that is happening, then your ride must be hella rough and your tachometer is showing the right thing (check whether or not your speedometer does the same thing?).

If this is not what you have experienced, then either something's wrong with the tacho itself, or with the sensor inside the engine?

I'm about 70% sure that the problem is the rectifier. I've googled the heck outta this problem and on a bike with an electronic tach, and no motor issues, a fluttering RPM needle is often due to voltage fluctuations due to the rectifier.

I have the bike in pieces right now so I just need to go through and re-connect all electrical connections (and blast some CRC QD cleaner on the contacts while I'm in there).

For what it's worth, I recently discovered a dead rectifier on my bike as well and I've experienced a silent tachometer on several occasions. I would start the bike and the tach wouldn't budge but would eventually jump back into action after a few minutes of riding. The manual has a pretty good description on how to test your rectifier.