Well, I've not seen this on the board before and a search yielded nothing similar, so I'm hoping someone can help. My tachometer looks to be reading about 2k high relative to what I believe the actual RPM is (the latter based on knowing the sounds of the engine at different RPM). So when it indicates I am at 12k RPM (redline), I'm pretty sure I'm only at about 10k - no way am I at redline anyway. When it says I am at 9k, where there usually is that sound of coming on the pipe, it feels like I am at about 7 - 7.5k. I know the sounds of the engine well enough to know the RPM reading is off, and just to complicate matters, this seems to be happening on an intermittent basis, so sometimes I'll go 50 miles and it will be fine. If there is any pattern, it is that when the RPM readings are too high, they stay too high continuously until I shut the bike off. On re-start, it may or may not start reading accurately, but if it is in accurate mode, it stays fine until I shut the bike down again, i.e., the intermittent aspect so far is a function of starting and restarting, not within a given running of the engine - not sure that is clear.

I checked around the tach for any signs of chaffed wires (none obvious) and cleaned up and tightened the ground, with no apparent effect. The engine seems to be running fine, even when the RPM is off. So can anyone suggest what else to check and the reason a tach would read high? Thanks for any help.
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-Jim
'93 MC21

Some manufacturers have (allegedly) been getting away with tricks like this for years!! Some even (allegedly) use the optimistic tacho as a selling point, maybe.

No help at all I know.

Suzuki and Kawasaki are the worst for that. TL1000 shows 11,200rpm but infact limiter cuts in at 10,000rpm. ZX636 will show 15,500 but cant even manage 13,500 when measured.

As for the NSR If you have checked the earths then I would swap the tach, they can be quite fraglie and can give up the ghost (race kit suzuki ones always die). Its very hard for one to read high due to a wiring fault as its triggered by the PGM, more often a tach will read low if something in the wiring is buggered, so you dont want to know what I would change after that.
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If I have to take the carbs off once more...

Thanks Stephen - I gather the "other" possibility for the intermittent high reading is the PGM? Against all odds, it would actually be easier for me to swap out the PGM than tachometer - I have a spare PGM but no extra tach. Worth swapping the PGM out? Thanks.
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-Jim
'93 MC21

That's what he was getting at Jim, so worth a go!
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Andy.
NSR-WORLD.COM

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It's possible that the electronics in the tacho have gone wonky. What you basically have is a frquency to vlotage convertor that monitors the signal from the PGM and then converts it to a voltage to move the dial on the tacho. The conversion is controlled by a set of resistors and capacitors and if any of those are past their sell by date then it's possible that you could get erratic readings.

I'd go for a tacho swap to see if the problem goes away.

Form tests I've done, the MC21 tacho is about 2~400 rpm optimistic, whereas the MC28 is about 500 rpm over the top.. If you wanted to be really anal, there's a potentiometer on the tacho control PCB that you could probably use to calibrate to the correct numbers using an accurate digital gauge.

Another bike that seemed very rev happy on the tacho was the '00 R6. 16,000+ on the clock, but only ever saw a max of 15,200rpm on the datalogger and that was on the down change.

Okay, thanks Matt. I'll have to keep my eyes open for another tachometer then. I see that the MC18 has a different part number from the MC21 tach, but does anyone know if the 18 will work for the 21, or whether any other tachometer cross-references? Thanks.
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-Jim
'93 MC21

The MC18 will work if the wiring is modified, but the units aren't interchangeable by simply swapping them!

The MC21 is an "all-in-one" unit (tacho and temp gauge) where are the MC18 uses all individual components. Therefore, although the '18 can be modified, it ain't straightforward.
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Andy.
NSR-WORLD.COM

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okay, - scratch that idea - thanks for the info though, Andy. Maybe I'll just ride it by sound then until I come across a 21 tachometer. I already don't look at the speedo much, so I guess I can learn to ignore the tacho too!
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-Jim
'93 MC21

I may have a '21 tach!

Try your other PGM first, just to rule it out.
_________________
Andy.
NSR-WORLD.COM

Please keep all responses to Forum posts on the Forum so that others may benefit.

Please DO NOT PM me for technical advice. My time is precious, and you will probably receive a faster response on the Forum anyway.

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will do - may take a while to determine, as it is an intermittent problem and my riding is limited to weekends, but that will give you plenty of time to see if you have one at least. Thanks!
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-Jim
'93 MC21