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Old 02-15-2011, 04:10 PM   #1
RichHead OP
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Airhead tachometer accuracy

Hey, I've got an r90/6 attached to a 4spd tranny and a tach/speedo from an r50/5. This may be a dumb question, but does this mean my tach reading should be way off?

Are these old tachs accurate anyway?

I'm trying to adjust my carbs and I'm wondering if I need to get a separate tach..
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Old 02-15-2011, 05:07 PM   #2
Wirespokes
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I believe the /5 and /6 tach both read 2:1 - meaning that if the cable spins at 1,000 rpm, the tach will read 2,000 rpm.

They're kinda close, usually within a couple hundred RPM. But unless they've been rebuilt or gone through and lubed, you never know without comparing to an accurate tach running at the same time.

As far as adjusting the carbs, there's a lot of leeway so don't sweat it. The range is something like 800 - 1,000 rpm. If it's too low it'll die when you blip the throttle, and too high it generates heat that won't be dissapating quickly. I prefer having them idle around 800, but do what feels right to you.
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Old 02-15-2011, 05:26 PM   #3
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Sweet, thank you. Yeah it didn't seem quite right.

So, if I want to set the idle to 1000 for example, the tach will read 2000?
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Old 02-15-2011, 05:41 PM   #4
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Uh, no - I was just trying to explain that both /5 and /6 mechanical tachs operate the same, even though they look different. So if you're using a /5 tach on a /6, it should still read correctly - well, as correct as a 40-year-old mechanical tach is going to read.
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Old 02-15-2011, 05:50 PM   #5
Tosh Togo
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Quote:
Originally Posted by RichHead View Post
Sweet, thank you. Yeah it didn't seem quite right.

So, if I want to set the idle to 1000 for example, the tach will read 2000?
No...read the 2nd post again: what he wrote was that in the case of both /5 and /6 mechanical tachs, the cable runs at half speed to what the tach-face displays. Unless things are grossly whack-o, the rev counter's announcements will match the crank speed.

It's the sort of thing that a lot of tach-drive designers did in an effort to make it more reliable, by having the most fragile part of the system run at half speed.




There's no rule unwritten rule that dictates a 1:1 tach-drive ratio.
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Old 02-15-2011, 06:11 PM   #6
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Ahh, ok. Got it.
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Old 02-15-2011, 09:25 PM   #7
Wirespokes
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Ok Tosh - what is it? Got another view you could show us? Looks sweeeet.
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