Airhead tachometer accuracy

Discussion in 'Airheads' started by RichHead, Feb 15, 2011.

  1. RichHead

    RichHead Adventurer

    Joined:
    Dec 1, 2008
    Oddometer:
    14
    Location:
    Bay Area
    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..
    #1
  2. Wirespokes

    Wirespokes Beemerholics Anonymous

    Joined:
    Jul 26, 2002
    Oddometer:
    8,586
    Location:
    Jackson's Bottom Oregon
    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.
    #2
  3. RichHead

    RichHead Adventurer

    Joined:
    Dec 1, 2008
    Oddometer:
    14
    Location:
    Bay Area
    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?
    #3
  4. Wirespokes

    Wirespokes Beemerholics Anonymous

    Joined:
    Jul 26, 2002
    Oddometer:
    8,586
    Location:
    Jackson's Bottom Oregon
    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.
    #4
  5. Tosh Togo

    Tosh Togo Long timer

    Joined:
    Nov 21, 2010
    Oddometer:
    1,594
    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.


    [​IMG]

    There's no rule unwritten rule that dictates a 1:1 tach-drive ratio. :D
    #5
  6. RichHead

    RichHead Adventurer

    Joined:
    Dec 1, 2008
    Oddometer:
    14
    Location:
    Bay Area
    Ahh, ok. Got it.
    #6
  7. Wirespokes

    Wirespokes Beemerholics Anonymous

    Joined:
    Jul 26, 2002
    Oddometer:
    8,586
    Location:
    Jackson's Bottom Oregon
    Ok Tosh - what is it? Got another view you could show us? Looks sweeeet.
    #7