Bonneville T120 Compression Test

Discussion in 'Bonneville' started by RickM46, Apr 18, 2025.

  1. RickM46

    RickM46 Active Member

    Oct 12, 2023
    64
    28
    US
    #1 RickM46, Apr 18, 2025
    Hi Guys,
    I have a great running 2023 T120 Bonne; just turned 800 miles (don't get out much).
    For the heck of it, thought I would take a compression test; I have a compression test gauge with adapters. The reading might be useful later as a baseline.
    Figured the process would be:
    Remove spark plug wires from plugs.
    Remove spark plugs.
    Test each cylinder by pressing starter for a few seconds.
    Record reading on each cylinder.
    Have not looked up what the factory reading should be.
    Any suggestions on the process?
     
  2. rpg51

    rpg51 Member

    Feb 3, 2024
    34
    18
    Vermont
    #2 rpg51, Apr 21, 2025 at 2:22 PM
    Cold motor.
     
    • Like Like x 1
  3. 243jimb

    243jimb Member

    Nov 27, 2018
    9
    8
    Canada
    #3 243jimb, Apr 21, 2025 at 4:49 PM
    Keep in mind that it uses a decompressor on one cylinder to assist in starting.
     
    • Useful Useful x 1
  4. Kinjane

    Kinjane Well-Known Member

    Oct 15, 2017
    251
    63
    Bristol, Land of Enger
    #4 Kinjane, Apr 21, 2025 at 9:09 PM
    TBH I’d be more interested in the throttle body balance readings.
    I’ve adjusted several Bonnevilles and the worst imbalance I’ve found so far was a 50hPa difference between left and right cylinders.
    TB balance is something Triumph says only needs checking after the first 10,000 miles!
     
    • Useful Useful x 1
  5. RickM46

    RickM46 Active Member

    Oct 12, 2023
    64
    28
    US
    #5 RickM46, Apr 21, 2025 at 9:54 PM
    Kinjane, is there any documentation that shows how to do the throttle body balance test?
     
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