Bonneville T120 Compression Test

Cold motor.

 

Keep in mind that it uses a decompressor on one cylinder to assist in starting.

 

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!

 

Can’t recall what was in the workshop manual but to be ‘in spec’ the readings need to be within 10hPa of each other.
AFAIR there is no recommended figure given as it will vary dependant on the idle speed.
The ECU seems to throw a little variance into the mix every few seconds which can be seen by checking the amount of time the injectors remain open so the pressure balance between the two cylinders seems quite volatile and difficult to equalise.
Hovever it is possible with practice and patience to get the cylinders balanced close enough to each other that the hPa readings remain both equal and constant.

One VERY Important thing to remember is that when the ignition is turned on the the balance screw accessible via the security hex bolt rotates as part of a larger assembly within the housing and if an adjusting tool were in position within the throttle bodies when this occurs irreparable and expensive damage is likely to occur along logging the appropriate fault code.

Adjustment should only be attempted on a hot engine already at idle prior to inserting the appropriate
tool.