While finding out how to repair my 1970s Honda c90 one of my many surprises was that the official shop manuals produced by Honda are still available. Unlike the Haynes and Clyne publications that were aimed at the general public, these are the instructions used by the Honda mechanics who mended and maintained these bikes when were first introduced. You and download digital copies of my workshop manuals here:
You might also find it useful to refer to the 1977-1982 CT90, CT110 Shop Manual which covers the same engine but has better illustrations than the the 1970s versions. The CT manual also shows the sub-transmission added to the later CTs and covers the 110 version that succeeded the 90 model.
Honda expected their model-specific manuals to be read in conjunction with the Honda Common Service manual, which explains the systems common to all their motorcycles. This manual is available on the excellent Vintage Honda Twins forum, as part of their document library.
The engines Honda used in S90, CL90, CD 90, and CT90 between the late 60s and early 80s were basically identical with the only major difference being that the the C 90 and CT 90 had an automatic clutch and the four speeds used on some of the models.
This was a very successful design – a modified version continued to be used on in the Japanese MD90 (Mail delivery) bike and was also the basis for the CT110, both of which were made well into the 2000s – and only a few minor improvements were made during the first 20 or so years it was in production:
Three versions of cam chain adjuster were used, with the early automatic oil damped versions being replaced in the 1970s with a simple manual adjuster provided with a lock nut to hold the adjuster in place after adjustment.
The original transmission, gear shift drum and gear shift forks were replaced with more robust versions in the late 60s.
The final change of note was that the Honda changed the C90-ZZ (1978-82) shift pattern from 1-0-2-3 to 0-1-2-3 and this engine has a different shift drum as a result.
The main cosmetic changes came with the introduction of the Z2 model. The Z2 was launched in the UK in 1977 and incorporated some features previously available on the special “deluxe” models sold in other markets, including foldable foot pegs, position light, an improved speedo with turn signal indicator and a new switch that activated the stop light when using the front brake. The choke was also moved to below the centre of the handlebar and the main switch to the legshield.
Note the 1971 workshop manual shows the wiring for the pre-1977 models, the wiring for the UK C90-Z (1977-82) models is included in the back of the owners manual linked above.