Honda c90 z2 - introduction

16th Nov 2019
Tags: carburettor,  bodywork,  Engine

Honda’s Super Cub step-through motorcycle is the most successful motor vehicle ever built and a staggering number – over 100 million – have been sold.

my Honda C90 z2 (1978)

Having sold my Vespa GTS300 I was on the look out for an inexpensive second hand replacement and I settled on the idea of getting an old Honda C90, an undisputed classic that – unlike most other vintage bikes – can still be found for around a £1000.  Given the massive numbers sold, I blithely assumed good used ones would be two-a-penny but it seems many of them have been ridden into the ground or have rusted to pieces and – where they have survived –  there is a small but enthusiastic group of c90 fans ready to snap them up.

I was lucky enough to find a low mileage 1978 C90 being sold a few miles from where I live.

Honda started making their first 50cc step through “super cub” bike in 1958 and  the very popular “90”  model, the CM90, was introduced in 1964.

Honda introduced the 50cc super cub in 1958, this was before they started naming their models using the engine capacity and the first cubs were called the C100 and C102.
Introduced in 1964, the 87cc CM90 OHV model was still on sale in 1966, the year that Honda unveiled their new 89cc OHC engine.

The CM90 used a pushrod overhead valve engine, based on the C200 Benly, but in 1966 Honda released an all-new overhead cam engine and called the new model the CM91.   The new engine improved power output and fuel economy and came with an automatic cam chain tensioner and better oil pump to improve the engine’s durability and reliability.

Honda CM91 (1966) – cosmetically bike is almost indistinguishable from the CM90 (note the “cub 90” branding on the tank now says “Honda 90”) but uses a brand new engine design.

Honda were so confident in the new engine that they offered a 2 year warranty, a guarantee that was unprecedented at the time.   Their confidence was not misplaced and the engine design remained in production for over 40 years[1].  My 1978 bike is one of the C90-Z models produced between 1977 and 1983.    Honda called the initial models ‘z2’ and the later models (which came with  a new gear change pattern) the ‘ZZ’.

C90Z2 – 1977

The c90s from the 70s sometimes come in for a bit of a bad press, owing mainly to their electrics, which were based on a 6v battery ignition design that Honda had first started using in the 1950s.    The bikes do not come with a regulator to control the amount of charge going to the battery, instead the design attempts to balance the alternator output so that it is producing enough power to run the electrics with just enough additional current to keep the battery topped up.   To this end the alternator is organised with three pairs of coils – the first pair are generating current whenever the engine is running, but the other two only come on once the lights are turned on (the extra power from these coils being needed to run the lights).

There is nothing really wrong with the approach, indeed Honda must have sold millions of bikes just like this in the 60s, however, maintenance issues can cause unfortunate running problems.  For instance, corrosion in the wiring connections can create resistance in the charging circuit that prevents the battery from charging reliably and – if the battery is not fully charged – then the ignition is effected and this can cause the bike to run badly or make it hard to start.

in 1982 Honda released the C90C with upgraded 12v electrics, CDI ignition and a new engine derived from the C70.

Despite the dodgy electrics, they still seem to have been very popular (Honda sold around 28,000 C90-Zs) and plenty of them seem to have survived in the UK.

My one did run for a bit during the return journey after I acquired it, but conked out after about half a mile on the way home so I pushed it the remainder of the way (6 miles!). Not an auspicious start, but I had already been warned that it had  been stored unused for the best part of a year and it was thus likely the fuel had gone bad (this problem is apparently far more common with modern fuel containing ethanol, and any lengthy period of storage can gum up the carburettor).

There are lots of instructions on how to dismantle and clean carburettors on the web, so I did as instructed and also cleaned out the fuel tank and charged the battery.  Much to my surprise it ran!   An oil change and a bit of work on the points, brakes followed and before long it was running reasonably well.

Apart from very basic maintenance tasks on cars and bikes, I had not done any automotive mechanical work before so this was exciting stuff! Emboldened by my early success, I decided to attempt a complete restoration.   Let’s see how it goes!

1. the last bike to use the engine – albeit with a modified version with a more modern charging system – appears to be the MD90 which was sold as a mail delivery bike in Japan and was made well into the 2000s ↩︎



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