Honda S90 - Restoration

This page shows some photos of the bike restoration.

The aim was to put the bike back into a usable condition, rather than restore every fine detail. It needed to be tidied up cosmetically and made safe. It would also need to pass the road-worthiness test at the garage.









These photographs show the bike in its original condition.

The missing parts included, the silencer, the drive chain, front brake cable, battery, indicators and the ignition switch.

Parts which required work were the seat, the paint and the wiring. The wiring in particular was very incomplete and had been modified by previous owners. The indicator and light switches were damaged too.









The bike was disassembled into component parts. The painted parts were sent for shot blasting to remove paint and any rust. Then they were repainted in a new shade of orange. All the parts came up well except for the rear swinging arm which revealed a bodged repair near the rear wheel. It looked like the swinging arm had been damaged by the sprocket bolts at some stage. Whatever the cause, a significant amount of metal in the swinging arm tube, had been worn away making it unsafe to use.









Thankfully someone on the S90 Yahoo Group, had a spare one for sale. It had already been powder coated in black and so rather than paint it orange, it was left black.









This image shows the tank after painting. The chrome polished up OK and 2 new tank badges were purchased from David Silver Spares. The badges were not to the original design but they looked much better than the old damaged ones.









This picture shows the frame, forks and wheel arches assembled.

The centre stand and foot brake lever were also in place. The foot brake had a new bush made up to reduce lateral movement at the pivot.

Click to enlarge

New foot brake bush made from bronze









The head-stock bearings were not in good shape and the shot blasting had damaged the outer races.

So an alternative taper roller bearing set was purchased, shown on the left here.









The headlamp mounting was repaired by screwing a brass block to the underside of the housing which was tapped to take an M4 screw. Then a brass tag was soldered to the underside of the lamp bezel to hold the assembly in place. The beam height adjuster nut had come of its mounting, so this was soldered back in place too.

New headlamp mounting plate (4mm hole)

Brass block on housing tapped M4









Click to Enlarge

This photograph shows a bit more progress.

The handlebars were fitted along with the headlight and clock. The airbox and rear number plate were also in place.

Some of the electrics had been installed including both handlebar switches, the lights and a new ignition switch.


The bike was rewired using a new wiring loom, but this loom was for a different revision of bike. Originally the bike would have had a multi-position ignition switch to operate the headlights, but the new loom was for a (more simple) 2 position ignition switch. Therefore a different (3 position) headlight switch was needed to operate the lights.

Click to Enlarge




New headlight Switch








Both the indicator switch and headlight switch needed to use the handlebars as an earth connection. However the bars were insulated from the rest of the bike because they were clamped via shock absorbing rubber mounts. So an earth lead was added to one of the bar clamps by drilling and tapping the end of the clamp bolt and attaching a wire with a small brass screw.

The other end of this wire was connected to the wiring loom earth circuit, which had a spare connection inside the healight cowel.

















The Engine

The engine was inspected to see if anything needed attention.

There were no signs of wear and tear in the gearbox, but the clutch had seized and needed to be unstuck.









The head was removed and inspected for faults. It needed cleaning and the valves were re-ground to make it good.

Cylinder head

Head after cleaning









Top view of cylinder and piston.

The cylinder bore looked in good order, as did the piston. The piston was marked as being 0.25mm oversized, so some work had already been done on the engine.

The bore measured 50.22mm diameter and the piston 50.12mm; a difference of 0.1mm which was at the limit of permissible wear.


 Cylinder Bore









The valve stems were measured to check for wear.

The exhaust valve measured 5.41mm against a specification of 5.435mm - 5.445mm, to be replaced if under 5.415mm.

The inlet valve measured 5.43mm against a specification of 5.455mm - 5.465mm, to be replaced if under 5.435mm.

Both valve stems measured just undersize, so they were replaced.

















Specification 31.8mm replace if under 30.6mm

The outer valve springs were checked for free length.

The Exhaust one was 30.89mm.

The inlet one was 30.65mm.

Both these were just about OK.









Specification 26.5mm replace if under 25.5mm

The inner valve springs were also checked.

The exhaust one was 26.88mm.

The inlet was 26.92mm.

Both these were OK.






Camshaft with no significant wear

The camshaft, cam followers, cam chain and rocker shafts were all OK. So the engine was re-assembled with a new gasket set.

The tappets were reset and the engine loaded back into the bike frame.


Fitting Tip

When removing and fitting the side cover to extract the camshaft, It is a good idea to remove the drive pin from the camshaft to avoid damaging the seal. The pin can be removed with needle nosed pliers.









New exhaust clamp

An original silencer was purchased from EBay. However the clamp to join it to the down-pipe was not available, so a new one was machined from a piece of bright-mild steel bar. The bar was machined to be a tight fit on the silencer pipe and a block of steel was brazed to the side for the pinch bolt. This block was then sliced through to allow tightening. The part was finished in high temperature paint.









This photo shows the new seat. It was for a later S90 model.

The rear mountings were the same but the front had to have a conversion plate made up as shown below. This plate was a spring fit under the bar behind the fuel tank.

New Seat

Seat adaptor plate










First Running Test

With the engine back in the bike a running test was performed although it still lacked wheels and a fuel tap, so a bottle was used to feed a small amount of petrol into the carb.

The bike was difficult to start due to a weak spark and some old fuel. The spark was improved with a new set of points and with some fresh petrol it eventually started.

It ran very roughly and there was smoke blowing from the crankcase breather. The plug was also covered in oil despite a short running time.

Bike running - note the smoke in the garage









The carburettor leaked badly and so a "Keyster" repair kit was purchased. Unfortunately none of the parts fitted, including the gasket; so the float bowl was sealed with a gasket made from tyre inner tube. However after a week this seal had swelled and began to leak. So another gasket was cut from cork.

Carburettor repair kit

The home-made cork gasket was cut by first making a print of the float bowl using ink and then cutting the inside and outside profiles with a knife.









Click to enlarge

The engine head was removed to look for causes for the burning oil.

The piston was inspected closely and the oil control ring was damaged in places. In fact all of the rings had imperfections/scratches on the surface and they had lost their springiness.

The piston was taken to an auto engineer who also saw signs that the engine had seized in the past and signs of corrosion in the bore. So a re-bore and oversized piston were used to make things good again.









This photo shows the new piston installed

Fitting Tip

It was easiest to fit the piston rings to the piston and then ease the piston into the bottom of the bore using just finger nails.

Then offer the cylinder, piston assembly up to the connecting rod and push the gudgen pin through.

This was much easier than fitting the piston to the con-rod first and then trying to fit the cylinder. Don't forget to but the base gasket in place before the cylinder though.









Second running test

With the wheels built, the bike was re-assembled to retest the engine.

The carburettor and exhaust were fitted but the bike still needed brakes, air filter, petrol tap, cables etc... to make it complete.

The engine ran much smoother this time, compression was noticeably higher and it ran with no visible smoke. The idle was quite steady although a little fast. Everything else in the engine sounded OK.

Click to enlarge









To finish fitting the wheels and brakes some parts were made up on the lathe including a spacer bush for the rear wheel and some brake nuts. Stainless was used for all parts









The leaking fuel tap was fixed by making one good one out of 3 incomplete ones.

Tap 1 was a brand new complete tap from Honda, but unfortunately it didn't have the second inlet to connect the other side of the fuel tank.

Tap 2 was the old leaking tap.

Tap 3 was a brand new tap body which Did have the second inlet on the rear.

So the internal parts from tap 1 were merged with the new tap body to make one good tap with all the right connections.

New tap in position










A bracket was made up from brass plate to hold the new horn assembly. This bracket was secured between the forks using 2 M6 screw holes which were already present in the fork yoke.

The horn switch worked by earthing against the handlebars, but these were insulated from the bike on their rubber mounts, so an earth wire was added to the handlebars to complete the circuit.





Finally after passing the UK road worthiness test, the bike was finished and put into use on the daily commute.





The finished Bike - Click to enlarge


500 Mile update

The bike has been used for 500 miles without major problem. There continue to be tuning tweaks particularly to the carburretor to get the bike running smooth and the chain has been tensioned once.

There was also a journey when the brake light switch stuck on an nearly flattened the battery, but the bike still made it home.