Rattus, The Rat Racer. GS750 1977 cafe racer rat-bike

5-10-16 by Sarah Bluck, OES Engineers, Planet-tharg.

Hi this is my old and rather lovely Suzuki GS750. This bike was bought by a very good friend of mine in the mid-late 1990's. Jon used to buy the occasional motorbike. and never usually paid more than 100 pounds for them.

So this started it's recent life as a cheap spare-motorcycle. Used by Jon and another friend of ours Dave. One year, Many moons ago. possibly about 13 years ago! the 1977 Suzuki GS750 came over to me. I used the glorious old beast several times for the mayday run, and it lived with me for a while..

(The Mayday run is a motorcycle run to the seaside town of Hastings on the UK south-coast along the old A21 route).

Time passes, and Jon and I lost touch with eachother, he moved to Derbyshire the 1977 GS750 went along too. There it stayed for many years in Jon's garage. Not used or started for ten years or so....

So this is a story really about an old motorbike that's put a smile on the faces of a few people. and how it's been born again. When Jon and I got back in touch about 5 years ago, one of the first things he said was.. "I've still got your GS750 here as and when you want it.." that was in about 2010 and now (October 2016) it's here, and back on the road again!

The Suzuki GS750 was always an oddball. It was the first of the real superbikes. The first in a long and continuing line of Japanese multi-cylinder motorcycles and even by todays standards is quite a beast!. 4-cylinder, Double overhead cam, 4 carbs, 5-speed constant-mesh gearbox. coil-contact ignition, and disc brakes front and rear. Even a kick-start for when you have a flat battery. Not too sad for a 1977 bike!

I had bought a Honda VT500E in pieces as a source of spares for my tidy VT500E road-bike. and as it was only ten miles or so from Jon in Derbyshire, I arranged to collect them both in the one journey. Jon's garage. Now there's a thing. 8x10 feet and there's a car, a trailer and 8 motorcycles in there along with loads of spare parts too. OK the car is a BMW Isetta bubble-car and most of the Motorbikes are of the monkey-bike type, so really tiny. It's odd that Jon likes these tiny vehicles, He's well over 6-feet tall. and makes them look like toys! We collected the GS750 and some of the spare parts that Jon had collected over the years, some from newer GS/GSX550 machines. Into the trailer it all went for it's journey back to Kent.



There were rusty spots on the fuel-tank, so knowing that this project would be completed in the back-garden I part-sprayed the fuel-tank and some of the frame with some black paint to protect it a bit from the seaside air here in North Kent. The engine casings would have originally been plain grey aluminium, but in the past the GS750 has been given a light goat of black paint. Most of which was peeling off to be replaced by fluffy white corroding aluminium. This too was happening to the barrels and cylinder-head. So I gave it all a good clean-up with a wire-brush and primed it all before applying fresh matt black heat resistant paint.


So here starts the process of getting the beast ready for the road again. It would have been so easy to just spray-over the rusty parts and pop fresh fuel in and hope for the best. BUT I don't work like that. After checking the oil and a quick check of the electrical system. I started the engine to make sure it was basically a sound proposition. If there were any engine issues I would need to address those issues first. Apart from a blowing exhaust, some wiring/switchgear problem and a bit of mis-firing all was in pretty sound condition.


The light-switches on the bars were faulty, and the regulator and rectifier system were rusted and in need of replacement. The GS750 uses a three-phase alternator and rectifier, with a seperate SCR/Zener regulator. One of the alternator phases is switched by the lights-switch so the regulator isn't over-worked. When the lights are switched on the loading on the electrical system increases and it needs the extra boost. so the lighting switch adds an extra alternator phase to the rectifier. This is not the way I like to see things done. so I ordered a modern rectifier-regulator module to replace the original seperate components and avoid the phase-switching through the lights-switch.

I knew from the start that I was going to replace the handlebars with clip-on type bars, and as I no-longer needed to route the charging system through the lighting switch, I also ordered a simple lighting/hooter/kill switch. In the spares box were two extra front master cylinders, so I chose the best of the three. Took off the forks for a bit of a tidy-up and service. and greased the bearings in the fork-stem. Also the tacho cable was broken and the original instruments looked a bit bulky, so I used a spare GS550 speedo and modified it a bit....


As you can see. the headlight is off too. it is an early type that uses an APF type lamp. 35w on dip/35w on high and I took this oppertunity to replace it with a simple looking fork-mounted headlight shell that uses a modern halogen H4 type headlight lamp, so it's amazingly brighter! Also As there was a spare top yoke in the spares-kit. I took an angle-grinder to one of them and machined-off the mounting-blocks for the handlebars and aluminium-welded the holes then smoothed them off.. As you can see it makes the front of the bike look very tidy!. But if I ever choose to go back to normal raised bars I can easily re-fit the other top-yoke and bars.



You may have noticed the Amal throttle-grip unit. Not the obvious choice for a cafe-racer type bike. But I like the slow-action and am used to the feel of it from my off-road motorbikes. I made a special throttle cable to connect it to the four carbs, and had a spare made at my local motorcycle shop. Also the return springs and the four slides in the Mikuni carbs are fairly heavy, so a slow-action throttle-grip makes it a little easier to get them all under control. The usual throttle assembly was rotted and damaged. There are minor dis-advantages with using this setup though. For a start. You need to twist a bit further for full-throttle, and there is no closing-cable, just the opener! So if anything holds the slides-up your ride will get very interesting bloody quickly!


As I mentioned earlier. I had a spare GS550 speedo in the bits-box. This unit is a slightly smaller diameter and uses the same gearing as the GS750 part. I wanted to keep the front of the GS Rat-Racer looking simple and clean so I took the GS550 speedo apart and did a few little jobs. First I wound it forward to the same mileage as the original GS750 speedo, And then I made-up some LEDs and limiter-resistors to go inside where the trip-meter was. Also I added small marks at 30, 60, 70, and 100MPH and engraved H N O for High, Neutral, and Oil these glow green when the backlight is on. Cool Eh...?


The exhausts were very rusty and the bolts / studs into the cylinder-head were rusted-in. After a week of spraying with penetrating-oil and taking a gas-torch to them I finally got the old exhaust-system off and was left with three broken-off bolts that needed a little more help. Also at this stage I had removed the brake calipers and rear master cylinder and had begun chasing-around for rebuld kits seals, pistons etc.. and of course braided stainless hoses. The rear caliper and master cylinder were easy with the parts being standard for the GS750 1977 bike. But the front calipers had been changed at some point in the past and are not standard 1977 GS750 issue!



I decided to use modern organic pads in the front and rear calipers, and had a few problems finding the repair- parts I needed for the front calipers. There were spare calipers in the bits-box. So I stripped them all, chose all of the best parts and used a matching-pair of Asin caliprs on the front These are very odd! they have a caliper-mount that sits inside the aluminium body of the main caliper-housing and "floats" on two steel pins. it's really quite a nasty design. If the top caliper mount-seal leaks, the whole sodding thing fills-up with rainwater roadsalt and spray and rots away! And the top seals are not easy to replace as they are not used on too many motorcycles. After 18 months or so the design was changed and the newer calipers were less problematic. Me bring a fussy moo though. I found all the parts I needed from three seperate suppliers and totally re-built the front-calipers with new seals, dust-seals, pistons, and top-boot. I also replaced the bleed-nipples with stainless steel types, and when I re-assembled used copper-grease in the mounting-body to keep the rot at bay.


Right, Getting there now. Delkavic stainless steel exhaust fitted and looks a treat. It sounds great too and if I want to be a silly moo I can always take the baffles out. The original seat was falling apart. The steel-frame rusty, vinyl ripped and foam rotting. I re-used a small part of the foam and the six plastic feet and got busy with a sheet of steel and an angle-grinder. I made a steel seat base. Riveted and welded on some strenghening-plates, covered it with foam cut to shape and glued-on some new vinyl. I fitted bolts for the front-fixing-hook and for the tailpiece before I covered it. And as I had a spare but leaking fibreglass fueltank for my vintage Ossa trials bike, I decided to use that as part of the tail-cone section. I welded a seat-support framework from some spare steel I had in the junkbox, and got the seat to fit properly first. Then welded a pair of lugs onto the top of the frame behind the rear shock mounts.


The long steel tongue from the rear of the seat mounting frame is to support the tail-cone, and to mount the rear light and number-plate. Without it the thin fibreglass would crack easily. I tend to do overkill jobs when I am making things like this. I know what a pain in the ass it is to have to re-make things again if you don't get it right first-time. As you can see I used my favorite tool again (angle grinder) on the Ossa fuel-tank and cut it to a rough-shape. Then bonded it to the frame with resin and GRP-sheet. Then started the few days of making the tail-cone and sanding it into shape.


I gave it a quick-blow-over with some matt-black and it looks quite crazed-rough and nasty.. Just perfect for the arse-end of this rat-racer project-bike... Things yet to do....maybe make some side-panels. and a few other little jobs.. But it goes!. I took the beast out today and gave it it's first road test. Did about 5 miles and booked it in for it's MOT-Test. It growls spits and takes off very nicely. The pictures below I took at the seafront here in Sheerness in North-Kent.