Rebuilding A Kawasaki GPZ1100 | Part 1

Date 14.7.2016

Presented by
  • Motorcycle Trader


What happens when a technical drawer goes back to the bikes he loved in the ‘80s and decides to recreate his old passion? Duncan Aitken-Bain took the plunge and here’s the first part of his fascinating journey rebuilding a Kawasaki GPZ1100.

Okay, where to start.

My task was to find a suitable machine to enjoy, that would satisfy my need to tinker and modify, be reliable, be affordable and be unique. I knew from the beginning it was a tall order. My developmental experiences in motorcycles reached their peak from 1979 until about 1987. In that period, my focus was on modified road-going machines from 1980 to 1984.

Locally, there was a hotbed of developmental work and racing going on. Superbikes were just getting off the ground and the mixture of engineering, tuning, racing expertise and stock road machines was producing a fascinating array of unique and experimental solutions to racing of this kind.

When I saw Graeme Crosby’s Z1R at Sandown Park, it was really quite something. I had read a little and seen one lousy, low-resolution image of it in an Aussie magazine. To see the machine in the flesh, pour over the modifications and take some images to study later, was intoxicating for me. The racing was thrilling, but the machines were magnetically mystifying. I was always much happier in the pits than in the stands. In spite of knowing what it’s like to race and be familiar with competing and being a competitor, the engineering, and thought behind the enterprise, has always been the thing that interested me the most.


I was looking for a project bike, initially a Suzuki GSX1100, although my timing wasn’t the best as the prices were rising and I had seen too many junkers. I looked almost daily for over six months. I’m not religious but, luckily, something happened for which I’ll forever be grateful. A bike dropped in my lap. Not only was it produced in the era I was looking for, it turned out to be about five minutes away from where I lived.

With some help from a good friend, I paid the seller a visit. SOLD – one 1984 Kawasaki GPZ1100. Yes, the Unitrack one. This example had a few nice modifications already, and I really liked the owner’s resolutions. It had a ZZR1200 swing arm and three-spoker wheel. This was coupled with a ZX6R front end and matching three-spoke wheel. It was bright green, had a nice pipe and, importantly, the fuel injection system had already been replaced by RX1000 34mm CVK carburettors.

The plan was to get it roadworthy, sort out the issues, then plan for the major overhaul. As it already had 80,000km on it and I wanted this to be a keeper, it would need the full overhaul. I don’t think I’m fussy or needlessly hung up on minor details but I like reliability and I’m not interested in appearance for the sake of looks or fashion. I know this view is likely to come back and haunt me in the future.

Practicality drives most of my decision making: functionality and serviceability.


After much work to sort the roadworthy issues out, one little gremlin surfaced in the transmission and I decided it was time to pull it off the road. It had lasted two weeks and the need for a teardown became overwhelming. Oh well, it had to be done.

Luckily, the stars aligned and the motorcycling gods thought I deserved a break, so an excellent mechanic presented himself, unaware of the hell I was about to put him through. Well, it was fortunate for me, anyway. Dale had already done a similar job on a 1983 GPZ1100 and amazing it is too. The paintwork overall and engine paint was exceptional. It wasn’t 100 per cent stock – no EFI and a four-into-one – but everything else was immaculate. I was lucky to find him.

Kawasaki GPZ100 Rebuild PT1 Dale’s finished bike.

I separated the bike’s various components and got the engine ready to deliver. Over a few conversations the task list grew and the project ballooned slowly. If these conversations were in the presence of alcohol, then I would have the luxury of blaming that but, alas, both of us are non-drinkers. Without this excuse, I’ve always referred to his workshop as the ‘Therapy Centre’.

It wasn’t long before I was made aware of the extent of the work necessary. Dale was very sensitive, knowing how little I actually knew needed to be done. His fall-back position was, “Here’s one I prepared earlier”.

The heart of the machine was in excellent condition, which was the first major problem solved. The frame needed work and the swingarm need to be replaced but the rest was pretty good. There were no major surprises, at least nothing structural.

Kawasaki GPZ100 Rebuild PT1

The engine was checked in detail and I mean great detail. The crankshaft, as it’s a pressed multi-piece part, needs to be assessed by the most expert of people. This was agreed not only by Dale but by two independent sources, one of who is the legendary John Treese who would later be working his magic on the ‘business end’ of the engine, the cylinder head.


At this point we were of the opinion that parts would be really hard to get. This is a huge concern for anyone tackling a 30-year-old machine. My view was that parts would be scarce and expensive. I wasn’t looking forward to scouring the globe for bits and pieces or relying on internet ‘experts’. I went looking online for technical information and came to the conclusion that I would have to start at the beginning and build as much technical information as possible. If you’re an academic, this process commences with a ‘literature review’.

The Haynes/Clymers versions of the GPZ1100 workshop manual don’t contain all the information pertinent to the GPZ1100 but I was able to find a hard copy of the genuine Kawasaki workshop manual online.

Next, I found the parts list exploded diagram which was a real godsend. After that, it was a case of firing up the computer to use the online parts finders. I discovered that Kawasaki parts which suit my model are also used in a wide array of machines, even current machines. This was the most important piece of information I had discovered. I can hear some of you sighing and rolling your eyes – an ignoramus, though, is someone who doesn’t know something you just learned yesterday.

Kawasaki GPZ100 Rebuild PT1

I have read and heard a lot of what I can only describe as urban myth regarding sourcing parts for my machine. It’s been extremely easy so far – in fact, I can’t think of a part we haven’t found and we’ve replaced a lot of things. I get chest pains when I think about it.

My scouring the world for bits went well and, after much price comparing, I received a load of new parts. First up was a Wiseco 1170 9.5:1 piston kit. The compression ratio may sound low but we wanted the bike to be serviceable and not dependent on special fuels or the risk of overheating.

An APE cam chain tensioner came next with a new/old stock Kawasaki cam chain, cam buckets, cam bearings, springs, valves and, my personal fave, the RS flat slides. You’ve got to have accurate fuel metering. The CVK carbies that came with the bike when I bought it were an interesting compromise. They had a Dynojet kit in them which makes the engine run rich between 2000 and 3800rpm and uneven running was an ongoing issue. They were reliable, though, and the bike always started very easily and ran extremely well.


The questions started. “What bars are you going to run?” “What are you going to do about paintwork?” “That fairing has to go you know.” The last of these isn’t actually a question but I agreed totally. Dale is very particular and very detailed in his approach: he’s not harsh in his opinions but he does let you know when reality can’t be disguised as anything other than what it is.

Kawasaki GPZ100 Rebuild PT1 Kawasaki GPZ100 Rebuild PT1 Duncan’s project bike – green 

With the engine having undergone its autopsy, the report diagnosed the need for some specialist treatment from Dr John Treese. This was going to be really fascinating. I had described what I was trying to create: I’m not drag racing, I’m not scraping the pegs on the boulevard and I’m not holding the tacho at just under redline from one set of lights to another. As I understood it, the 1984 model had the best flowing head, largest stock valves, a triple hemispherical combustion chamber roof design and slightly steeper valve angles.

Kawasaki GPZ100 Rebuild PT1  Kawasaki GPZ100 Rebuild PT1

Making the most of the stock breathing would be a refinement. With a slight increase in compression and capacity, the larger valves, improved breathing from some porting would be enhancing and refining the basic design, making more of what existed in the beginning.

Dale and Dr Treese did their patient consultation routine and made their treatment plans. The head would be an inpatient at Dr Treese’s rooms for the procedure and Dale would let me know how the operation went and when the patient could be picked up. Cool. I had some valves on order – exhausts are 33 mm and the inlet size was being deliberated over by Dr Treese and, as he was making all of the technical decisions regarding the head and its performance, I waited for instructions from Dale.

“He’s doing the inlets seats – the exhausts are fine.”

Excellent – another few days…

“He wants to see if you can get 40mm blanks for it.”

“Okay, will do.”

“He isn’t going to replace the valve guides so don’t go looking for replacements. He’s going to sleeve and bore the existing guides – then they’ll be perfect.”

In the meantime, there was much cleaning and repairing. I also had a pesky decision to make. The machine originally had gold wheels and it was rattle-can warm gold which looked like paint. This had to go. I was after the beige, brown, soft metallic gold look: a cool, early Marvic or, even better, the Lester Morris magnesium.

Dale did his ‘thang’ and, after a few consults and humming and erring, he came up with his preferred colour combination. In the sunlight, the wheels are just awesome. Mission accomplished. The bodywork is such a bonkers bright green that the colour choice of the wheels really had to be right. Dale understood my concerns. “I know you’re into function and practicality and not fashion, but this has to be right, has to be complimentary, has to be very classy.” I totally agreed with that.


**End of Part One**

NEXT TIME: Duncan, Dale and Dr Treese get close with the engine rebuild but are thwarted by bearing dramas.