The 1974 GT550

DonorSome time ago, I picked up a 1974 GT550 which came to me with the engine stuck. As a part of the deal, I also was given two donor bikes of the same model, but different years. Basically, the bike was complete, and just looked too good to pass over even though the cost to repair was probably going to be more than its value. As I did with my GT750 project, I’ve started a web site to capture additional details as the repair work on this GT550 progresses.

As was often the case, the donor bikes had been left outside with the carbs and exhaust pipes removed, so water, snow and whatever was blowing by had made their way into the engine cylinders and they were solidly stuck and not looking very good at all – as per the photo to the left.  After letting the engines soak for several weeks with penetrating oil,  I pulled them apart to see what could be saved, Interestingly, both donor engines appear to have failed due to holed pistons, so naturally I was curious whether the project GT550 had the same problem. As well, I had been told that this model of Suzuki had a fragile starter, but when I pulled these apart both donor bikes showed no sign at all of problems. One of them even still had its starter motor fitted, and when I’d pulled it out, I tried it with a power source, and it spun freely.

At the end of the day, I had about one and a half engines with serviceable bits and pieces suitable as spares – one actually had a set of first over pistons which are quite rare to find now. The barrels were in rough shape, but at least could possibly be bored out, and one set of engine side covers were worth keeping

The next step was to strip down the project bike and see what its problem was. As it turned out, it had more than one. Once the head was off I was initially relieved to not see any holes in the pistons, however the centre cylinder had a lot of small metal particles on the top of the piston as well as in the exhaust port, and once the barrels had been pulled off it was clear where they had come from per the photos below left and centre. The centre piston’s gudgeon pin had snapped, and then the piston had essentially self destructed. The bits falling down from the broken piston were lodged between the crank throws and the case which was why the engine wouldn’t turn over. As well, when I pulled off the clutch basket, the starter clutch assembly basically fall apart as the centre boss had come completely apart. In the photo below to the right, you can see the broken one to the left, and what it’s supposed to look like to the right.

Cylinder Starter

Suzuki completely redesigned the starter clutch assembly in 1975, the year after my bike was built and obviously the updated version had never been installed.  The service bulletin which details the changes and new part numbers is GT-26, dated August 15, 1975 and a copy can be found here. I have two ‘good’ old style starter clutch assemblies from the donor bikes as neither of them had been updated either,  but I wasn’t too happy with doing this as I could reasonably expect the same failure mode again at some point – and as I was putting this on the road for my wife, I doubt she would be impressed if it quit working !

I am looking to see whether I can track down a new style starter clutch assembly and we’ll see what happens. At the moment, I’m waiting on parts, seals and gaskets before I start to put the engine back together

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