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1977 GT500 Engine

  1. Assessment
  2. Oil Pump and Lines
  3. Magneto and Coils
  4. Carburettors
  5. GT500 Project Menu


So the time had come to take a closer look at the two GT500 engines I had - one was a 1976, and the other a late 1977. I scraped the worst of the accumulated grime from them but did not bother to do a real cleaning as the plan is to do a full strip, see what I had, and then clean and rebuild. The first issue was the clutch side cover on the 1976 engine as I could not get the shift lever off. I had looked at it very briefly months ago and had just been giving it a shot of penetrating oil whenever I walked past it in the shop. Now that I had it up on the bench, I finally took a close look at it, and soon realised that the reason it wouldn't come loose was that the shifter arm had been welded to the shifter shaft !!! I have to admit, I hadn't seen that particular bodge before, and it didn't bode well for the condition of the rest of the engine.

After a few minutes of grinding, it was sorted and I was able to get the shifter off and then the left side cover. The drive chain had obviously come loose at some point as there was the typical damage to the case and neutral indicator housing. After that, it was time to pull the heads off and take a look at how bad the barrels and pistons were. Thankfully, the 1976 engine actually seemed to be in pretty good shape internally - the barrels were still on their original bore and there were no obvious signs of oil starvation or scoring. The crankshaft felt good with no roughness in any of the bearings and there were no signs of excessive play in the con-rods and small and big ends. Better still, there was no sign of gear pitting on the gear tooth faces in the transmission, and the clutch and clutch plates all looked almost new.

The 1977 engine was a different story entirely.

When I pulled the heads on the 1977 engine. I was quite surprised to find that one barrel was at second over, and the other was still on standard bore ! As well, there were obvious signs of fairly serious engine damage in the past as the left cylinder head had a deep gouge in it - and then most surprising of all, the right cylinder head was actually cracked the width of the cylinder right through the spark plug hole, so both heads are pretty much scrap.

The barrels also had been played with - the intakes had both been opened up, probably to try and make the engine breathe a bit better which in itself is not a bad thing, but the liner on the left cylinder that had been bored to second over was cracked in the area of the intake transfer ports - so that is scrap also.

The crankshaft seems OK as well as the con-rods and the small and big ends, but the engine must have been really unbalanced with having the cylinder bores unmatched and probably buzzed like crazy at speed. It also will have stressed the crankshaft bearings so they will need to be looked at carefully

The other problem I found, was that whoever rebuilt the engine last time hadn't remembered to bend up the locking tab on the clutch boss nut, and so at some point the clutch basket had come loose and had 'reshaped' the inside of the right side engine cover. As a result, both of those were not much use either, and several of the clutch plates were chewed and the metal friction plates were blued.

Thankfully the gear clusters were in good shape and it at least shifted freely.

The good news out of this is that I did have one set of barrels and heads which seemed OK from the 1976 engine together with the clutch and engine side covers, so I was able to get at least one complete engine from the two. I also have four con-rods which seem to be in good shape which is very nice as these specific ones have not been available anywhere as NOS for years. As of this writing, they are being re-produced in small numbers in the UK by Pete O'Dell. The crankshaft bearings likewise pretty much are not available from Suzuki - you can have them made at considerable cost, and they also are being reproduced by Pete O'Dell and a few others, but it was very nice to find that, at first glance at least, the original crankshaft bearings look to be re-usable. I also have two complete sets of gear clusters, and as the transmission was same from 1968 through till 1977, that may be useful on a future project I'm considering. So of the two engines I started with, by the time all was said and done there wasn't much left over !

One thing to note with the 'GT' crankcases is that the transmission oil starvation problem common to the earlier crankcases had been fixed. This is addressed in US Suzuki Technical bulletin T-3 which can be found here. With the GT crankcases, the rubber oil dam talked about in the service bulletin was no longer required as the lower casting had been updated, and the oil quantity corrected on the upper casting by the oil fill hole. It isn't clear to me though whether the over fill check bolt hole in the right side cover had been relocated - I suspect not, but this needs to be confirmed.

Joe Haseloh at RPM Services in De Winton rebuilt the crankshaft for me and also bored the cylinders, and the engine is now back together. I used a gasket set and pistons from an outfit in Japan called Cruzinimage on fleabay - I've dealt with them previously and been happy with what they offer. I have left the cases unpainted as per original - not as nice as some people do them, but I like the 'original' look. I did have to take the engine apart and re-assemble it again as I realised that I was missing a thrust washer from the oil pump drive shaft. There was only one in each of the two engines that I dismantled, but a close examination of the parts listings actually showed that there should have been two of them. As the engines had both been apart previously by previous owners, I couldn't be certain whether it was a late change by Suzuki to move to having just one thrust washer, or whether one was just lost on both engines during previous rebuilds. In the event, once I had it apart again, I was able to confirm that a second washer was required so it was worth checking before I'd put the engine into the frame !

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Oil Pump and Lines

Owners of Suzuki twins with oil injection from this era often report air leaks in the oil injector lines. Unlike the triples, the twins use a split feed line between the pump and the point of injection, and it seems that after 35+ years. whatever Suzuki used to glue these together is starting to fail, or possibly it is just hair-line cracks developing. Whatever the case, I am putting a drop or two of epoxy on the joint areas (marked in the photo to the right), and will also be flushing out the lines with methyl hydrate to make sure the check valves at the end of each line are also working properly. I've included a photo to the right of what they look like disassembled. The same check valve design is used on the Suzuki triples and care must be taken to not damage the seat or small spring, so blowing these out with compressed air is a recipe for disaster as new ones haven't been available from Suzuki for quite some time.
Update: the epoxy didn't work as well as I'd hoped, and I did have trouble with air leaks in the injector lines at the splitter so I ended up sourcing NOS lines from Marcel in The Netherlands. Fast service, and recommended ! I will try again to re-glue the fittings at some point, but for now what I have is working.

Continuing on with the oil pump - the two that I had were in poor shape and so I needed to be dis-assembled and replace the oil seals, and of course these were not offered by Suzuki as the pumps themselves were never intended to be repaired. It took a bit of sleuthing to figure out what will actually fit and where to buy them. This is a single piston style pump, and while a lot simpler than the one used on the Suzuki triples it is also pretty robust and the seals are all easily replaceable if you can find the correct sizes.

The photo to the left shows the disassembled pump (just click for a larger view. I had trouble locating 1/2 mm sizes locally, but after a bit of searching found a place in the USA called MFGSupply that had stock at reasonable prices.

NOTE: I've marked the one fitting that is a left hand thread - you have been warned !!!!!

With the pump all back together and installed, it is working great with no leaks so I'm happy.

Magneto and Coils

The coils on the magneto and timing plate were damaged with bare spots in the insulation on the wires, and they were also coming apart as the cardboard spacers had turned to powder and were long gone leaving the coils to flop back and forth on the armatures. Luckily new coils are still available from Suzuki in the USA (not in Canada) so I was able to buy a full replacement set. I could have just rewound the coils as of course the armatures themselves are steel - rather than fiddle with it, I just bought the new ones and justified the cost to myself on the basis that it would be one less thing to be worrying about later.

On the T/GT500's and of course many other bikes also, the magneto coils and in this case also the pick-up trigger coils used for the electronic ignition, are soldered in, and given it had been quite a few years since I had wielded a soldering gun in anger, I took my time. The photo to the right shows the completed job with the new sleeving installed.


At some point during its long sleep, the bike had obviously been left laying for a while on its left side, as the left carburettor was literally full of dried dirt. It took me ages to get the fuel inlet and the other small passageways and jets cleaned out using repeated soaks in my ultrasonic cleaner, followed by solvent washes and air blowing. One of the floats wouldn't - float that is - and so had to be replaced, and a part in the throttle slide was missing so while I was waiting for a new one to arrive I made one from some scrap aluminium. The air inlet and outlet boots are new, and of course so are the o-rings, gaskets and seals. After many hours of fiddling and adjusting though I was ready to try them on the engine - while it did start, and even ran 'OK' they clearly still weren't right so I spent another whole day cleaning them yet again and at the end of that, at last had some success !

I do still plan to replace one needle as it is slightly pitted, but the engine now pulls cleanly through the rev range so any additional changes can wait till after the engine is well broken in.

Update: I sourced a pair of NOS needles ,from Reiner in Germany - to be honest I didn't notice a change in how the engine performs, but then I also don't have another one to compare against. It does run well though, so other than some minor tweaking to make starting a bit easier (the choke rod latch doesn't work well, so it often slips off choke when starting it) I think its good enough.

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