SUZUKI GT550
Stan the Man again on tuning this Suzuki triple.
114
Ihave fond memories of the Suzuki GT550 – 40 years ago I had a 1976 Candy Red GT550A. I did some tuning to it and it was a rocket! Three cylinders, electric start two-stroke, I loved it, I even considered putting the engine in my race sidecar outfit. Over the years I have worked on very few GT550 engines and I have never ridden another one since. Recently a good customer of mine, Nigel Foster, asked me to overhaul, renovate and tune the engine and gearbox on the GT550 that he had bought on ebay. Nigel has a collection of GTS and I have built all the engines. He said that the 550 would complete the collection, so I couldn’t say no! Nigel arrived at my workshop with the engine, it was partly stripped and covered with corrosion it looked a real dog... The 550 engine is really straightforward to strip. Undo the 12 head nuts; they are 10mm with 14mm spanner size and remove the one-piece cylinder head. What a mess: there were hammer marks all over the tops of the barrels and most of the head studs were stripped in the barrels. To remove the barrels there were another 12 nuts, 10mm with 14mm heads. With the barrels off the next damage showed up, the base of the barrels had lever marks all over them: why? I later skimmed the tops of the barrels and the bases but there were broken fins as well and Nigel managed to buy a better set. Now onto the bottom-end. Remove the left-hand side generator cover and remove the three screws which hold the stator. There is a special puller for the rotor. On most engines the rotor has to be removed to get the crankcases apart but not the GT550, when apart the rotor will still be on the end of the crank so when I send the crank off to Grampian to be rebuilt they remove the rotor when pressing the crank apart! Now over to the right-hand side of the engine. Remove the outer clutch casing, remove the six clutch spring screws, bend back the tab washer and with a clutch holding tool remove the 32mm centre nut and the crank 32mm nut. Remove the clutch and primary gear. Now remove the top crankcase bolts, there are seven of 8mm, 12mm headed bolts and four of 6mm, 10mm headed bolts. Turn the engine upside down and remove the bottom crankcase bolts, there are 13
of 8mm, 12mm headed bolts and hiding under the starter motor another four 8mm Allen bolts. I removed the starter motor cover plate and removed the starter which promptly fell apart! The starter clutch had jammed and the starter must have been spinning away at engine revs until it disintegrated. So, if you’re buying and the seller says he always starts it with the kickstart – steer clear! With all the bolts removed, the top crankcase lifts off. My advice would be to take photos of the gearbox before you start removing it. The gear shafts with the gears on lift out simply but if you are having the cases aqua-blasted, as these were, the selector drum and selector shafts etc. are a bit tricky to remember how to replace them at a later date. With everything stripped out it was time to take all the side-cases to the polishers, all the nuts and bolts to the platers, the crankcases to be aqua-blasted, the heads and barrels to be bead-blasted and the crank to Grampian. I gave a list of the parts needed to Nigel for him to source on the internet. When the parts were back it was time for tuning and here I had a stroke of luck, as when looking through all my spec sheets I came across the original figures I had written down from tuning my GT550 from 40 years ago! The cylinder head is a one-piece casting and is too large to swing in the lathe so I set it up on the mill. Unlike most other two-stroke engines from that period onwards, the 550 does not have squish heads, just domed combustion chambers. I skimmed 0.5mm off of the head face to raise the compression. To skim any more would have made the combustion chambers smaller diameters. The exhaust ports are reasonably easy to work on, the ports are straight and fairly short. Surprisingly, the top of the port is quite square so when I widened the port from 37mm wide to 39mm I left the sides the standard height. I raised the top of the exhaust port from 40mm (measured from the top cylinder face) to 37mm but made it a more gentle curved shape which would aid ring life and also smooth-out the mid range power that raising the port by 3mm may have caused. The transfer ports were horrible castings. With a right-angle porting tool I just concentrated on getting them all the same height and flowing them. Likewise the bottoms of the transfer port windows.
I usually say that there is very little to be gained from flowing the collection areas at the bottoms of the barrels but on the 550 the liners are an unmachined step 10mm thick, so yes flow them. The inlet ports are tricky to work on, the ports are fairly long and there is a bridge the full length of the port; in addition there is the oil feed in each port. If I was tuning the engine for maximum power and was using larger carbs I would have removed the bridges and the oil pipes and the oil-pump and pipes and run it on pre-mix but this 550 was for use on the road. I machined the inlet port windows with the right-angle porting tool from inside the cylinder. I tidied up the top of the ports and lowered the bottoms from 95mm from the top to 97mm. For a longer and earlier inlet port timing I machined 3mm from the inlet side of the piston skirt. For any readers wanting to do the tuning themselves that don’t have the right-angle porting equipment you can shorten the piston by 5mm. Don’t forget, only shorten the piston on the inlet side. If you shorten it all around on, say, the lathe, at top dead centre the bottom of the piston will uncover the exhaust port and you will lose all the incoming gases straight out the port. When writing a tuning article about any particular engine I usually cover big-bore conversions as well. To my knowledge nobody has ever done a big-bore GT550 so I looked into the possibilities. I worked out that if you use a 65mm TS185 piston it will bring the capacity out to 640cc. The existing engine is 61mm x 64mm so that will make it a slightly oversquare 65mm x 64mm. The TS185 pistons are 2mm higher from the gudgeon-pin to the top; Grampian do a 1.5mm spacer gasket so you would need this plus a standard 0.5mm gasket each side of it. I must stress I have never carried out this conversion so if you decide to try it out yourself be careful and check out if there are any snags. The classic motorcycle fraternity owes a great deal to Grampian Motors; without them there would far fewer classic two-strokes on the road. If engine parts are no longer available, Grampian will look at a way around it or have the parts made. The GT550 crank is a good example, there are no longer any parts available for them. They have sourced main bearings and had crank seals made. The con-rod kits are from the KTM 200 Enduro bike and a great improvement over standard. The con-rods are 1mm longer than the Suzuki rods so Grampians have had cylinder base gaskets made that are 1mm thicker. The parts were sourced by Nigel on the internet; they must be right if they were on the internet! Of course they weren’t right at all but I got around it. With all the parts back from the platers and aqua blasters, and the crank back and with my tuning finished, it was time for reassembly. The crankcases had been stripped out completely to have them aqua-blasted, so first of all they had to be reassembled. If you had just stripped the engine to overhaul it this wouldn’t have been necessary. I am not going to tell you at each step to clean and oil all moving parts and to grease all seals – just make sure you do it. The first thing to do is to fit the oil pump drive so that you don’t forget it later. On most older Suzuki two-stroke engines the gearbox bearings locate in the crankcases with small steel dowels pressed into the lower case. Nearly every time somebody has previously fitted gear-shafts without locating the bearings onto the dowels, the result is that the dowels have been pressed into the case and then won’t locate the bearings. If the dowels can be
seen from the other end where they have been pushed through the alloy, you can punch them back through with a hammer and drift and then peen the alloy over to retain them. If the dowel has been embedded into a blind hole you can’t do this. One way I have found is to use an arc welder to just tack weld something onto the dowel so that the dowel can be pulled out, then put a ball bearing or something similar in the hole before putting the dowel back, or make a longer dowel. I said earlier in the article to take a photo of the gearbox and selector mechanism before you remove it, one reason is because the photo will make more sense than me trying to put it into words. Remove the stopper plate for the kick-start shaft; this will allow the kick-start spring to unwind and make it a lot easier to locate the shaft without the spring trying to interfere. Fit the selector drum and selectors. Locate the gear shafts onto the locating dowels and C-locating clips and the crank pegs into their positions. Double check everything is as per the photo and smear gasket sealer on the gasket faces, I always use Yamabond or Threebond, never use a silicon sealer or instant gasket. Lightly tighten the bolts on the top crankcase, turn the engine upside-down. This is when I put the engine on my camera and crushed it and is why the photos of the tuning which I took with a crappy borrowed camera are so poor! Sorry all! So, fit the 8mm bolts and the four Allen bolts and torque them down. While the engine is upside-down fit the selector drum detent plunger and spring and the drain plug. Fit the starter motor and cover. Turn the engine the right-way-up and retrieve the crushed camera! Tighten the top crankcase bolts. Turn the crank and make sure it turns smoothly. First the right-hand side of the engine. Turn the kickstart shaft until there is the right amount of spring tension and refit the stopper plate. Make sure the ratchet works; if it does not engage, push the shaft in and try it, the clutch outer case pushes the shaft in when it is fitted. Fit the selector drum stopper plates, fit the oil-guide plate, fit the crank primary gear and fit the starter idler gear bracket/ shaft. Now fit the larger starter gear and needle roller bearing. Fit the clutch complete with starter clutch assembly, then fit the shim/spacer on the clutch shaft and fit the clutch hub with a new lock washer, tighten the clutch nut while holding the hub with a holding tool and bend the lock washer over. Fit the clutch plates and check everything over. Turn the shafts and make sure they turn smoothly. Fit the gear lever and go through the gears, it’s easier if someone turns the gear shafts while you do it. When you are satisfied all the transmission is okay, fit the outer clutch casing, make sure the peg on the crank primary gear lines up with the cut-out in the points drive. On the left-hand side of the engine fit the rotor and stator, don’t fit the outer cover yet, you will need to use the rotor bolt to turn the engine over later. Fit the oil-pump and make sure all the little O-rings are in place and the drive pin. It’s time to fit the little ends and piston assemblies so now it’s time to fit the special base gaskets. Fit the centre barrel first and then the outer ones and tighten the barrel nuts. Now fit the one piece head and torque down the head nuts evenly. Turn the engine over and make sure it turns smoothly. Fit the points assemblies and time the engine as per the manual. Fit the outer covers and stand back and admire a very elegant looking engine!