Stan the Man again on tun­ing this Suzuki triple.

Classic Motorcycle Mechanics - - CONTENTS - WORDS AND PHO­TOS: STAN STEPHENS


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!

We need to raise, widen and re­shape the ex­haust port.

Here we are skim­ming the head on the mill.

Fit the bear­ing lo­cat­ing C-re­tainer.

Here we have the gear­box as­sem­bled.

Time to lo­cate the clutch seal peg.

As said, do check the photo for se­lec­tor mech­a­nism po­si­tions.

The ma­chined in­let port and trans­fers and the flowed bot­tom of the liner.

Here we can see the starter gears, se­lec­tor shaft and kick-start shaft and stop-per and crank pri­mary gear.

Cases as­sem­bled with the starter and se­lec­tor plunger fit­ted.

The crank is fit­ted with pegs lo­cated.

Here is the large starter gear/clutch and the trans­mis­sion clutch with spring shock ab­sorbers.

Now we are torque­ing up the clutch nut.

Now it’s time to fit the bar­rels and the head.

Here it is: all fin­ished and hand­some!

Lin­ing up the steel peg on the crank gear with cut-out.

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