Boiling hot!
Within Kettle Club circles, Phil Baldwin’s 1000cc four-cylinder version of Suzuki’s GT750 has become legendary. John Nutting takes ‘The Beast’ for a ride and comes back surprised
Some reckon that when Suzuki’s engineers were developing their first proper superbike back in the late 1960s, they built a fourcylinder two-stroke liquid-cooled version of the 500cc Cobra twin, which was then the Hamamatsu factory’s biggest bike.
The story goes that they bottled out of offering a 1000cc four, chopped off one of the pots and ended up with a 750cc machine which would have been on the superbike trend at the time, and the GT750 emerged. Nothing much wrong with that, because the Kettle, as it became affectionately called, is a fine touring machine. But the theory that the bigger bike existed lingers, and certainly does in the mind of Kettle Club stalwart Phil Baldwin, who turned it into reality by building what he thinks it might have been like. So here I am threading my way through the twisty lanes of north Kent on that very machine trying to keep up with Phil on one of his GT750 triples, a minter he’s owned for many years.
Amongst fellow Kettle Club members the GT1000, as it is badged, has become legendary. They have rightly called it The Beast and when the opportunity comes on a bit of open road I open it up and realise why: the gap to Phil suddenly shrinks in a tsunami wave of silky acceleration. What’s also remarkable about Phil’s GT1000 is that he’s managed to make
the engine look uncannily more authentic than the triple it’s based on. The cylinder block and heads have been combined so smoothly and symmetrically that there’s barely any indication of a join, and likewise with the cases. He’s also built the frame and bodywork so that it matches the extra width of the engine – but you have to look closely to see where the additional material has been added, but more of that later. That’s the point, says Phil, who started with a self-imposed budget of just £3000 to pay for anything he couldn’t do himself, which as it turns out isn’t much, because he’s so multi-skilled. “It wasn’t built for performance or handling, but to look like a bike from Suzuki’s GT range,” he says. “I started it six years ago as a trial to see if I could get it to work. I was quite surprised when I built it up with old parts kicking around the shed and I had a water-tight running engine. I then put it to one side and didn’t look at it until a year later.”
Actually it was a bit more complicated than that. Suzuki’s 750cc engine has three pairs of flywheels pressed together with a helical drive gear to the clutch between the centre and right-hand cranks. On top of the horizontally-split crankcases, there’s a one-piece cylinder block – the first to be liquidcooled on a Japanese road bike – topped by a one-piece cylinder head. Porting was simple, but to save width the left exhausts were rotated outwards so the transfer ports snuggled more closely while the other cylinder was rotated to the right. “The right-hand side is the only side you can add the cylinder,” says Phil. “You have two left-hand pistons and one right hand, and the left-hand exhaust ports come out of the left side, and right-hand one comes out of the middle. You can add one to that but if you added to the other side you have to go even further. It just didn’t work out. I just looked and looked at it and it was the only way to do it really.”
To add the extra capacity, Phil cut off a right-hand cylinder, head and cases from one set and mated them to another crankcase. He pressed up the additional crankshaft, machining off the drive gear to join them up. “As luck would have it, all this lined up perfectly, so that saved me having to make a new main-shaft,” he recalls.
“Now I had tested my mods I needed to build the engine properly, so took it apart and rebuilt it with new crankshaft parts. I machined the crankcase, cylinder and head parts to fit and tack welded them with a MIG spool gun to hold them in place.” Phil then cut a vee into the joins so he could get more deeply into the water jacket. The parts were then heated up uniformly before properly TIG welding them together. All the mating surfaces were machined to fit and a special gasket cut to fit.
Phil explained the challenge of welding up large castings. “As you know, even when it’s bolted together and you let go of the bolts it’s all going to move slightly,” he says. “So I put a shaft all the way through the bearings with a tool in the end and machined the right-hand one out bigger on the journal and then put a sleeve on the end of the crank to bring it back down to the same size. Originally I just machined it a small amount and just put shims just to try it. Now I’ve changed it and it’s got a sleeve on it.”
While the three crankpins on the original crankshaft are spaced at 120 degrees, the fourcylinder’s flywheels were pressed up at 90-degree intervals, giving four firing strokes, unlike the other biggest four-cylinder two-stroke,
Yamaha’s TZ750 racer, in which the inner and outer pots fired at
180 degrees. With the cylinders overbored by 1mm and 71mm-diameter pistons fitted, overall capacity works out at 1013.5cc. There was also a lot of finishing necessary to match up the exterior of the castings, along with the mounting lugs, to get that authentic look. But if you are not completely familiar with the GT750 you have to look more closely.
For a start, the outer crankcase covers are much narrower, so much so that the four-cylinder engine is only half-an-inch wider than the triple. This was achieved by fitting a slimmer generator from a Yamaha 600 Fazer on the right-hand side, while a cover welded up from 2mm aluminium sheet also
fills in the volume outside the clutch housing. On the left, the cover is also abbreviated because it has to house only the ignition trigger coils, which Phil has adapted from an aftermarket electronic system that fires double-ended high-tension coils from a GS model, so it fires twice per cycle and there is a wasted spark at bottom dead centre.
That gives the GT1000 a novel sound, still a bit like the triple with its deep crackly note, only more so and with a busier feel. The engine exhausts through four expansion-chamber-style pipes, neatly welded up by Phil from rolled stainless steel sheet and feeding a set of four silencers.
The GT750, particularly the early models, is flexible, normally pulling cleanly from low revs, but the GT1000 was less so. That could be a blessing because there could be so much torque, but in fact it’s likely to be a result of the fixed ignition timing. No matter, for when that open road presents itself, opening up the GT1000 is a real joy, and revving to 7000rpm or more through the gears is largely unnecessary. The carburetion also retains that slightly erratic feel when shutting off and cruising at a lower throttle opening, but that’s much of the fun of riding a big two-stroke.
When he first built the bike, Phil used the larger CV Mikuni carbs from the M models, but arranging a linkage for the right-hand carb proved troublesome, so he reverted to the smaller-bore early slide-type carbs with the right-hand one mounted on a longer cylinder stub. They work just fine, despite the extra cable. When you’re seated, the view of the instruments is all GT750, complete with the central temperature gauge, but a glance down reveals that the radiator has more volume. This is an aftermarket item and matches revised coolant plumbing in the engine. Phil has fitted a fan to the rad, but the engine’s never got hot enough to activate its use.
Riding south on lanes through the Medway valley, we ended up at Aylesford and joined the A229, climbing the North Downs in the return to Rochester. Phil tore off into the distance on the dual carriageway, leaving a trail of blue smoke from the 750, but even though I’d been caught on the hop, the GT1000 swiftly reduced the gap. I knocked it off at an indicated 80mph and glanced at the rev meter to see the needle at a modest 4000 in fifth. The GT1000 is geared very high.
Like much else on the GT1000, Phil has changed the transmission and final drive. The five-speed gearbox is from a later GSX750, which fits with minor modification, the benefit being that the lower ratios are closer, and you don’t get that sudden shock when slipping down into first. Also the final drive ratio has been raised with a smaller 41-T rear sprocket (two down on the stocker), a modified Honda item.
Although the four-cylinder engine has more bulk, it doesn’t appear so. That’s because it’s mounted in a frame which has been fabricated to be 50mm wider across the swingarm pivot, while positioning the engine to the left by a few millimetres to get the chain run right. Phil started with a jig based on a rolled steel joist and used the original 1492mm wheelbase. At the front the frame tubes are splayed out more so the exhaust pipes look symmetrical, and likewise at the rear. But the assembly doesn’t appear wider because the seat has also been widened by adding the same amount to the pan and stretching the cover across to fit. It’s all very subtle, and means that modern 17-inch wheels don’t look out of place.
At the rear there’s a wheel from a ZXR750 Kawasaki with a wide 190/55 Continental Sport Attack tyre, mounted into a matching Kawasaki aluminium-alloy swingarm, shortened and modified to take conventional, but longer than stock springdamper units. Phil says: “Rear shocks are Girlings at the moment but they’re not going to be on there much longer. They’re just not right, but the others from China look right but are just like pogo sticks. I don’t know really yet what to put on it. But I want it to look Kettle-ish!” Phil was concerned about a lack of cornering clearance, so the front end – a complete telescopic fork and wheel fitted with a matching 120/70 Continental tyre from a Suzuki 600 Bandit – has been raised with the addition of screw-in extenders at the top of the legs.
At speed the bike sits securely, a comfortable platform for cruising as the exhaust note is lost in the wind. I didn’t have any problems with clearance – I wasn’t about to push it that hard in any case
– but I found the steering, well, interesting. Whether this was due to the wider rear tyre, the altered front end geometry or the additional weight of the engine, but when tipping into roundabouts, the bike had a tendency to drop in too readily, and lacked the neutrality of the stock bike’s navigation.