Yamaha 700cc Four
There has rarely been a series of machines more desirable, or mysterious, than the big Yamaha racing fours of the mid 70s. Before the types introduction, 750cc racing machines were based upon modified road bikes, the rules demanded a minimum of 200 models would have to be built to satisfy homologation requirements, so using an existing machine upon which to base your racer on made good economic sense. Suzuki, Honda and Kawasaki, along with Triumph BSA and others, all used their latest roadsters to compete in the newly formed Formula 750 class, Yamaha’s interpretation of the rules led to a different path being taken. They didn’t have a suitable large capacity machine with which to compete anyway so they simply put the big TZ into mass production, producing enough to meet the demands of the rule books and with it, changing the face of modern day racing forever.
For those fortunate to have ridden the real McCoy, the ride is nothing less than the reputation purports to be. Riding is one thing however, go several steps further around the tacho dial and you enter an all-together different world. For the elite, able to fully use the power on tap, the rewards are clear, a full trophy cabinet and for a special few, world renown and fame. The TZ became an instant legend, with Giacomo Agostini on board, the factory version won first time out at the 1974 Daytona 200. What followed after that event was an almost total domination of big capacity racing, a TZ750 of one form or another carried on winning at the Daytona race until 1982. The American officials tried to ban it when it first appeared but, as it was completely legal albeit not truly within the spirit of the rules, they introduced measures to restrict it by limiting the thirsty engines inlet diameter. These measures, although stunting the power output, were never truly effective and the big four just carried on being the racers ideal tool, usable, powerful and sweet handling.
Many individuals have tried to replicate the format for road use but few have captured the true spirit of the dinosaur TZ 750. Even Yamaha themselves gave the idea a wide berth and never came close to giving the road rider the tremendous pushing power that the TZ engine gave the select few professional racers. Martin has captured much of the essence that the big four-cylinder racer exhibits. Ok, the power is down by around 30 percent from the fire-breathing missile that is the TZ, but the standard wet clutch of the RD twin couldn’t handle much more without serious complaint.
Travelling like a King
By joining together two RD twin cylinder crank casings Martin and his supporters have moved into an area not so distant from the world of the Big TZ. Internally the race machine shares little with the roadster engines, externally however, the look is quite convincing. The ride isn’t so far off either. Immediately the feel was familiar to me, and those many races on board my own TZ’s soon came flooding back. That low down grunt, seldom seen in any kind of road machine, was there by the bucket load. No matter where about in the rev range you make that call to go, a cart load of horses are there at all times ready to answer. That distinctive exhaust note that can only be created by four separate end can is also authentic; his machine really could be a TZ on the road. The way the engine picks up from low down in the rev range, and just keeps on pulling, is addictive. The engines voracious appetite for ratios soon eats up the six-speed gearbox, while those four separate end cans sing a sweet tune seldom heard in these four stroke times. Accelerating has rarely been this much fun.
Chassis wise the tubing is a largely unmolested RD400, and yet it handles the extra weight and power admirably. In fact I would go so far as to say it handles far better than any standard RD twin I’ve ever ridden. Gone is that mid corner wallow so typical of the lightweight twins when ridden hard and in its place is a sure, rock steady cornering experience. The up rated suspension, particularly at the front end, has created a machine that handles like a modern day sports bike while the extra length and strength found in the swing arm area keeps the powerful and torquey motor firmly in check.
The test ride nearly didn’t last too long as after the first run past the camera I grabbed a big hand full of right lever with immediate, and shocking, effect. I soon let go of the lever and rethought my approach with this area of the bikes performance, stunning is one word that could be used, overkill and professionals need only apply are others equally as apt. There is simply no way to be light with the brakes, the FZR1000 set up hanging off each side of the front wheel take no prisoners as it hauls the race rep up from speed. Even from top speed the brakes feel as if they could lock the front with the slightest increase in bar pressure.
Unlike many other special builders Martin has widely chosen to keep the original size carburettors and this has proved worthwhile. Personally I would have been tempted to go for the 34mm Mikunis as fitted to the real McKoy but this would have lost so much of the tractability evident with this machine. Its perfectly driveable and yet let it get anywhere near the start of the real power and, like its pure bred cousin, it will rip your arms off and most likely beat you with the soggy ends. The big air-cooled four feels just like its water-cooled, race bred, stable mate, the grunt, the willingness to rev and even the sound are just about there.
Martin Bell, along with his close circle of imaginative and creative friends, has created a complete and ready machine that could easily have been produced by Yamaha such is the finish, look and usability. Knowing Martin as I do, I should imagine that in his eyes it will never be quite “there” and he will continue developing the big four well past this already complete stage into something even more so.
Martin’s recipe for the perfect race rep
“I started mid-October 2003 with some sort of crazy idea that I was pondering over for two or three years. I wanted to build a race replica RD, trying to keep that seventies look that I love, but I didn’t want to do the usual upside down forks, single sided s/arm- nothing wrong with that, but everybody does the same sort of thing. I also wanted to know what a four cylinder RD would sound and feel like. If anybody out there can remember the flying pig, yes Ham Yam Racing, this was a bike shop in Newcastle that transformed new RD’s into race bikes, racing tank and seat, rear sets, pipes, and full fairings and yet fully road legal!. Well this is exactly what I wanted to create with my bike, that Ham Yam look not the TZ look or the Kenny Roberts. The number 4 on the fairing is for four cylinder, now’t else for those who keep asking.
I originally sat down in my garage with two engine casings, wondering if I could create a four cylinder. Weighing the pro’s and cons up, it seemed too hard to do. Idea’s soon turned to reality with the help of a good friend’s machinery and much welcome advice, such as Kingsley Davies (technical advisor for the RD club) and Andy Nicholson (Engineering wiz, welder, machinist) All deserve a mention for their time and have a go attitude. I machined 19mm off between the two casings so that crankshaft would fit in. Armstrong Racing (Newcastle) did the crankshaft for me. Then I used a 62mm dummy shaft, to put in place where the crankshaft goes to hold the two casings straight, and 62mm reduced to 31mm in the rear of the gearbox. I bolted it all together and Tig welded it – sounds easy doesn’t it? The left side of the gearbox all required cutting away but leaving the pushrod and gear selector housings in place.
The first engine started life as a 500 using two 250cc top ends, I cut all the fins of both centre barrels, plus a bit more and one fin off each of the centre heads. The transmission remained very much standard, other than extending the output shaft, using 7mm thick pipe and grub screws. Machined and polished where the oil seal goes, then a billet bearing housing was used for the extended shaft, which is in the empty left/gearbox, with a hole cut for chain entry. Gear shaft and pushrod where extended by what ever amount was required to get it all to work.
A Newtronics electronic ignition, with two ZXR600 coils, firing 1&3 and 2&4 together, provides the sparks. Other minor problems involved machining the tick over screws down on the inner two carburettors for clearance and making up a one into four throttle cable, using a modified GT380 unit.
With the engine coming along nicely I turned my attentions to the chassis, cutting all the lugs off that I didn’t need, keeping the weight down as well as strengthening all the mounts. New brackets where added for the Yamaha R1 pegs & hangers which now use Raaske levers. Fairing is off a TZ250N, modified, stretched & cut to fit the extra width, mixed with a Maxton alloy race tank. A period TZ race seat and not so, Suzuki SV front mudguard cut & shaped add the finishing touches to the racey look. RG500 forks & yolks were modified to accept the huge FZR1000 discs & calipers, which in turn are bolted onto 17-inch TZR250 wheels. An aluminium, LC350, swinging arm slotted straight in, once the under & top supports where cut off to allow the exhausts to pass lugs where then welded in place to allow the fitting of standard Yamaha twin rear shocks.
Once the 500cc version was sorted I began to think of going the whole Hog by taking the capacity out to 700cc or above. RD400E barrel’s where machined down to match the 54mm stroke of the 350cc to make 700cc, then RD250 heads machined out to 64mm to match the cylinders. Each cylinder was then blue printed to ensure good, safe power while RD250carb slides where machined into 400 ones. People keep asking how have you done that? My reply usually is lots & lots of super glue but to be truthful you need lots of good friends favours.
I just love this bike it brings me back to my youth when the smell of two stoke oil in the air was so intoxicating you could get drunk it. The bike just puts a smile on my face every time I go on it. When the power band comes in, it’s just an awesome power rush.”
Yamaha RD700 Specifications
- Engine – air cooled four cylinder two stroke
- Capacity – 694cc
- Bore & stroke – 64 x 54mm
- Carburetion – 4 x 28 mm Mikuni
- Max Power – 84bhp @ 8000rpm
- Torque – 50 ft-lb @ 6500rpm
- Ignition – Newtronics electronic
- Transmission – 6 speed chain final drive
- Frame – steel tube twin cradle
- Suspension – 38 mm telescopic forks twin shock rear
- Wheels – 100/80/17 130/80/17
- Brakes – 2 x 320mm disc 4 piston calipers, 220mm disc twin piston caliper
- Wheelbase – 1375mm
- Weight – 118kgs
- Fuel capacity – 20 litre
- Top speed – 140mph
Yamaha RD700 Gallery