1977 YAMAHA RD400- Project

1977 YAMAHA RD400- Project

Source: Cycle Guide of 1977

Dearly beloved, you can begin to wave good-bye to all those two-stroke roadsters we've known and loved for years. Kawasaki's KH400 triple has been laid away, and Suzuki has rung up the flower car for their two-stroke roadsters. The four-strokes have won the sales derby; more important, they have a better chance of winning the government emission tests. But before you get all teary and blow your nose, you should know that's all the bad news.

The good news is that RD400 Yamaha is still with us, and will be present or accounted for on the performance scene until at least 1980. And however attractive the old Kawasaki KH400 was, and how-ever utilitarian and -sturdy the GT-380 Suzuki was, there can be no doubt that the Yamaha RD two-strokes have been the most charismatic little rockets built in the last decade. So it has come to pass, as we climb the mountain of bureaucratic red-tape and finally see below us the Valley of the Shadow of Clean Air, that the first name in two-stroke performers shall be the last to go. Hallelujah: we can still do the reed-induction boogie.

If there's one word that describes the Yamaha RD400 series, the word is intense. The all-disc brakes are intensely powerful, an RD tradition since the motorcycles were disc/drum 350s. The RD Yamahas have always been quick steering motorcycles, thanks to a 52.5-inch wheelbase, 27.5 degrees of rake, 4.3 inches of trail, and a wet weight of 379 pounds. This kind of motorcycle isn't really guided into and manipulated through a corner; rather, RD Yamahas simply dart into the entry and out the exits.

If you weigh 185 pounds with riding gear, you're likely to dispense with a great deal of body English. Whatever you put into the bike, it tends to translate that input, amplify it, and feed it back to you. Call it intensity.

The engine has a high degree of sensitivity. The RD400 seemingly magnifies a quarter-inch turn at the twistgrip to a yards-long leap at the rear wheel. The engine is like the brakes: a little at the controls does a lot at the wheels. In about the first tenth-mile, the RD400E serves notice: be careful and well, or be foolish and sorry.

It's the way the two-stroke engine delivers its power that separates the RD400 from all the four-stroke twins. The four-stroke twins seem to accelerate by pulling themselves through the rev-range; though revs build with determination, no 400cc four-stroke twin goes through its rev-range with a blinding rush. By comparison, the RD400E delivers power in an explosive, instantaneous way. The revs build so rapidly that the tachometer always seems to be one shift behind.

Furthermore, the RD400E jumped from 25.34 horsepower at 6000 rpm to 31.11 at 6500 rpm. That's what you call coming in with a bang.

Our tune-up included resetting the ignition timing to the specified 2.30 mm BTDC. The timing had slipped to 1.95 mm BTDC on the left cylinder, 1.99 mm on the right. We fitted NGK B-7ES spark plugs, a range hotter than the deposit-coated B8ES plugs that came out of the bike.

The tune up brought the performance up, but not much. The RD400E turned a 14.72/89.87 mph quarter—an improvement, but not the big one we expected. It was a 105 degrees at the dragstrip leaving the bike over-rich and the spark still seemed weak, but all excuses aside, the 14.7-second quarter disappointed us.

RD performance arrives at the rear wheel courtesy of an amazingly simple powerplant. In these days of 400cc four-stroke twins with one or two overhead camshafts, two or three valves-per-cylinder, and full complement of contra-rotating balancers, Yamaha's two-stroke twin seems like a monument to simplicity. The 64mm x 62mm (bore and stroke) cylinders draw air in through two 28mm Mikuni carburetors and two reed-valve assemblies. Reed valves have been used since the days of the old RD350 to spread out the power. Yamaha has succeeded in this respect, though the engine definitely begins to perk harder (as the dyno reveals) when the rev-counter needle swings past 6500 rpm.

The most interesting thing in the intake system is the RD400's anti-cackling, anti-surging modifications to the reed petals (uppers and lowers have different stiffnesses), the cylinders (.080-inch passageways in the cylinders lead from a point above the exhaust port windows, through the liner walls, and into the exhaust ports), and pistons (slots in the pistons' exhaust skirts open the crankcase to the exhaust port when the pistons are at TDC). The dynamics of this intake, crankcase and exhaust fiddling are complicated, but the result is pretty simple. It eliminates the surging associated with high-performance two-strokes running at partial throttle, say 45 mph in fifth gear, by weakening the out-of-synch resonating pulses that go through a two-stroke's intake and exhaust system just before the engine gets on its pipe, and all the pulsing comes into synch.

Pistons for these reed-valve twins have windows in the intake sides of the pistons. Slotted and windowed pistons are more likely to collapse with prolonged use and get rattly. Moreover, much of the area in a reed-valve cylinder is taken up by ports, so that the liners offer less contact area for the pistons than might otherwise be the case. The ports-everywhere liners and shrinking pistons eventually produce enough racket to be noticed. This two-stroke clatter is nothing too serious, just annoying and normal.

Yamaha went to a great deal of trouble to silence the RD400 series; indeed, on the induction side, a mammoth air-box filter-unit was designed to hold down the noise and still pass a sufficient volume of air to maintain the aspiration-requirements of the engine. The sheer size of the airbox necessitated a redesign of the frame behind the engine and below the saddle. Despite Yamaha's efforts, the RD400E is still a noisy machine at start-up and idle. There's a fair amount of intake honk that is supplemented by piston/ cylinder noise and well-defined pop-pops from the exhaust pipes. But if you like two-strokes, it's not a bad cacophony at all.

Two-stroke twins have a vibration problem that's not as severe as the 400cc four-stroke twins. Or at least the two-stroke's problems make solutions easier. The low-amplitude, high-frequency vibrations in the RD400 are mainly isolated by rubber-mounting the engine. This allows the engine to vibrate furiously on its soft mounts, but the vibration stays away from the rider. In an effort to be thorough, Yamaha has rubber-mounted the footpeg holders, so only by touching the engine is the rider aware of the unit's activity. Rubber joints between the head pipes (that bolt directly to the shaky engine) and the mufflers—solidly attached to the frame—flex enough to keep the exhaust system intact.

In the suspension department, our main complaint centers on the rebound damping of the shock absorbers. The damping gets limp after you've been thrashing around on bumpy back-country roads, and that causes the RD400 to feel vague and rubbery in a corner. Riding the Yamaha really quickly is a task that takes a lot of concentration, and it's the kind of bike that you like to get through a corner in one swoop with no corrections at mid-passage. It does nothing for your concentration or tidiness to discover that the RD is beginning to waggle.

There are other reasons to keep your focus while jetting with the RD400E. The disc brakes work with an almost ferocious effectiveness; unless you exercise a certain amount of care, you could overbrake and lose the bike. Applied somewhere near their limit, the brakes will let you plunge so deep into a corner that you stop breathing from entrance to apex. Hopping on the gas too hard in a first- or second-(and sometimes third-) gear corner can get the front end very light, and you must recognize that a hot throttle hand can unload the front end enough to make steering highly problematical. The Yokohama Speed Master tires stick very well under acceleration, braking, and cornering loads; nevertheless, given the forcefulness with which the RD400E can be ridden, a rider must make sure he doesn't pass through the tires' margins of safety.

To be sure, the RD400E can toot down the freeway, offering most all those convenience features typical of Japanese motorcycles, including the trick Yamaha directional signal that measures distance and shut itself off automatically. Still, if you're the kind of guy who eyes the freeway exits looking for an interesting road to add to your collection, then you'll intuitively understand the RD400E. And you'll rejoice in the Good News that this high-performance two-stroke is still alive and well in 1978
Source: Cycle Guide of 1977

We picked this puppy up last year from a friend of the shop. It was in decent condition and would start... kind of. We will be going for a tracker style on this baby. Our plans so far are keep the stock tank, lighten the frame, new tracker style fiberglass seat pan, new sub frame, new electronics tray, and relocate the oil tank.

We've already stripped her down to the frame and started tearing into the motor. The cylinders will get an overbore for sure, probably 66mm making it a 425cc. After the seat hoop, brackets, electronics tray and oil tank are all welded on we'll be sending the frame off for a thorough cleaning before painting.

**Update Aug 22nd 2014**
We skipped a few as far as posts showing progress, but heres a few shots of the nearly finished bike. The frame has been powder coated in semi-gloss black while the tank and custom fiberglass seat pan have been treated to a Ford color 'Toreador Red'. All electronics and controls have been installed including a new Koso gauge, left and right control switches, new featherlight battery, and a hidden running/ brake light. A new low rise bar has been fitted as well as Kamikaze rear sets.

The oil tank is now hidden under the seat along with the reworked wiring harness

In the power department we rebuilt her top to bottom leaving the transmission as is. This included overbore to 66mm, new pistons, rings and the like. As well as new bearings, seals, and gaskets. The carbs were thoroughly cleaned by vapor blasting and rebuilt to accomodate new pod filters and a DG performance exhaust.

For suspension we upgraded to RaceTech springs in front which turn on new tapered steering head bearings and added adjustable preload. Then we dropped the front 1in. Out back we installed new adjustable Assault shocks that are 1/2in longer than stock to give the bike a more aggressive stance and put a bit more weight on the front tire for better grip in the turns and less likely to wheelie when you whack on the throttle.

New reproduction brake calipers where put on front and back as well as new master cylinders. The rotors have been drilled and countersunk to reduce weight and keep brake temps down. For better brake feel and better stopping power, the brake lines are now custom braided stainless steel coated in red.

**Update Aug 30th 2014**
We have installed the last of the parts and fired her up for the first time. She runs great. We will be changing the pilot jet sizes but other than shes ready to roll. Now we just have to get her inspected and titled which usually takes 3 weeks or so and she'll be ready to go. The photo shoot is next week so check back for some shots of the finished product.

**Update Sep 5th 2014**
Dunskys! Rejetted, inspected, titled, and photographed. Click here to see the finished product.