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This article was written by Robert Miller with assistance from Arlie Rauch .
First recognize the simplicity of the design. This motorcycle rewards a small amount of maintenance with a large amount of trouble-free miles. The XJ650 is a good design for a turbocharged motorcycle and can be improved upon to achieve motorcycling excellence. Second, always warm up and cool down the engine preceding and after use of boost. Leave the engine alone. It has more than enough power to get even the most experienced of riders in deep trouble. It is the rest of the components of the motorcycle that require upgrading to the level of the engine.
Suspension:
As supplied by the factory, the air suspension was more fad than function. Air has been proven to be a poor hydraulic medium by contributing to the contamination of shock oil and premature seal failure. The Koni model 7610 has been a good replacement.
The forks have too little rebound dampening and too much compression dampening. The fork springs should be replaced with progressively wound units. Revalving is the cure, but short of that, heavyweight fork oil helps. The forks are not strong enough and exhibit a lot of flex and will unnerve riders spoiled by large diameter cartridge and upside down types. Add an aftermarket brace.
Steering:
The placement of needle bearings in the steering head provides increased feedback from the front tire and more precise steering inputs. The generic bearing # is 25Y-48YS/ 30Y-48KS and may be available from a commercial distributor.
Brakes:
The single-acting piston, dual front, calipers are probably the weakest link in an otherwise very competent motorcycle. In stock form they are downright scary. This motorcycle goes too fast too fast not to have the best possible brakes. Braided stainless or Kevlar lines and high friction pad material are a must- and then braking is only adequate.
Exhaust:
Unfortunately the mufflers are not stainless steel (like the header pipes) and are angled downward. This results in loosened metal collecting at the turbo and wastegate outlets. Remove, inspect and clean any debris from inside the mufflers on a regular basis. The crossover pipes between the turbo and the mufflers will deteriorate with age. Luckily, they are not critical to engine performance.
Cables:
The choke cable, as installed from the factory, is improperly routed. Proper routing can be accomplished after removing the fairing side panels and the gas tank. Route the cable along the frame backbone so the choke lever operates freely throughout its full range. Otherwise, choke lever will operate improperly and result in poor cold weather starting. The clutch cable will show a wear point where it is in contact with the left side fitting of the air line between the forks.
Turbocharger:
The Mitsubishi TC03-06A is designed to withstand 210,000 rpm with a turbine diameter of only 39mm. It is the smallest unit ever fitted to a production motorcycle. It exhibits lag, but this is because most of the time (below 70 mph and 5000 rpm) this engine operates at low (5 psi) boost. The unit produces produces usable boost from 6000-9000 rpm afterwhich the two-stage wastegate begins to open.
Debris, road dirt, and water bombard the wastegate location. The actuator pivot lever tends to seize and cause improper wastegate actuation. This will result in poor engine performance and be characterized by a wavering boost needle and inability to achieve engine redline. On those models with the Power-Up Kit, repeated wastegate actuator failure can be eliminated by disconnecting the intake manifold pressure line at the pressure restrictor located inside the fairing above the right footpeg. This pressure restrictor is a 1" circular plastic piece that restricts manifold pressure to the wastegate actuator thus tricking the wastegate into opening at a higher pressure than the stock arrangement of running the wastegate actuator pressure line directly from the turbo housing to the actuator.
Plug both the restrictor line and the line running to the actuator. With the wastegate now disconnected, the poppet valve in the surge tank will open at 15 psi and protect the engine from overboost.
The 1982 model operated only to 7 psi. The factory upgraded engine performance on the 1983 model by providing a Power-Up Kit (See Yamaha Service Bulletin, “Power-Up Kit”, January 1983). The restrictor plate in the right side muffler, the plastic pressure restrictor, and a blind plug on the turbo housing evidences installation of the Power-Up Kit. The Power-Up Kit allows up to 12 psi and provides a substantial increase in performance.
Do not allow anyone but a qualified technician to repair your turbocharger unit (there are about five in the States - each specailizing in a different model) or you will be sorry. Turbocharger bearing wear is also evidenced by a severe decrease in acceleration as engine approaches maximum boost under load (6000-9000 rpm). Turbocharger bearing wear is not the reason for engine oil pumping from exhaust pipe or into intake tract.
Failure of the oil return line check valve causes this and can be fixed by removing line and clearing any debris from under ball-check valve. This usually occurs after long periods of nonuse. After any period of nonuse, engine and turbocharger oil pressure must be present to prevent turbocharger bearing failure. Do this by removing and grounding sparkplugs and cranking the engine repeatedly for 5-10 second intervals. This turbocharger can destroy itself in seconds without proper lubrication.
Also, if you notice a large puddle of oil underneath the turbocharger and the mufflers are blowing out white smoke, and especially if the engine quits after running for a period of time, then there is likely a worn turbocharger. The turbocharger can actually circulate engine oil through the air induction into the carburetors and back into the engine. Engine oil can also actually pass through the turbocharger into the mufflers so that they eventually begin to fill with oil. Mounting the turbocharger low and behind the engine encourages this. Anyway, these symptoms suggest the need for a new or rebuilt turbocharger.
When a worn turbocharger eventually causes the engine to quit (probably because of circulating oil through the carburetors), some have tried to blame the fuel regulator. Most likely the fuel regulator is fine. It is easy to test the fuel regulator by removing it, running water through it and applying a little air pressure through the proper opening. That way you can tell whether the diaphram is operating. A worn turbocharger may lead a person to believe the fuel regulator is bad, but a new or rebuilt turbocharger will solve the problem, and the fuel regulator will seem to be ok. That's good, because those little units are quite costly.
Engine:
The engine produces very little power below 6000 rpm and has peak output of 71 hp across the narrow range of 7500-8500 rpm. The "horsepower hit" occurs between 6000-7000 rpm when the boost builds and the engine responds with a jump from 42-65. Usuable power is from 5500-8500 rpm when the engine more than doubles its horsepower (from 32-71). Below 6000 rpm, it feels on par with modern 750cc in-line fours operated in the same range. From 6000-9000 rpm it feels like the same 750 at 9000-12,000 rpm.
These numbers don't reveal the Yamaha's driveability. It will accelerate forcefully from 25 mph (1500 rpm) to 125 mph (9000 rpm) without a gear change. The smallish 30mm carburetors and resulting high velocity intake, efficient combustion chambers and computerized ignition system with knock sensor all contribute to its smooth acceleration.
Conclusions:
A sane riding position, low maintenance, and total reliability are the main features of this motorcycle. What it lacks in total performance it more than makes up for in exclusivity. The wiggle from the rear end as the tire breaks loose momentarily (at 85 mph!) as the boost hits at 6000 rpm after a top gear roll-on, the sight of other riders pulling over to see why they haven't been able to lose the motorcycle behind them; and the bewildered looks from modern sportbikers who, acknowledge it as one of their own, but can't quite figure out what it is, are experiences of pure joy, and just some of the benefits of owning a Yamaha Turbo. Get one for yourself and find out the others.
Last Modified March 06, 2007, 18:41:38 Eastern Standard Time
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