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Yamaha XJ650 Turbo, Part Three
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After investigating the background to Yamaha's 650 turbo, Roger Prosper bought a Seca. Then he had to fix it...

Mechanically, the bike was hurting from the mileage it had done. I was the fifth owner of the machine, and the amount of care lavished on it by the previous four was in dispute. There was a worrying buzzing coming from the motor between 3000 and 5000 rpm. Since this bike was probably the only machine I would own for the foreseeable future, I decided it had to be fixed, but I put it off for as long as possible. Only when the starter clutch finally refused to engage (easy to discern with the loud marbles-in-a-coffee-can rattle) I decided the time was ripe to open up the engine.

Sometimes it is best to let old machines die gracefully, as their individual components all seem to settle into an equal decrepitude that allows them to keep running for miles and miles in their hobbled state. Once this equilibrium is upset, one can easily find oneself chasing down problem after problem trying to restore this equilibrium. Such was the case with the XJ.

Well, at least it looks comfortable. 1982 Yamaha XJ650 Turbo

The design of an XJ Yamaha requires the motor to be pulled and the cases split to replace the starter clutch. Anecdotal stories exist where sufficiently skilled mechanics have been able to go I through the alternator housing with very long pliers and screwdrivers to do the job in-situ, but the details on how to do this are so chilling I decided it was better to go the whole route, since I was pretty much 100% guaranteed to drop something inside the engine anyway.

The motor was a mess. The whole back of the engine was caked in oil and dirt, which had leaked out from the pressure pipe leading to the carburettors. The XJ has its turbo mounted down low, behind the crankcase, which allows oil to gravity feed into it when it is left sitting for a long period of time. There is a check valve mounted at the front of the engine where the oil feed line taps into the main oil gallery, but it tends to be problematic in operation after a while, as the ball and seat inside it wears. The turbo itself has seals on the shaft which resemble small piston rings, which require boost pressure for sealing, so they do little to stop oil from leaking into the turbo compressor housing when not running, which leads to smoky start-ups. The bike actually burped about a shot-glass of oil onto my leg when I removed the rubber air inlet hose during the initial teardown…

The source of the buzzing turned out to be the camchain rubbing against the valve cover. The front guide had broken about two inches down from the top, inducing enough slack in the chain to let it contact the cover. A new camchain and guide were in order, and a new rear guide was installed as well.

Removing the head revealed combustion chambers and piston crowns caked with hard, thick carbon. That oil leak-through had done its dirty business. It took about an hour of hard scraping and polishing to get the chambers and pistons reasonably clean. The piston rings were worn shiny and very sharp (ow!), so they too needed to be replaced.

The bottom end was in good condition. All the shell bearings were shiny and had no perceptible wear patches. The transmission looked to be in decent shape, with no worn gear dogs or shifting forks. The gears and crankshaft were the only components that didn't have to come out to get the starter clutch out.

I swear the clutch parts that came out looked no more worn than the new ones I put it, which was unsatisfying to say the least. A seasoned Yamaha mechanic later told me it might have been the synthetic oil I had put in the bike that caused it to slip. The wear on these parts would seem to back up his statement!

All the chains in the lower end seemed to be terrifyingly slack, but to be honest, this whole repair was costing me so much in time and money that I was getting frustrated.

Turbo stuff on eBay.co.uk

Summer was passing by, I had parts everywhere in plastic bins and in the trunk of my car, there was a stink of chemicals and gasoline everywhere, and I had only an old television stand to use was a workbench. It was either staggeringly hot and humid while I was working, or I was scrambling to collect all my tools and parts when the clouds opened up.

I confess to calling the local breakers to see what they would give me for the bike in parts, but it turns out they would have charged me to come collect it! Talk about falling from grace.

I bit the bullet and completed the repairs. Lifting the 650 lump back into the bike showed me where much of the bike's mass came from. It's a heavy old motor. I am glad to say the repair was a success. The engine started fine, didn't self-destruct, and ran reliably the rest of the summer. So for my troubles, I had an old, portly, poor handling, peaky turbo bike, that no longer buzzed and started reliably. I dared not add up the cost of the repairs, since I'm sure it would have given me palpitations. I rode it for the last two months of the riding season and parked it.

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Next episode: so was Roger going to live happily ever after with his XJ turbo?



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