Service

SERVICE

BMW cylinder coating

Could you address the issue of special coatings on aluminum alloy cylinder walls? The BMW K100 coats the cylinders with “Scanimet.” How well does it wear? When it’s time to rebuild are the walls recoated? How many miles can I expect with an engine using this kind of technology? I’ve also heard the K100 smokes a lot—how come? I need to know before I seriously consider buying this machine. Don Thompson San Francisco, California

Two types of l inert ess cylinder coatings are in common use: chrome plating, and a composite plating that uses nickel for the plating medium but embeds in it tiny ceramic silicon carbide particles. The German piston company Mahle developed the nickel/ silicon carbide process, and uses the brand name Nikasil. BMW's coating is also nickel/silicon carbide, but done bv a different company with a different brand name.

Nikasil-type coatings have proven very wear resistant, more so than chrome plating; and barring some drastic mechanical failure that would gouge the cylinder wall, K100 cylinder walls should outlast at least one set of pistons and rings. In other words, it should be a 100,000-mile-plus coating, but only time will show if KlOOs live up to those expectations.

As far as KlOOs smoking, they do sometimes when first started, and for a simple reason. When a Kl00 is parked on its sidestand for a period of time, oil tends to run down the cylinder walls, leak past the piston rings (especially if by chance the end-gaps are facing down), and make its way into the combustion chamber. Then, when the motorcycle is started, a white cloud of smoke announces it to the world.

BMW is well aware of this phenomenon and is studying ways to reduce its magnitude. The compan y claims that the condition is at its worst when the bike is new and diminishes greatly as miles are accumulated. In the meanwhile, the smoking can avoided by simply parking a K100 on the centerstand, and not the sidestand.

Replacement mufflers, cheap

I own a 1977 CB550K. The problem is that the baffles are shot in the exhaust system. I went to my local Honda dealer and asked if the baffles can be replaced without replacing the whole system. He said that they couldn’t, and that a stock replacement exhaust system would cost $600. I couldn’t believe my ears on that price.

Is there any way to replace the stock baffles? It not, what do you think of my choice of a Wolf 4-into1 exhaust system as an alternative? These cost around $300. Gerry Kimmett Deseronto, Ontario, Canada

Your dealer is right; the baffles can 7 be replaced separately. There is, however, an alternative. Jardine Exhausts makes replacement mufflers using a 4into-2 design that slip over the stock headpipes. They re available through Rocky Cycle in the U.S., as well as other distributors, and a set for your bike costs under $150.

A 4-into-l system is also a reasonable alternative, and its reduced cost when compared to stock pipes may explain as many 4-into-l sales as any performance gains. We have no specific experience with the Wolf exhausts, so we can 7 make any useful comment on its qualities. We suggest you ask around at local shops and see what they say about the Wolf pipes.

A performance Vision

I have a problem. I’m in the process of blueprinting the engine on my XZ550 Vision, and I want to increase its performance. But I can’t find any manufacturer that makes any performance accessories such as pistons, air filters, etc. Do you know of any sources for these parts.

I'm also thinking of having the carburetors bored out? Would that increase performance? Richard Rives Jackson, New Jersey

Unfortunately, we don't know of an V’ suppliers of hot-rod Vision parts; they simply haven't been in much demand, even though the Vision engine has the potential to put out more power than it currently makes.

We'd recommend against boring out the Vision 's downdraft carburetors. Ridding the Vision of its > mid-range stumble was a major project for Yamaha, and its carburetors are more complex than most motorcycle curbs. Unless you really are a carburetor expert, or want to become one, this is one modification that 's likel y to be more trouble than it's worth.

RZ350 ignition timing

The timing on RZ350s retards as rpm rises to about 16 degrees BTDC at 9000 rpm. It then drops steeply to about 9 degrees BTDC at 10.000 rpm. w hich is about its maximum rpm limit with stock exhaust pipes. Would advanced timing help it to rev or make more power when other exhausts are fitted?

My know ledge of solid-state ignition systems is not good. As far as I understand, the CDI unit is responsible for the advance and retard curve and the timing is not adjustable on the backing plate. Is there any way to stop the retarding at higher rpm? Electronically or otherwise? Paul Ward Durban. South Africa

The RZ350 ignition-timing curve is almost certainly wrong to work with engine tuning that raises the power peak past 10,000 rpm. Generally, twostroke engines run best with more spark advance at medium speeds, then with gradually less advance until a minimum is reached at the torque peak. Further spark retardation, as with the drop from 16 degrees to 9 degrees on the RZ, is essentially there as overrev protection.

How to obtain the correct advance curve is a harder question to answer. If you 're willing to sacrifice streetahility, it 's possible to fit an y of the several different TZ roadrace ignitions to the RZ, all of which have advance curves more suitable for 10,000-rpm-plus engines. But that will mean giving up the alternator. Advancing the timing by moving the ignition pickups is possible, but that might lead to too much advance at low speeds, causing detonation problems. It might well he possible to fit an old points-style ignition from an RD, giving a single fixed timing, hut that seems less than desirable as an overall solution. The most likel y source of the right advance curve would he to use a GDI box from an RZ250RR, a higher-output 250 continued from page 104 Yamaha recently released in some markets. We suspect this CDI has an ignition curve more suitable to higher engine rpm, and that it would interchange with the 350 CDI box. Are there any RZ racers out there who've tried this? If you 'll write us and tell of your experience, we'll report the result in a future Service column.

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More on the turnout blues

In the February, 1985, Service column, a letter appeared about problems a Honda Magna owner had encountered after installing a set of turnout-style aftermarket exhaust pipes on his bike. His Magna wouldn't rev past 6500 rpm with the replacement pipes, and we recommended a change to richer jets. We now think that may be the wrong advice.

Jerry Jardine, the owner of the company that makes Jardine exhausts, read our reply and called us to help clarify the matter. He said that many times during his own exhaust-system testing, he had encountered the same condition that the Magna owner had experienced, and that it wasn 't an ything a jetting change would cure. Jardine believes the problem to be caused by the fluttering of the slides in the Magna's CVcarbs, and a very rich condition at the engine speed where the flutter occurs. Jardine thinks that the smaller-than-stock volume of the turnout mufflers is at the heart of the problem, causing a strong resonant pulse to be reflected back up through the exhaust system and to interfere with the carburet ion.

If that is the case, there are a few possible cures, but onl y one realistic one. Either slide-throttle carburetors, or camshafts with less overlap—and, therefore, less interaction between exhaust effects—should eliminate the problem, the reasonable solution is to switch back to the stock exhaust system, or to a larger (and with any luck, more fully-tested) aftermarket exhaust.