The Service Dept

THE SERVICE DEPT

JODY NICHOLAS

LEAVE IT ALONE

My problem is with a 1965 BMW R69S. It's been a wonderful machine and has put down the last 28,000 miles with never even a fit of hard starting. However, I recently had the misfortune to hit a Chevy with one of our blind motorists aboard her-no sweat, it was his fault; he pays-but it beat me and my black Frau in pretty bad, she requiring a new front end and I a few months' mending for my left leg.

While we're down for repairs I thought I would change the rear drive ratio to obtain a bit more pep. I recently switched to one of those new Con tinen tal racing-section tires, a 4.00-1 8. The larger section (from 3.50 to 4.00) had a gearing-up effect which destroyed acceleration and made her sluggish on top. I'd simply return to my old size except that the bike handles so much better with the Conti-more stable, more secure over tracks and in corners. So to keep the good handling I thought I'd change to a rear gearset used in the R5OS: 7/25, or 3.58:1. This would yield an overall ratio of 5.31:1. Is this too much of a drop? The present gearset, at 3.13:1, 8/25 (4.82.J overall) is most definitely too high. What do you advise? There is also an 8/26 available.

Reid Sprague Hialeah, Fla.

The increase in tire size from a 3.50-18 to .•a 4.00-18 shouldn't alter your final overall gear ratio by more than 0.20. In other words, your final ratio of 4.82:1 should now be no higher (lower numerically) than 4.62: 1 or so, which is roughly the equivalent of re ducing the rear sprocket size by be tween one and two teeth on a chain driven motorcycle.

Rear-end gearsets for BMW motor cycles are very expensive, are getting scarce for your model, and are extreme ly difficult to install properly to prevent gear whine and premature wear of the components. There are a number of very thin shims which must be correctly placed during assembly to prevent a malfunction, and it takes a highly skilled mechanic to do the job properly.

I would suggest that you spend your money on a ring and valve job to restore the machine's performance and forget changing the gearset unless you plan on installing a sidecar.

WEAK COIL

lam having a problem with my 1971 Yamaha 350-cc R5B and am writing you for advice. The bike's standard plug is Japan `s NGK B-8HC. I ran the B-8HCs at 0.022-in, gaps (according to the manual) for the first 1000 miles, re placing the plugs at 500 miles. After approximately 200-300 miles, the plugs would frequently quit firing and then foul, accompanied sometimes by back fire. By 500 miles the electrode would be extremely worn. At 1000 miles the dealer recommended I switch to NGK B-9HCs with the gap at 0.025 in. The same symptoms occurred, even after varying the gap. The plugs simply aren reliable over 200 miles.

I seldom lug the engine, never idle it, and clean the plugs frequently. The fuel system and carburetors have been checked, the timing is right and the Autolube is properly adjusted. Two dealers are as puzzled as I am. I inquired about switching to Champions or an other American brand, and the dealer recommended that if I did I should be very sure that the American plugs com pare with the NGK's operating tempera ture. There were several two-stroke Yamahas in their shops with holes in the piston crowns to attest to that.

• . Is there a comparable and possi bly more reliable American plug? Could carbon in the cylinder have anything to do with the problem? The bike has over 2000 miles now, and except for the plug problem, I am very pleased with the R5B.

Steve Duermeyer Newport, R.I.

Because of their high state of "tune," some two-stroke motorcycles are very hard on spark plugs. Many other twostrokers also consume spark plugs at a high rate, but the problem often lies with the failure of the high tension coil. When this happens, other signs appear such as hard starting (even with new spark plugs), misfiring and often pre mature erosion of the spark plug's elec trodes.

It's obvious that all spark plugs are not of the same quality, but the listed heat ranges seem to be pretty close. The NGK plug has an excellent reputation, and is indeed a good quality spark plug. However, Champion has recently intro duced a new series of spark plugs especially for the two-cycle engine which works in four-cycle engines just as well. This new series is called the Gold Palladium. In these spark plugs, the center electrode is much smaller in diameter than in a conventional plug, and is made of a semi-precious metal alloy which has several desirable quali ties.

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The design of this spark plug requires less ignition voltage to get clean run ning, and the tiny center electrode makes more space available in the firing area to better scavenge fuel deposits and reduce the possibility of spark plug fouling. But don't overlook the coil... it could be malfunctioning.

THERE'S A REASON

I am a new subscriber to CYCLE WORLD, and a newcomer to motor cycling. My current bike is an older model Honda 300.

My reason for writing is a question concerning the warnings for tampering with stock exhaust systems. While I may be green to motorcycles, I do have some experience with sports cars and hot rods. And back pressure does not yield horsepower, at least not in a four-cycle engine.

Charles Swann Glasgow, Ky.

The reasons for the warnings about tampering with a motorcycle's exhaust system are few, but are important.

First of all, a change in the exhaust system will often require a change in the size of the carburetor(s) jets, often to a larger size. If run at high speeds with too lean jets, an engine can overheat and be severely damaged. Burned valves, burning holes in the pistons and piston seizure are the worst things that can possibly happen.

Next, by changing the standard ex haust system by removing the baffles, for example, you will increase the sound level of your machine. While it may not be a large increase, you will become less popular with the public, which is be coming increasingly aware of the motor cycle.

Thirdly, your assumption that less back pressure means more power is simply not the rule. All internal com bustion engines currently being used in motorcycles are designed to operate best using a system incorporating regu lated back pressure. If this were not the case, and if noise were not a factor, the easiest and simplest method of discharg ing exhaust gases would be directly out the exhaust port or by using a very short exhaust stack.

It is true that altering the stock exhaust system for reduced back pres sure may yield increased horsepower at some point in the engine's rev range, but it's a pretty safe bet that the overall performance will suffer.