Service

SERVICE

Len Vucci

We welcome your technical questions and comments, and will publish those we think are of interest to our readers. Because of the volume of mail received, we cannot return any personal replies. Please limit your "Service" letters to technical subjects only, and keep them as brief as possible. Send them to: "Service," CYCLE WORLD, 1499 Monrovia Ave., Newport Beach, Calif. 92663.

BMW FUEL ECONOMY

Recently I purchased a BMW R100/7 and, mindful of some of the low-octane gas that I had to put into my R90S, had low-compression base gaskets installed. By my calculations, this changed the compression ratio from 9.1 to 8.3. As yet the rest of the engine is standard.

Do you have any suggestions as to changes in spark plugs, main jets, ignition timing, etc., or should I leave these as is? The bike runs fine, but fuel consumption (at 37 miles per U.S. gallon) is greater than I'd hoped for.

Hugh Turnbull Coquitlam, B.C. Canada

The installation of a base gasket is, according to Butler & Smith, BM W's West Coast distributor, an acceptable method to lower static compression and allows the use of lower octane gas. Because several different gasket and cylinder configurations have been produced, however, use of the wrong gasket could result in excessively low

compression and reduced performance and/ or fuel economy, as you've described.

Preand early-1976 R90 engines utilized a 0.5mm gasket between cylinder and crankcase. In preparation for the forthcoming R100 displacement increase, later R90 engines had cylinders with thicker spigots (the portion of the cylinder which slips into the crankcase) and correspondingly larger bores in the crankcase. The cylinder is also longer than early R90 units, and does not use a base gasket. This longer cylinder is also used in most current R100 models, but has a larger bore to accommodate the RIOO's bigger pistons. Butler & Smith also related that some new models, destined for certain overseas areas, will have an even longer cylinder than the current R100. Lowered compression for using poorer quality fuels is the chief reason.

Following BM W's suit, you can lower the compression on most late Beemers by using the appropriate gasket set.

Preand early-1976: Replace the stock 0.5mm gasket with a special 1.2mm thick low-octane gasket, part #11 11 1 257 092.

Late 1976 R90 and R100: Use the 0.5mm thick R90 gasket, part #11 11 1 255 001. The gasket bore must first be enlarged with a rotary grinder or file.

Either of these modifications will lower compression by roughly Vi point, and should be all that's necessary to utilize contemporary fuels. But if the 1.2mm thick gasket is installed in a late R90 or R100 engine, the loss in compression is enough to affect power output across the rpm range. More throttle would be necessary at any given power level, which would result in poorer fuel economy. Restoration of fuel economy to proper levels—40-45 mpg, typically—is not possible by external means, if you've installed the wrong base gaskets. The problem, not just symptoms, must be treated, and that means installing the correct base gaskets.

If the poor gas mileage is caused by other than the base gasket modification, normal maintenance measures should be taken. Use the factory recommended plugs, jets, and timing, and ensure the air cleaner is serviced properly. The jet needles may be lowered one notch from the normal center (#3) position to lean the midrange, but no other modifications are recommended.

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If low fuel mileage persists, have a competent mechanic thoroughly check the engine andfuel system for possible mechanical deficiencies.

SR500 TIPS

I own a 1977 Yamaha SR500E which is operated at sea level. I was immediately disappointed (as mentioned in your January test) in the engine's inability to pull high gear at less than 3000 rpm without severe surging and misfiring. I was able to clear up this problem entirely, at a cost of about $2.

I replaced the stock #25 pilot jet with a #55, and the #300 main jet with a #290. The idle mixture screw was set at lA turn. The engine will now pull down to 2000 rpm without protest; gas mileage has dropped about 2 mpg (to 60) in suburban commuting. Starting continues to require only one kick using the "two-stroke twist" when warm, and the idle is steady.

Can you tell me why Yamaha warns against using tubeless tires on their cast aluminum wheels? Also, what can I do to decrease the drag of the brake pucks on the rear disc? I've heard this is a common Yamaha problem.

Richard Harmon So. San Francisco, Calif.

Thanks for relating your thumper therapy. We checked with Yamaha's technical department about dragging pucks, and we're told that the problem was non-existent, to the best of their knowledge. We've had two SRs in for testing, and know several riders who own SR500s; none exhibited the tendency toward rear brake drag.

The problem seems to be peculiar to your bike, and is probably caused by some foreign matter in the caliper cylinders. A little bit of crud between the piston and wall can prevent the puck from retracting as it should, and cause it to continually drag on the disc.

The fix is simple. Disassemble the rear caliper unit and clean it thoroughly (see the brake-fix article in the January, 1978 issue of CYCLE WORLD). Any irregularities on the surfaces of the piston or cylinder walls should be smoothed with #400 abrasive paper. Clean the parts once more, and reassemble the unit after coating the components with clean, fresh brake fluid. Install the caliper on the bike, bleed the system, and the operation will be drag-free. SI