| Feature | Specification |
|---|---|
| Production Years | 1973–1976 |
| Engine Type | Air-cooled, four-stroke boxer twin |
| Engine Oil | SAE 20W-50 Mineral (2.25 L) |
| Front Tire | 3.25 H 19 (1.9 bar / 27.5 psi) |
| Rear Tire | 4.00 H 18 (2.1 bar / 30.5 psi) |
| Battery | DIN 52816 (12V 28Ah) |
| Spark Plugs | Bosch W5DC / NGK BP7ES |
Historical Engineering Highlight: The BMW R 90 S holds the distinction of being the first production motorcycle in the world to feature a factory-fitted cockpit fairing. Beyond its aerodynamic prowess, it introduced the legendary “Smoke” paint finishes—Silver Smoke and later Daytona Orange—applied by hand, ensuring that no two R 90 S units were identical.
The Engineering Paradigm
Launched in 1973, the R 90 S was BMW’s answer to the Japanese “Superbike” invasion. While competitors focused on raw four-cylinder power, BMW refined the Boxer twin to 898cc, utilizing 38mm Dell’Orto pumper carburetors to deliver a sophisticated 67 hp. This setup provided a linear power delivery and a top speed exceeding 120 mph, making it a formidable endurance racer, famously winning the first-ever AMA Superbike Championship in 1976.
Maintenance Guide & Technical Specs
1. Lubrication System
The R 90 S relies on a high-quality mineral-based lubricant to protect its air-cooled internals. We recommend SAE 20W-50 Mineral oil. The total refill capacity is 2.25 liters. Given the lack of a modern radiator, regular oil changes every 3,000 miles are critical to prevent thermal degradation of the lubricant.
2. Tires and Handling
Maintaining the correct contact patch is vital for the R 90 S’s telelever-precursor handling characteristics. The front requires a 3.25 H 19 tire at 1.9 bar (27.5 psi), while the rear utilizes a 4.00 H 18 tire at 2.1 bar (30.5 psi). These pressures are optimized for solo riding; for touring with luggage, a slight increase in rear pressure is advised.
3. Ignition and Electrical
To ensure crisp combustion, the Boxer engine utilizes Bosch W5DC or NGK BP7ES spark plugs. The electrical demand of the 1970s charging system is supported by a substantial DIN 52816 (12V 28Ah) battery, which provides the necessary cranking amps for the high-compression pistons.
Editor’s Verdict
The BMW R 90 S is more than a vintage motorcycle; it is a mechanical masterpiece that defined the Sport-Touring category. Its blend of reliability, aesthetic audacity, and functional aerodynamics makes it a blue-chip investment for collectors and a joy for the technical enthusiast. At MotoSpecsLab, we rate the R 90 S a 4.9/5 for its historical significance and enduring build quality.
Frequently Asked Questions
Q: Can I use synthetic oil in my R 90 S?
A: While modern synthetics offer great protection, many specialists recommend sticking to high-quality mineral oils to ensure seal compatibility and proper clutch/transmission interface in these vintage blocks.
Q: What is the significance of the Dell’Orto carburetors?
A: Unlike the vacuum-type Bing carburetors found on other BMW models of the era, the Dell’Orto PHM 38 units feature accelerator pumps, providing immediate throttle response and a more aggressive power curve.