Race to Rebuild: The BMW R90/6 Part 2
After deconstructive surgery, the transformation of our 1974 BMW R90/6 starts.
The 1974 BMW R90/6 as purchased.
Photo By MC Staff
Deconstructive surgery is fun. Compared to the exacting process of carefully assembling bits and pieces into a functioning whole, there’s huge and immediate satisfaction in taking something whole and, scant hours later, ending up with a pile of parts scattered around the shop. Ahh, progress!
It’s a good thing we think that way, because deconstruction definitely defines the current phase in the Motorcycle Classics/Dairyland Cycle Race to Rebuild.
Chalk it up to an insanely busy summer, but we’re running a good month or more behind schedule. If things were going right, by this time we’d be showing you sketches and already acquired pieces to define the projected end product in our build.
That’s not to suggest we haven’t made any headway. In a break from previous builds where we’ve just jumped in whole hog, discovering what surprises awaited us as they presented themselves (the holed piston and trashed forks on our CB500 project come to mind) this time we’re heading into our teardown phase with a little more care. We really do learn from our mistakes. Maybe.
For example, we know we’re going to give the engine on our 1974 BMW R90/6 some mild massaging. Nothing radical, mind you, just a few fun and sensible upgrades to give the bike a bit more personality and a leaner look with the benefit of enhanced performance. When it comes to the engine, we’re looking at items such as upgraded carburetors and filters, a custom exhaust and electronic ignition.
On the bench
Since we want to inject a little extra oomph into our Beemer, we decided it’d be fun to put the BMW R90/6 on a dyno so we can document how our project bike ran when we got it against how it runs when we’re done. Fortunately, local dyno expert Kent Clawson likes us, and agreed to take our bike through a session on the chassis dyno.
For our dyno run we tested the bike exactly as received, and the results showed our old Beemer to be pretty healthy. When new, BMW claimed as much as 67.5 horsepower at the crank at 6,700rpm (some reports claimed 60 horsepower at 6,500rpm). That’s taking the measurement at the crank, but we did our testing on a chassis dyno, which measures power at the rear wheel. Factoring in power losses from the transmission, driveshaft and general variations from one engine to another, dyno man Clawson said to expect up to a 20 percent reduction for rear wheel horsepower, with a perfectly tuned bike. In dire need of a tune-up, running rich and on unknown octane fuel likely months old, our Beemer put out 44.54 horses at 6,500rpm and 41.51ft/lb of torque at 4,200rpm, roughly 34 percent down on horsepower and 20 percent down on torque from factory claims. When we’re done with our planned mods, we’re hoping to get within spitting distance of the factory claim, at the rear wheel. Cross your fingers.
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