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Bike Tale - Posted 21sh August 2015
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BMW R80/7 - Part 16: In Which Nothing Happens

There are now 110,000 miles on the odometer of Martin Gelder's Franken-BMW R80/R100 hybrid, and probably quite a few more under its wheels. Nothing's really gone wrong, but that doesn't stop him looking for things to fix...

It's been a little over two years since the last update on my high-milage R80 / R100 hybrid airhead BMW. Why so long? Well, nothing much has happened.

That's not to say that the bike hasn't been ridden; as well as winter run-around duties it's been to numerous local and not so local bike shows and classic events, it's had a dirty weekend in Brighton, a couple of runs to the wonderful if wet Wirral, it's been photographed next to many bridges, it's done just-before-closing-time dashes to the library, post office and bank (got to get your priorities right) and it's been ridden purely for the enjoyment of riding.

It's been serviced a couple of times, but I can't remember it needing more than a slight tweak of the tappets and juggle of the carbs (airhead BMWs always need their carbs juggling...). I've even cleaned it once or twice.

BMW R80/7

It just... works.

It's probably back at the point that my previous airhead BMWs were when I was using them regularly as my main transport; other than routine maintenance, I just got on with riding them. After the Big Trip to Spain (RealClassic magazine issue 110 and a follow up here) the bike ended up with refreshed carbs and a revitalised ignition system, and those two tweaks seem to have made all the difference to day to day running. It's not that it was unreliable or unpredictable before, it's just that everything seems to stay in tune longer now.

Not that the bike isn't without faults, mind. It's 38 or so years old and has done more than 110,000 miles. The handling is a bit... loose. It's very nimble and quick turning, and it's stable at speed, but it doesn't feel as planted on the road as a good one should. Cleaning the wheel bearings helped a bit, as did a new rear tyre. The front tyre is due for renewal before the next MOT and I'll change the fork oil at the same time and check the steering head bearings. All those little incremental things add up.

BMW R80/7
BMW R80s on Now...

Another noticeable issue is the amount of slack you feel in the transmission when going from a closed throttle to an open one, and vice-versa. With a clumsy throttle hand it's very noticeable, with care and forethought it's not so bad. Again, I think it's probably a lot of incremental wear rather than one component with a significant problem.

The time to act on this may have come, though. Returning from the Brooklands Ton Up day I noticed that the trip meter wasn't recording as many miles as I was covering. Every now and then it would freeze and miss a couple of miles. Not the sort of thing you'd notice riding locally, but more obvious on a 200 mile round trip.. It's apparently a known problem, and perhaps it explains why my fuel economy has gradually been worsening; if the trip meter isn't counting as many miles as I'm covering, then obviously I'll be using more fuel than I think I should be.

BMW R80/7

I could dismantle and maybe even fix the wayward trip meter, and possible have a go at recalibrating the speedometer at the same time; it reads at least 10% above actual road speed when compared with a GPS speedometer, sometimes more. Why take the simple and cheap route, however, when a more complicated option is available.

As my regular reader will remember, my R80/7 has been fitted with R100 pistons and barrels, taking it out to 980cc and giving a significant and noticeable boost to mid range throttle response. The only downside to this simple conversion has been a slight increase in engine vibration at around an indicated 70-75 mph. It's more of a tingle than an actual vibration, and it's diminished as the miles have piled on, but it's there and I can feel it. Below 70 mph, fine. Above 75 mph, better, and at an indicated 85 or so everything smoothes out.

I'm still using the standard R80/7 32/10 final drive ratio but I could swap the bevel box for a reconditioned 32/11 unit from an R100S or R100RS. This would drop the engine revs at 70-75mph by about 500 rpm (or raise the road speed at the buzzy rev-range by about 7mph) making for more relaxed and smoother motorway and dual carriageway cruising and, as a by-product making first gear more useful. With the increased grunt from the bigger bore engine, first gear is now almost redundant; there's certainly enough power now to run higher overall gearing.

BMW R80/7

Changing the final gearing would, of course, mean replacing the speedometer as these are driven off the gearbox and so are themselves geared to suit the final drive. Reconditioned units are readily available.

Fitting a reconditioned bevel box would also hopefully remove some of the slack between throttle and rear wheel. There's a shock absorber in the (later model) shaft that I fitted and that might be adding its own bit of play to the whole assembly; it had done more than 80,000 miles before I fitted it. The gearbox itself is good, but if a reconditioned bevel box and shaft don't tighten things up then a rebuilt gearbox might be next. The modular nature of the airhead boxers means that parts availability is really good and there are enough specialists around to make fixes and swaps a breeze.

Other future projects include sticking the bike back on the dyno to see how much more (if any!) power it's now feeding to the back wheel, and possibly having the cylinder heads fitted with unleaded-proof exhaust valve seats.

But mostly, I'll just be doing some more riding. That, after all, is what airheads do best.


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