Tuesday, September 9, 2008, 09:51 AM ( 4926 views ) - BMW Cafe Racer - Posted by Administrator
"The Isle of Vashon TT is this Sunday," Uncle Phred remarked casually to me Friday night. "If you have something running, bring it down and we'll ride together."

It doesn't take more than that to motivate a last burst of effort to get a motorcycle running. The r60/5 cafe racer was coming along, sure, but not so well. I'd ordered a new range of jets to help with the carb adjustments, and was busy with real life leaving time open to do the work. By my calculations if everything went off well, I could just get it done in time. If not, I was prepared, but I wasn't beaten yet.

When I got to jetting, that went fairly well. It didn't look good to friends and neighbors who dropped by to watch me fiddling with carbs and little brass parts. It seemed like I was doing the same thing over and over, and so I was. They were all puzzled- I thought BMW's ran well? And so they do, especially when you keep them in original stock condition, as I hadn't.

In this case I was essentially adapting an unknown carb to an unknown engine, starting somewhere at random. As it turned out, it was a freak coincidence that it had started so well from the first. I realized this when i noticed I'd installed the carb slides backwards, by switching the right and left cables when I'd routed them. This wasn't really right at all and when i corrected it the jetting was too far off to even run. I had gotten it to idle nicely (the first step, you work upwards) but it wouldn't run at all without the choke on.

If it seems ambitious that I'd expect to jet a carb in a few hours before a ride when it hadn't really run at all yet, well, maybe. But I had taken one precaution this time- in addition to the Twinmax carb sync meter (which is really two rubber hoses that connect to vacuum lines with a gauge to show the balance between the carbs) that makes it so much easier to balance the carbs, in addition to that, I'd ordered a K&N air/fuel meter.

A few nights before, I'd drilled a 3/4" hole in my exhaust collector pipe, yes right through the lovely black ceramic coating of the Lufty 2-into-1 exhaust. Then I'd welded in a threaded fitting, screwed in the oxygen sensor, and mounted the exhaust back on the bike. When it was all done I could remove the sensor and screw in a cap, and get another fitting to weld into another pipe.

That made carb jetting amazingly easy. After revving the motor for 45 seconds or so (hey, I needed to break it in anyway) it would get warm enough and the meter would show a reading. It was often surprising, too. Like, you see belching black smoke and you think it's rich, but I saw it in odd cases when it was really running too lean, too- I think it would 'lope' with richness and blow it out the tailpipe. Well, I guarantee that's sent me off adjusting it the wrong direction in the past.

The sun was down and it was becoming saturday night when I finally finished. I had a good strong idle that was just a little rich, and I had a slightly rich power band and then it would lean out a little bit on cruise, just like it ought to be. Except of course that I hadn't actually ridden it more than a block or so. As it had gotten dark I moved up the street to an empty parking lot away from my neighbors so I wouldn't have to deal with anyone being angry and my incessant revving.

Finally it seemed to be running well enough that I could probably take it out and get it warmed up really and have the next go-around. I wheeled it slowly back toward the garage.

But I wasn't happy, and I wasn't headed out for a test ride. Bad symptoms that had started appearaing the night before were now erupting into full-scale problem. The bike was makign a bad grinding noise, in neutral or in gear. It seemed to be coming from the back of the transmission, and confirmed my worst fears about the output shaft bearing on this junkyard 5-speed.

It was 10:30 at night, and I weighed my options. I could just call it quits and ride the RS the next day, although it wasn't nearly so much a fun old project. But I wasn't beaten yet. I could swap the 5-speed out and put the old 4-speed back in. That would work.

In brief, the next hour or two was spent tearing the rear end apart, unbolting the fneder and battery box and disconnecting hte u-joint and removing the pivot pins and detaching the rear end from the swingarm. In short, taking everything apart so all the puzzle pieces are seperate. OK? REady, go!

And the weird thing was now I really couldn't detect where the horrible grinding noise I'd heard was coming from. I had fully expected the gearbox output shaft to flop around loose in my hand and ooze silver slime on me in troublesome ways, but it didn't. Seemed all pretty normal in fact. Ditto the rear end, and the driveshaft. Through process of elimination I concluded it was a broken U-joint, but you know, it was fine.

(It did help that I'd kinda been through here a couple of times, so I had an idea of what was 'OK'. Like, there's a frightening amount of play in an airhead gearbox. But that's 'OK'.)

So, I aligned everything perfectly and bolted it up and said 'Before, it made grinding noises when I pushed it in neutral. If it doesn't make that noise, maybe it's fixed.' Of course it made the horrible grinding noise again.

Ack. So this time I lie on the bike, rolling slowly back and forth, pressing my hands over the parts, trying to sense where the noise is coming from. It had really felt before like it was coming from the very back of the gearbox... but maybe not...?

Finally I concluded that it sounded like it was coming from inside the swingarm tube. The driveshaft was grinding around in there. But why? I pulled back the rubber boot and noticed that there was a half inch clearance between u-joint and tube on one side, and nearly none on the other. Hm, that's not right. Maybe the swingarm's mis-adjusted? No, and you can't adjsut it far enough to correct this.

OK, so then the gearbox is mis-aligned in the frame. Why? Well... ok, there's a spacer on one side of the engine stud, but not the other. Hmm... I seem to recall that's correct... unless I got it on the wrong side! Duh! So I pulled the stud and removed the spacer and put it on the other side.

Well that was ever stranger, becaue now it was the same problem in the opposite direction- the u-joint would grind against the outside side of the swingarm tube. Weirder and weirder.

So I stopped and followed the adivce of the great Bob Berg, who said 'look at it for a half hour before you call me'. So I looked and thought about it. I felt under the R100RS, sure enough there's a spacer on the right-hand side. Yes, the spacer was supposed to be on the right side all along.

But... the spacer on the RS seemed to be thinner than the one I had. Sure enough, I looked through my parts box and found two more spacers that were half as wide as the one I was trying to use. I installed the correct spacer (about 5mm wide instead of 10mm) and sure enough, the problem went away COMPLETELY.

In this state, I put it to bed for the night and got a few hours of sleep. Would it make it? I would find out. I was planning to raise the needle clip one notch, then go, and I did. And indeed that morning it started and ran and I rode it, fully a couple of miles before the first set of plugs started fouling. I stopped at a local farmer's produce stand, still closed up in the misty dawn light, and raised the clip one more notch.

This time it ran with gusto, and I was away. It made the thiry-mile ride to Uncle Phred's house without incident, and started flawlessly on the way out, though I changed the plugs several times during the day. That night I arrived home safe and sound, sore as hell. And the next day when I fired it up it made all the aches and pains hurt just right.

It has work to do yet, I'm not done refining the jetting for midrange. And then there's the top end main jets that I haven't even touched yet, that will unleash the power.

Meanwhile, a couple hundred miles of riding later, it seems that the cylinders and rings are getting to know each other, and I'm getting power I wasn't before. Now, it's nice and strong at 3-4k rpm, and quite muscular at 4-5k. But between 6 and 7k you really can feel the power of the racing cam getting into teh power band. It revs handily up to 8k rpm, and no doubt more than that. I'm treating it as break-in, and treating it both rough and gently as a result.

This is good news, a new chapter in the r60/5 cafe racer!

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