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Old 10-04-2012, 06:16 PM   #1
headtube OP
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Excessive vibration

This is my first ever BMW (79 R100 RS) which I purchased 3 weeks ago. I really don't know much about how it should feel. With that said, I took my new ride out for a spin. I noticed that at stop lights the bike shakes side to side (as in pistons wanting to come through the cylinder head... like out of balance), and when the throttle is blipped it vibrates at low RPM's and rattles the fairing something bad, even though every fastener is tight. However, cruising anywhere past 4000 rpm it's all good. It almost feels like it's missing an engine mount. It's not BTW. Any ideas what this might be?
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Old 10-04-2012, 06:23 PM   #2
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Other than carb balance, which is going to be the most oft repeated suggestion, there is a torque value for the nuts attaching the engine to the frame. There are two through studs on the bottom. They also carry things like the foot pegs and center stand and muffler clamps. Those nuts on the ends of the engine mounting studs are torqued to 55 ft/lbs.
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Old 10-04-2012, 06:47 PM   #3
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My vote is for a carb adjustment and cable synch. These bikes will idle fairly smooth but with some side to side rocking motion. It's normal to have some vibration at 3200 RPM and 4000 RPM. But other than that, it should be very smooth.
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Old 10-04-2012, 07:40 PM   #4
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Badly out of sync. I see it all the time. Set up right, they are not paint shakers.
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Old 10-05-2012, 03:23 AM   #5
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Would a bent rod cause this?
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Old 10-05-2012, 09:21 AM   #6
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Would a bent rod cause this?
Ouch! I believe there would be other indications, like odd noise/s. I'd better get those carbs synchronized.
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Old 10-05-2012, 10:08 AM   #7
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Headtube-not trying to induce panic; asking the collective.
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Old 10-05-2012, 10:43 AM   #8
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Connecting rods can happen from hydrolock of the piston because of a petcock not turned off. Doesn't really happen all the time but when it does I think we hear about it because the rider is so perplexed. We hear about one of these every year it seems.

There may be no symptoms of a bent connecting rod until the owner does a compression check and discovers low compression on one side. It really has to be checked for before the engine comes apart because you may not see it until you look for it. The check is to look for TDC at the same time on both sides.

Connecting rods can also be twisted.

I straightened mine by hand when assembling the engine.
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Old 10-05-2012, 06:03 PM   #9
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Connecting rods can also be twisted.

I straightened mine by hand when assembling the engine.
You did WHAT?
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Old 10-05-2012, 04:44 PM   #10
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The once hydrolocked motor would have some gas in the oil, no?
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Old 10-05-2012, 06:36 PM   #11
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I had several sets of connecting rods to chose from when reassembling my R90/6 engine several years ago. I used a pair of parallels resting on the flat surface of the block and the wrist pin in the small end, I rotated the crank with the alt bolt until the wrist pin could contact the parallels. I could see where the rods were a little bent because the parallels didn't both stay up when held by the wrist pin pressing against them. I placed a long rod in the small end and bent the rods till they were straight. I performed this little trick on both sides several times, until the parallels were only lightly contacted and I wasn't holding them up by excessive force but they were contacted by the wrist pins at the same time.

Been running engine now for 3 or 4 years maybe. It had a lot of vibration the first year but that has gotten better over time. It is pretty smooth these days.

I suppose I should have checked for an even TDC between the two sides. I never did but since then I have found the tools to do that. I have a degree wheel and a correctly fitting TDC bolt set up. Still engine is running pretty good so I probably won't do that right now.

If you really want to do a correct rod reconditioning then send them to a shop that does rod reconditioning. I don't think my little operation is as good as the real job but I don't think it hurt things any and may have made them better.
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Old 10-06-2012, 12:36 AM   #12
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Thank you Professor Disston. Maybe this explains the big difference I noticed in how difficult it was to remove and reinstall the wrist pins the three times I've done that on an airhead. Sometimes easy, sometimes harder n' hell. Sounds like the alignment on them things can be off by more than I thought.
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Old 10-06-2012, 01:10 PM   #13
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Thank you Professor Disston. Maybe this explains the big difference I noticed in how difficult it was to remove and reinstall the wrist pins the three times I've done that on an airhead. Sometimes easy, sometimes harder n' hell. Sounds like the alignment on them things can be off by more than I thought.
That phenomenon is mostly not about the twist in the rods. It's varying interference fit in the pistons and varying pin/bushing fit and bushing bevel. I have sized enough wrist pin bushings on a Sunnen rod honing machine to know for sure that if you get that down to .0003 or .0004" like it is suppose to be sometimes you would swear the pin is never going to go in with the rod and pin right there in your hands! Then you try and try again and it falls right in with plenty of room to slide back and forth.

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Old 10-06-2012, 10:03 PM   #14
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You will definitely feel that side to side vibe at idle and up to about 2000RPM. Beyond that, you probably need a carb balance.

Those twin disks should work very well. If you have the original rubber brake hoses, replace them with some quality braided stainless. Speigler makes a pretty nice kit, that includes the line for the rear disk as well. Change out the original fluid and reverse bleed the system. You will find a HUGE difference. Won't be as good as some of the new bikes, but will definitely improve over the stock OEM setup.
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Old 10-06-2012, 11:01 PM   #15
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The Airheads do vibrate more than I think they should. But they all do it I think. Since I have only ever been on any Airhead that was mine and that I tuned, except once, I've always been paranoid that this is a personal problem. Well, I decided a long time ago I wouldn't let that stop me and so i ride the bike I built and tuned.

Mine vibrates mostly between 3000 and 4000 rpm. And idle is not always very smooth, but sometimes it's better than others. If I get tired of the racket the trans makes in neutral at a light I will pull the clutch in. But mostly I just let it rattle away.

You should of heard them when we had the Turkey valves. On top of the rattle at idle there was a honking noise from the engine breather.

I think the /2s were truly smooth.

Check the eccentric adjustment of the caliper mounting. The pins also need to be cleaned and greased. The pins are extracted from under the cap on the bottom of the fork tube. The eccentric is held in place by a stout spring. To get it out after removing the cap and the spring a small bolt (8mm maybe) is threaded into the end, it's drilled and threaded for this, and the eccentric is pulled out. If it's been a long time since these were pulled the grease can be very dry and the part hard to remove. Use ordinary wheel bearing grease.

The adjustment is explained better in the manual than I think I can tell it but I don't do it the way the manual says to anyway. The manual has something about an involved process of marking the rotor and adjusting the eccentric so the brake pads wipe the markings off. All really nice I guess. I just watch how the parts are moving and set the caliper to it's closest position. My method may not be correct.
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