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headtube 10-04-2012 06:16 PM

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?

disston 10-04-2012 06:23 PM

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.

Wirespokes 10-04-2012 06:47 PM

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.

supershaft 10-04-2012 07:40 PM

Badly out of sync. I see it all the time. Set up right, they are not paint shakers. :D

ozmoses 10-05-2012 03:23 AM

Would a bent rod cause this?

headtube 10-05-2012 09:21 AM


Originally Posted by ozmoses (Post 19752043)
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.

ozmoses 10-05-2012 10:08 AM

Headtube-not trying to induce panic; asking the collective.

disston 10-05-2012 10:43 AM

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.

ozmoses 10-05-2012 04:44 PM

The once hydrolocked motor would have some gas in the oil, no?

Horsehockey 10-05-2012 06:03 PM


Originally Posted by disston (Post 19754695)
Connecting rods can also be twisted.

I straightened mine by hand when assembling the engine.

You did WHAT? :photog

disston 10-05-2012 06:36 PM

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.

Horsehockey 10-06-2012 12:36 AM

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. :bow

disston 10-06-2012 12:52 AM

I got the idea from the BMW shop manual where they show rods being straightened on a special mandrill. I think I am doing the same thing with the crank being one part of the mandrill tool and the parallel sides of the engine block being another.

For the bending or straightening operation I remember now that I had a long thin steel rod that fit inside the wrist pin. I said in the last post I stuck a tommy bar in the small end. Well sort of, but in the hollow center of the wrist pin so I didn't do any damage to the bushing.

The rods bend easily and hold a bend nicely. Bend, test, bend, test. Done.

Parallels are easy to find on Ebay. I have a little bit of an assortment. They are just rectangular pieces of steel with flat and parallel sides. The ones that are calibrated and marked are parallels as opposed to the plain pieces of bar stock that's not the real tool.

DaveBall 10-06-2012 06:32 AM

Headtube, As this is your first Airhead, what other bikes have you ridden? If you are used to something like a modern, silky smooth inline 4 or such, you will find that in comparison the BMW will have a lot more vibration at idle. A lot can be attributed to the very heavy flywheel that is stock in your model year, plus the fact that you have 2 good size jam cans bashing back and forth at 180 degrees. It is perfectly normal to have some rocking side to side at idle. They will smooth right out as the revs climb. Don't ever lug it around under 3000RPM. They really don't like that, and it can cause expensive repairs.

What I am saying here is that if you are not used to it, you may find it to be a little disconcerting. But, what you are experiencing just might be perfectly normal and those of us who have ridden these beasties for many years just don't notice it anymore. If I were you, I would find some local Airhead riders and talk to them. Compare their bikes to yours and see if the vibes are similar. It is impossible to diagnose a vibration over the internet.

Oh, and I also would suggest learning how to balance the carbs. You can get pretty close by ear, to the point where it will run much better and smoother. Check around your area for someone that knows airheads or see if a local club puts on Tech Days. Lots do and you can learn all kinds of stuff real quick. But once you attend one, or join a local Airhead group, you will become addicted to airheads and before you know it, you will have a garage full of them.

Uncle Pollo 10-06-2012 10:12 AM

I vote for someone to look at it.

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