1991 R100RT Engine vibration

[1505]

I have been doing my own maintenance for the last 35 years and over 300K miles. The 66 R69S, 74 R90/6, 78 R100/7 all ran very smooth. The recently purchased 91 R100RT with 27K on it has engine vibration. It vibrates so much I have trouble telling between cars and SUVs in the mirrors. Vibration goes away when the clutch is pulled in and engine goes to idol. The timing looks good, carbs are ballanced using a carb stick and compression is the same on both cylinders. Looking through the timing hole the flywheel looks like it is a lot less massive than on previous bikes. This bike is a lot more responsive and faster than the R100/7. Whats causing the vibration?

[magoo]

Your obsevations on the fly wheel are correct. In the eary 80's the airheads recieved a lighter flywheel. This gives a better throttle response, at the cost of more vibration. This was done to add more "pep" with the motors being restricted by EPA regs.

You said that you synced the carbs, did you do it at idle or in your cruising rpm range?

Doug Mc Gee

[flash412]

First, are your engine mounting bolts TIGHT?

Next, you don't need carb stix to sync the carbs RIGHT.

If you don't have a throttle lock, have a friend help you by holding the throttle ROCK STEADY at whatever RPM you cruise. Take the cable for each carb, one in each hand, a couple inches above the carb. Hold the cable between your thumb and the first two fingers, with your thumb in the middle. Now press your thumb between the two fingers to bend the cable some (not enough to hurt it). Do it on one side and then the other. If one side gains RPM when you do it and the other doesn't... your carbs are out of sync at the RPM where you need them to be sync'ed the most.

Credit for the development of this tuning method goes to retired BMW airhead rider, Edward Foote, tuner extrordinaire, of Precision Piano Works, Nashville, Tennessee.

[pmdave]

There will be somewhat more vibration with the later (light) flywheels, but it shouldn't be much more than your other R100. The advantage is quicker shifting and quicker deceleration when you roll off the throttle.

I'm thinking the clutch is out of balance. The flywheel and pressure plate are supposed to be balanced together, and assembled in that relative position. It's possible that the components failed to be balanced, or that the assembler dropped something and lost the balance positions, or that someone did a clutch job before you got the bike and didn't mark the components before disassembling.

Squeezing the clutch and letting the engine drop to idle slows down the flywheel and clutch components, which would explain why the vibration goes away as rotating inertia decays. In other words, it's in the engine or clutch, not the transmission, wheels, or final drive.

If you suspect the flywheel/clutch assembly is not in balance, it will need to be removed and taken to a machine shop for balancing. If possible, each component should be balanced separately. And, once you get things balanced, it's a good idea to mark them anyway. (say, a couple of punch marks at the same location on each piece.)

pmdave

[pmdave]

Idea other:

It is also possible the rear thrust bearing slipped off the pin during assembly, and is dragging on the crank. Try turning the engine over with the plugs removed and transmission in neutral. It should turn evenly with no drag at the same point of rotation. (except for the cams operating, of course)

If the crank seems to drag in the bearings, it would be worthwhile to check crank end play.

pmdave

[bprigge]

I rode a 'for sale' '82 R100 last year for about 20 minutes. It shook a lot. The owner said she didn't see it as a problem. I bought an R75/6 instead, no vibes above 3k RPM. :) Bruce

[pmdave]

Of course it wasn't a problem. She was selling it.

pmdave :thumb

[nrpetersen]

Maybe something has happened to the spring in the driveline damper (I assume R1100 has one like the older R bikes).

Find another similar bike and put them both in 1st gear with the engine off on the center stand with the back wheel held off the ground. You should be able to feel the driveline spring compliance as you push and pull on the rear wheel tire patch. Do the spring rates seem similar? If yours feels much harder, I'll bet that's your problem. Those springs are subject to a lot of fatigue - especially at lower engine RPMs and at higher driveshaft operating torques.

The clue might be if you are only getting vibration when the engine is driving or retarding the bike.

[kbasa]

Thanks for the tip, Flash. That's a neat idea.

[flash412]

Thanks for the tip, Flash. That's a neat idea.The guy Edward showed it to, with a set of vacuum gauges and a bunch of switches and tubes and stuff, didn't think it was so neat, especially when Edward's method worked better, no matter what the gauges displayed.

"In theory there is no difference between theory and practice. In practice there is." - Flash Gordon, M.D.

[kbasa]

The guy Edward showed it to, with a set of vacuum gauges and a bunch of switches and tubes and stuff, didn't think it was so neat, especially when Edward's method worked better, no matter what the gauges displayed.

"In theory there is no difference between theory and practice. In practice there is." - Flash Gordon, M.D.

:ha

Flash is me and MrsK's doctor.

[RandyB]

I'm thinking the clutch is out of balance. The flywheel and pressure plate are supposed to be balanced together, and assembled in that relative position. It's possible that the components failed to be balanced, or that the assembler dropped something and lost the balance positions, or that someone did a clutch job before you got the bike and didn't mark the components before disassembling.
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Is it possible for it to come out of balance in the bike? My R100RT just started vibrating not long ago. Haven't checked engine mounts yet.

I also did a cable check but it only worsened the vibration. Same for both carbs.