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oilhead rough running


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I've only done about 2000klm on my "new" (to me anyway) '96 1100RT (95k klm old), and am curious about how smooth the bike should be at about 4000RPM and above? I have had the throttles balanced, and the bike is very smooth up to about 3700RPM, them a harsh rumble comes in through the footpegs and seat. It makes no difference with the throttle open or closed, and does go smooth with the clutch in and back at idle at the same road speed(about 120KLM/HR)-doesn't seem to be road speed, just engine speed related.

I am a little perplexed as I have heard so much about how smooth these beasts are. Hopefully, this is not a sigh of major grief..... confused.gif

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Not really enough information to diagnose. If it goes away at around 4500 or so, I would say it is normal. Just about all BMW's have a little rumble to them in the RPM range you describe (3800-4300). Has to do with a secondary imbalance in the motor due to the offset crank pins. It is there on the older bikes (airheads) as well, but on those, a far heavier flywheel helps to mask it.


Simple solution, if your riding frequently finds you at that RPM, is to downshift one gear and just ride at a higher RPM. It won't hurt the motor nor will it materially affect your fuel consumption. I tend to think of 4000 RPM as a minimum.

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The rpm range mentioned does have a bit more vibration but to describe it as a "harsh rumble" makes me think the throttle body sync should be checked again. Also check the cable condition, make sure the throttle pullys are free of debris, no air leaks at the seals etc.

I got far better results with TB syncs once I started doing my own. I've been known to strap on the twinmax and take a ride to check the sync at speed and under a normal load.

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Sounds like it may well need a rebalancing!

When I did mine, I used a Morgan Carbtune and balanced it running the bike through 5k rpm's using my Throttlemeister to lock in and check the balance at each 1k interval. My bike is now way smoother between 3500-5k (when it used to be 'rumbly' about 3500 and then 5k and smooth at 4k) and now only gets a little rumbly above 6k.


I believe most folks on this board will tell you that spending a few minutes checking and balancing your TB's THOROUGHLY reaps great rewards thumbsup.gif.


BTW - Overfilling your crankcase can also induce 'the rumblies' as well eek.gif

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