Roll-on / roll-off bucking or lurching

[Steve]

My 1979 R65 now has 175,000 miles on it. I do my own maintenance, so I am thrilled to to discover this club dedicated to just that model! Perhaps you can help me with an annoyance.

I’m quite sure my bike did not originally do this, and that it developed slowly over time. Say I am coasting in gear with the throttle closed. It can be in any gear at any RPM but it is most annoying at low speed. When I crack the throttle open any amount the bike lunges forward. When I close it again it decelerates in reciprocal manner, with a jolting suddenness. Thus I call it the “roll-on / roll-off bucking or lurching”. It happens 100% of the time. There is no clunk or noise. It is severe enough that when rounding a corner at low speed it disturbs my balance.

I have twice completely rebuilt the carburetors and replaced the throttle cables. This did not help. The idle circuit is clear of obstruction, so it’s not that. Someone said it is “drivetrain lash.” But when I put it on the centerstand, in gear, and rotate the rear wheel it only turns two inches measured at the tread. Internet postings, for various makes of motorcycle, indicate that this isn't excessive. What say you?

[Bob_Roller]

Hi Steve !!
Welcome to the site .
You have  an unusual issue, never seen it here before .

I did a search online there's a few areas you can check . 


https://thingsmoto.com/motorcycles/jerk/#:~:text=A%20motorcycle%20can%20jerk%20under%20many%20circumstances%20such,engine%2C%20clutch%2C%20transmission%20and%20final%20drive%2C%20and%20electrical.

Here's one thing that may be unique to BMW's there is a slip joint in the driveshaft, kind of a spring loaded ball and socket coupling, parts #4 nd 5 .

https://www.realoem.com/bmw/enUS/showparts?id=0363-USA-03-1979-248-BMW-R65&diagId=23_0257

[Barry]

Someone said it is “drivetrain lash.” But when I put it on the centerstand, in gear, and rotate the rear wheel it only turns two inches measured at the tread. Internet postings, for various makes of motorcycle, indicate that this isn't excessive. What say you?

Some backlash is normal.  What gear were you in ?    It occurred to me that measuring in 5th gear would be more realistic.

I've just measured mine at 27,500 miles and I get 1" in 1st gear and 2" in 5th gear.

[georgesgiralt]

Hello,
If you bike is fitted with, check the spring coupling in the driveshaft. The spring get weaker and the bell housing has burrs the nut has to bypass with either a noise or a strange effect in the drive.
You can't check it without the engine power applied because the spring is very strong. Only way is to dismantle and check the components.
At your mileage, I suggest buying the new parts and replacing them.
Beware, the spring is very tough and you need a proper compression jig to remove the retaining clip. Use a pipe wrench to present the new spring when refitting and a screwdriver for the final push in place. IT is a funny thing, so not too much coffee before trying it....
Just my 2˘

P.S.: while you are at it, check the joint for play and wear. IT would be funny to reassemble everything and have to dismantle again in a few weeks time ....

[Steve]

For the 2 inches of drivetrain slop it was in first gear. In 5th gear it's 4 inches. A lot, huh? And thanks to the suggestion as to which drive shaft parts may be responsible.