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.

[Steve]

Could it be a sticky ignition advance? If so, what to do? R65s have a "spark trigger." The Repair Manual says, "Never disassemble the spark trigger, always replace entire unit." It is part number 12 11 1 244 088, now superseded by 12112413906, which costs $620. However, MAXBMW also sells a "repair kit for ignition sensor" for $330. How can you repair something that isn't supposed to be disassembled?

[dogshome]

Hiya, if you have an old skool timing light. You can easily check if the advance is working properly. Some jitter is normal, but rapid changes or the mark moving about more than jitter will tell you.

The driveshaft spring winding up makes a noise on my bike. Having made an electronic carb balancer and fitted Hektik ignition, I've never had a click or clunk again.

Of course you know the r65 idle screws should only be 1/4 turn out, the balance is set on the other screws at idle and the cables just off idle, and how to set your static timing.

[dogshome]

Ps you can easily rebuild the spark trigger (or electronic bean can). Get a simple timing light. It should flash on the static mark at idle, gradually moving towards ful advance over 2500 rpm ish. If it leaps up or bounces around, you should rebuild your bean can.

[Tony Smith]

I very much doubt your problem is electronic in nature.

As others have suggest your  bike's behaviour might be due to a failed shock absorbing device in either the drive shaft or the gear box. But I doubt it as they do not tw d to fail in that way.

If I were to take a guess I would start by looking at the output splines in the fi al drive and in the rear wheel.

[Steve]

I'm up to five good ideas for what could be wrong with my bike, and they are all are beyond me, and I don't have a mechanic around here I trust. Bummer.

[dogshome]

Splines can be seen easily if you know how to take the back wheel out. Take some pics for us if you can get wheel out. That might come without much noise.

Lash in the final drive would come with spinning, whirring, clicking, whining or growling or vibration if really bad. You may have some whining and simply got used to it. There is oil in there and in the shaft?


My money is on worn bean can or slightly misadjusted carbs (I shall probably regret that! But it's my 5c 😁 ). No noise is reported , just a snatch.

[Steve]

So you all know, the consensus pointed to either the advance mechanism getting sticky (for that I will soon be taking apart the "bean can", lubing it, and putting in new springs) or something in the drive train such as the drive shaft coupling or the final drive. I will do the advance mechanism first because with so many miles it should probably be done anyway. If it doesn't help, maybe this winter I will take apart everything aft of the transmission and look for wear in there.
I tried looking for advance mechanism stickiness with a timing light but it's all too blurry. Don't know if that is another bike problem or a cheap timing light.

[Steve]

This is an update on my roll-on/roll-off bucking problem. I rebuilt my “points in a can!” Thanks to the info via Steven Rankin - I can provide the link if anyone wants it. The can came apart with difficulty - hammering on the shaft from back to front. Lots of crinkly black dust fell out. Any grease had disappeared. Cleaned, greased, installed new springs and O-ring (Euro Moto Electric), replaced the cannister, re-set points and timing. (I am now using a dwell meter, not just setting the points gap. I’m feeling very scientific!) All this has seemed very much worth doing. The marks are now less blurry in the timing light. I feel it is advancing smoothly. But it has not affected the bucking! My bike now hauls ass, but then it always did. My points ignition system has never been finicky. Whatever adjusting I do it runs equally great. Except for the bucking, which is an annoyance at low speeds, like stop-and-go traffic. I will continue with other suggested fixes over the winter.

[dogshome]

I see 'beyond me' is changing to 'no problemo' 

So show us your splines!!! 

[Steve]

The rear drive spines or the tranny shaft spines?