R100rs 1979
 

I've just replaced the bottom end on my bike due to big end wear (or poor maintenance on my behalf)
After a few problems on the initial shake down I was quite pleased the the way it ran. Unfortunatley on the third trip out, I felt an twing at the rear end, I first thought it has slipped out of gear, (I also considered the centre stand spring failing and dragging as this was how it felt.) I have checked all the gear linkage, rechecked the drive shaft bolts and stub axles and driven it again.
Last time out I was quite gentle with the acceleration and very positive with the gear changes and all seemed well until I was nearley home when I gave it a bit more throttle and again a nasty faulter in the power delivery.
My thoughts suggest a worn gearbox, but considering all the items I have disturbed, it seems very unfortunate that the 'box should develope a fault right now.
I think the ignition timing is a little advanced, could this cause a problem like this ??
I would appreciate any suggestions.

Julian Delaney

Could you please describe the -problem- more clearly?

It is simply impossible to understand what is going on for you.

Clutch slip at speed when torque is increased is a common problem.

Worn shock-absorber in shafts and gearboxes may cause accelleration problems...

Worn splines may cause weird phenomenas...

Loose wheel-axels may cause problems...

Loose shocks will cause problems...

Loose Subframe will caused phenomenas...

Clutch leaver not adjusted correctly may cause sudden clutch slipping...

Swingbearing not set propperly or worn-out may cause perculiar happenings...

A bad rim may cause weir stuff...

...and so on...


What have you done with the bike?
- new bottom-end (=½ new engine).
- what else did you do?
- cast or sprocket wheels?

And, by the way, there is no way in... that you can -feel- a dragging central- or side stand. You hear it, but thats all you do - I know for sure; mine is dragging (poor service on my part an excellent on the road salts part...).
Perhaps you try to describe that odd weird feeling of a "braking" somewhere in-spite that there isn't anything obvious causing this? Had that a lot the last year... feels like the tires "glues" to the tar...

Do NOT set the timing beyond the full advance marker... the exhaust fumes get really hot... that will in time damage the exhaust valve and may result in a separation between the disc and valvestem... my right exhaust valve did this to me back in 2006 at 5200rpm.
Reset the timing to the correct! Ok... - the bike will not start as easilly and it will not rev as happilly; if that is a major down for you - change the Bings to Mikuni, Keihan or dell'Orto (I recommend 36-38mm).

more details
 

Yes, the complete bottom end has been replaced, so the points have been taken from the original engine and aligned as best i could without proper equipment, all the necessary ancillaries, carbs, starter motor, alternater, diod board, fairing,tank battery etc. have been out and back in, the old clutch transferred etc.
The noise/sensation felt through the frame is, alas, very similar to a previous gearbox problem, that is a sudden twitch at the rear and a 'mechanical' scraping sound.
I've just had a thought, the clutch did slip a bit last time out, but there is no tension on the cable and the friction plate is fairly new - could that be loose at the fly wheel ?- it was torqued up to the manual - but funny things happen !

Sorry to be so vague!

Thanks for taking an interest, regards Julian Delaney

You did use -new- stretch bolts for the flywheel and shaft?!...

The gearbox version you have can leak a bit more hypoid than then 1981-on... so a small slippage may be caused.

But... you feel it through the frame... that is not very reassuring...

Twitch at the rear???
Machanical scraping sound???

How is the bevel?
Have you had it apart?
Did the large nut in the shaft end of the bevel sit tight?
What is the play of the bevels and ballbearings?

What is the condition of the brakeshoes (or do you have a disc-brake)?

Is the shaft firmly attached to the gearbox?
Feel over the gaitor to ensure that all 4 bolts are still up there...


Do this test:
- have the frontwheel pushed down into the ground lifting the rear clear off the ground. (I prefer to do this on a lift and secure the front with truck load straps).
- start the engine. Try to get an even 3000rpm. Put it in 1st and then shift up and down trhough the gears.
- Use a stetochope (= a large screwdriver, tire iron, steel-rod, what ever till do); place it against the side of the gearbox (both sides, front and rear).
IF there is a major problem inside the gearbox you will hear it! Should you have a bad bearing... it sounds like a stone quarry(!) i.e. really bad; and as to lift the stetoscop off the gearbox the sound "vanishes".
This test should be done every 1500-2000miles and whenever you feel like it.

You will also hear what is going on in the shaft and bevelbox this way.
There are magnetic plugs in you gearbox, shaft, finaldrive and engine - right?
If not - Wunderlich GmbH | complete your BMW carries all needed sizes. Art.-Nr. 8154010
Magnet-Ölablassschraube*
Louis have a wide variety of magnetic plugs:
www.louis.de - Motorrad & Freizeit
http://www.louis.de/_30603859026886e...nr_gr=10002912

As the twitch and metal scaping - does it happen only in accelleration or will it occur as you decellerate or keep even speed?

How are the wheelbearings?
When did you regrease them last?
Was the play correct?

The drive shaft bolts and big end bolts were replaced, the fly wheel was already fitted to exchange engine and not disturbed, the clutch used the old bolts.

The brakes are both disc.

The drive shaft bolts have been re checked.

The Bevel was not disturbed.

I will check the wheel bearings and listen to the gearbox tomorrow.

Thanks again for your assistance.

Your question is indeed tricky - the fault will be rediculessly obvious when located -;)

Note BMW "sickness" such as:
- oil leaking into the shaft from the gearbox.
- oil leaking into shaft from the bevelbox => lose nut => bevel about to get really badly damage.

In a way you kind of describe what happened to my 32/10 bevel - the bevel part connecting to the shaft suffered one bad ballbearing causing excessive travel. It was seen only as a bit more metal on the magnet than could be expected. My mechanic said I was over concerned... I drove -many- miles with a really bad bevel... broke some teeth... and what prompted me to check was that backing the bike became weird.
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If I had not used Omega 690 80W/90 the bevel would have jammed for me... the oil was still in very good shape after 40-45 000km incl the "broken" ballbearing(!) - point, change oil to look at the magnet(!) every 5-8000km or there about = very cheap "insurance".

Check you disc-brake. Check that you did install all parts correctly and bleeded the caliper fully. A warped disc / worn-out (common BMW sickness) could cause the sounds (but that kind of requires you to have touched the brake-pedal...).

more descriptions
 

i'll look ast the bevel too - but at the back of my mind is the thought that the fault should really be to do with something I've disturbed.

the sensation is similar, (if you have ever had the misfortune to ride such a worn out bike) to a chain that is so stretched that it climbs and jumps the sprocket, i.e a rhythmic twang !

does this help or just make things stranger !!

Regards Julian Delaney

No that info did not make it more complicated - it simplyfied matters.

It is not the clutch.
It is not the gearbox (less you have the shock-absorber on the out-going shaft).
It it not the splines in the shaft.

It may be the shockabsorber in the shaft - it happens, rare but still.
It is highly likely to be a damaged bevelbox, very much like mine.

See, a clutchdrag - you would have noticed revving, hence excluded.
Ballbearings act differently; they will grind/rumble and make such noice that you will stop driving.
Splines just loose tracktion and sounds like a slow machine-gun (12mm at a distance).

What you are left with are:
- spring shockabsorber gone bust; not snapping over but giving way too much play; checked by turning the rearwheel in gear. A few inches of play is normal.
- bevelbox bearings and/or teeth bad, really bad. => acute dismantling before a seazure occurs (that -will- lock you rearwheel and send you skidding which may be an unpleasant experience, like into oncomming traffic...).

Use the stetocope method and listen to you bevel; a gentel whining is all there should be.
Change oil in the finaldrive to study the condition of the used oil, the magnet for any metal.

Motorworks (BMW spares supplier) seemed to think it was the bevel too, I am going to grasp the nettle over the weekend and make all the checks you have suggested. An added complication is I've just fried the diode board by not having the nuts tight enough on the earth points !!!!- ordered a new one from the above, this bike has become a money pit !!.

Thanks again Julian

So sorry to hear you shorted your diodboard. The needed parts costs a few bucks, but soldering the new diods are said to be a trycky business.

I forgot one simple test you should do.
- Grap the gaitor at the u-joint. Get a firm grip on the u-joint, then turn the rearwheel. The ONLY motion you should feel is the play in the splines between the rearwheel and the final drive. IF you have more play than that (which feels less than the click of a ball-pen) something may be not so good... there should be absolutetly no detectable play between the bevel (crown sprocket?) and the u-joint - not even the slightes that you can feel with your hands. The play is measurable but with oil in the bevel and shaft you should not be able to feel it.

I've stripped the rear of the bike, cleaned the back wheel caliper etc, and given it a good looking over, the swing arm feels nice and everything seems as it should be.
I've decide whilst I can't do any dynamic testing until I get the electrics back together that I may as well restore the sub frame and get it back in good order,
I will try your later suggestion and let you know the results, just turning the wheel seems very smooth and the oil in the bevel looks like new - I don't want to drain it until I can listen to it running .

Thanks again

I've pulled the gaitor back and rocked the wheel back and forth, I can move the wheel about 3mm only (measured on the tyre thread) before the universal joint moves, which feels to me like the slack on the splines between the wheel and the bevel. The diode board should come Monday , i'll try the rest of the test then. I've got my eye on a gearbox on Ebay just in case !

Julian

Always drain the oil-compartments - if you clean the out-side well, it is possible to re-use the oils if need be.

3mm play is -very- good! That sounds like >80% of the spline-thickness still remains -:)
IF the bevel would have had excessive play then you would most likely have felt two stops - you did not mention that so I assume that the play in the bevel is ok; however, do not let this fool you!!!

You need to look at the oil - and you have fooled youself by looking down into the filling hole... that is -exactly- what I did to, and I also dipped a finger in the oil to check it more thoroughly; my oil still looked ok and felt fully operational at 40 000km in spite the worn-out ballbearing and teeth damages (!).

If the magnetic plug is clean - no metal; and the oil is clean (no debris nor water) and no metal parts followed with the oil out - and you can not provoke any oddeties rotating the rearwheel forth and back; then I would assume the bevel to be ok.

Sorry I forgot that you need to provoke the bevel by firmly rotating the wheel backwards as well; and you need to spinn it by hand in reverse less you have a reverse gearbox.
As you press the rotation backwards you pull the the bevel sprockets to each other instead of pressing them apart as normal driving will. If there are any bad sprockets you will feel them like a kind of snapping motion, soft to hard. There rearward spinning should feel just as smooth as the normal frontward spinning - if any doubt; open the bevel and check! Broken teeth does not have to follow the oil out... contradicting my opinion above...

EUREKA, although it reflects badly on my rebuild skills, I've got it !
I've been turning the wheel as you suggested and found the drive shaft snagging, after fishing about inside the gaitor I've just pulled out a redundant drive shaft bolt !, this must have been floating around and occasionally getting snagged in the U joint.

Thanks for all your time and help, I'll hopefully get the bike back together early this week (after having a look a the U joint !)

I'll let you know how it goes.

Julian

... -:D !!!

Now that was indeed a new angle of the problem!
:wave: