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).

__________________
Drive Safely,
Albert

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?

__________________
Drive Safely,
Albert

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...).

__________________
Drive Safely,
Albert