Loud clunk when accelerating in 1st gear.

[Mrducati]

My 1989 r100st is making a loud clunk when accelerating in 1st. If I pull away slowly it will not do this. The sound is pretty violent. Like banging a hammer on an anvil. What can it be? Only in first gear. Do I need a new gearbox?

[Rob Frankham]

Probably to do with the drive shaft... check the spring damper in the shaft, the bolts that connect the shaft to the output flange on the gearbox and the Hooke joint (UJ) at the gearbox end of the driveshaft.

Oh, and check that the main stand springs are present and in good condition... it's not unkown for the stand to be dropping slightly under acceleration only to back back against the stop...

Also, if you're having difficulty placing the sound, it's worth checking the head bearings. Loose bearings can make a clunk under acceleration.

If your getting nowhere, it might be desirable to drop the gearbox and check the clutch disc.

I wouldn't start looking at the gearbox until I'd checked everything else...

Rob

[Mrducati]

Thank you so much for advice Rob! I am very much hoping you are right about it been driveshaft related. Quite scared to open up the gearbox. It’s not in my comfort zone.

it's worth checking the head bearings. Loose bearings can make a clunk under acceleration.

Rob

What do you mean with head bearing? Certainly not in the cylinder head… Where are they located?

[SteveD]

Steering head.

My first thought was the trans output flange bolts x 4.

[Rob Frankham]

Apologies... should have been clearer. I meant the steering head... hence the bit about not being sure where the noise is coming from. Under brisk acceleration, the front of the bike rises (bear in mind that the acceleration available in first is greater than that in any other gear) which is the same as saying that weight is taken away from the front suspension. If that weight removal is enough to remove downward pressure from the front suspension and if the steering head bearings are loose, this will make quite a loud clunk. Putting this forward as a 'possible' not a 'likely'. Worth checking.

Rob

[Mrducati]

Thank you so much for clarification Rob and Steve! I am so thankful for you pointing me to a plausible solution. I doubt it can be the stand or steering head bearing. It is way too loud for that. It’s only one bang and only in first gear.

[gspd]

Mrducati-
Hard to diagnose remotely. Could be anything. Literally.
My internet guess?
(and don't hold me to it)
hmmm...
Maybe first gear is only partially engaging and then banging fully home when fully under load.
Check that the shifter lever travel (and linkage if applicable) is not being partially restricted on it's way down to first gear.
If that's OK, drain your transmission oil when hot and check it for chunks of metal.

[Mrducati]

Maybe first gear is only partially engaging and then banging fully home when fully under load.
Check that the shifter lever travel (and linkage if applicable) is not being partially restricted on it's way down to first gear.
If that's OK, drain your transmission oil when hot and check it for chunks of metal.

It sounds and feels excactly like it could be this!

[jagarra]

Does your bike have linkage going from shifter to an arm on transmission?