Help & advice re. recurrent slow-speed tank slappers, pl

[derillius24]

Hi.

My dad bought an old BMW R100 last year in order to modify it for my profoundly disabled younger brother. The pillion seat modification is great, a really pretty job, and we all had our first ride out together last weekend (the wee man loved it!). However, just since then (it's been used with the seat modification for hundreds of miles) my dad has tried to take it to work to show friends twice in the last two days and both times has had to stop and turn around following near 'offs' after violent slow speed tank-slappers.

He took it very easy this morning, trundling along the street at 20mph or so and as soon as he even suggested a whiff of throttle the front started to shake it's head in anger.

Does anybody have any idea what could be causing this? The tyre pressures have since been checked and are absolutely correct and there is nothing glaringly obvious with the front end of the bike.

Any ideas/suggestions much appreciated,

Cheers,

Mark.
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CG125, CBF250, DRZ400-SM, Z750, Street Triple R, Tuono V4R

[P.]

Seat modification added significant weight to the rear?

[derillius24]

The bike's been ridden for months with the seat modification in place without any issues, the problem just came about overnight.

Cheers,

Mark.
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CG125, CBF250, DRZ400-SM, Z750, Street Triple R, Tuono V4R

[Rowey]

If you have adjustable suspension set it all to standard. If it's still doing it, I'd have the front and rear socks off for a rebuild. Have you lowered the seat or the rear of the bike? This will upset the geometry. If the rear has been lowered it's good to drop the front forks a bit to compensate
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ZX6R C6F.
Never argue with an idiot. They'll only bring you down to their level, and beat you with experience.

[Tarmacsurfer]

I'd have thought head bearings and wheel alignment/balance were the next obvious things to check.
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I'm immortal. Well, so far.