Since putting my '99 M1 back together I've noticed that the engine is jerky on the overrun, low rev & throttle settings and the idle speed seems to need playing about with frequently.
I'm thinking that the usual fight to get the carb off might have disturbed the manifold /head seals which are original.
Am I on the right track here - and if I am, my manual says that the '00 models had a one piece clamp and seal. Would I be better fitting these or does the '99 part supercede to these anyway?
Do I absolutely have to take the carb off the manifold to swap these out?

Advice appreciated.

I would say that if you've recently disturbed the manifold, then it would be a good thing to replace them. If the carb rubber is original, there is a good chance that it has perished over ten years. The manifold is easy to remove, but you need a cut down Allen key. While mine was off, I used a rotary burr to remove a bit more material where the bolt heads are, as when you losoen them, they start to dig in to the radius of the manifold. This makes re-assembly easier.
HTH.

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I started out with nothing and still got most of it left.