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Hello again!

I hope I might have found it!

There's a restrictor in the balance pipe of the exhaust, like a core plug with an 8-10mm hole in it.

It must have been floating around, because there's no clean face where it could have been mounted, but it was wedged in at 45° to the pipe.

Does anyone know what that restriction is supposed to achieve? Do I throw it away, or try to wedge it back in? I'd rather ditch it unless it achieves something useful.

Anyway, I think that's hopeful, just the exhaust studs to extract now...

I think the heads will need to come off for stud removal.

Heat and pentrating oil aren't touching them, and the bits I can get a grip of keep breaking off.

I tried drilling one, using the exhaust and a bush to get the drill in the right place, but I'm not happy with it, so it'll have to be heads off.

Before you pull the heads off, try turning up (or getting them made) a proper drill bush.
20mm of steel bar drilled and tapped to stud / thread size one end and a smaller pilot drill size the other end.
Wind it onto the remains of the stud and then drill. If properly centered, then you can drill out all the stud except for the thread which will need picking out of the thread in the head like a biro spring.

OR Mig/Tig weld a nut onto the remains of the stud thread then you can use a socket. The heat helps free of the corroded threads.

If the above fail, then its off with the heads and down to an engineering place and pay up - and get studs with nuts made up, don't use bolts again.
Best of luck with it.

any news on what the fault was?