Hello Folks can you please advise the internal dimensions of the flexible fuel pipe from the fuel pump to the carb ( on a 650cc).

Also there appears to be a leak near the rear manifold on the exhaust I cannot see any obvious point where it is leaking but can hear the popping and feel the exhaust escaping if I put my hand round the right hand side of the engine at the rear. Do I have to lift the engine to diagnose / rectify this problem ? The part of the manifold that I can see appears to be OK.

I used 6mm or 1/4" i.d. flexible pipe.

Chances are that you are best to remove the engine or the head to sort out the exhaust leak. If it's the gasket at the head/elbow there's a chance you have stripped threads there as this is quite common.

With a bit of luck, some ingenuity and a touch of double-jointedness, you could replace both the gaskets in-situ, (if that's all that's wrong) but it will be a task with no pleasure contained in it at all.:bang:

Peter is correct in that the exhaust rectification is best done with the engine out, or at least, with the head removed. There is a fair chance that when the exhaust-manifold 'elbow' is removed (if that is where the leakage is), when removing the bolts that secure the little manifold in place, the threads will be full of aluminium from the cylinder head. This has to be repaired, and I have found that the best way is to use the "Self-tapping steel inserts" from either "MEMFAST" or THE STEEL INSERT COMPANY". These inserts are threaded both internally and externally and with the use of a very simple (homemade) tool are easy to fit. With a bit of luck you should be able to use the inserts with the 8mm internal thread. These inserts are almost identical to the Abarth supplied (and recommended by) inserts that we use to fit at Radbourne Racing, even on the brand new cars that we were modifying.

Might just be one of the gaskets between manifold and head or exhaust if you are lucky. Get a better location on the leak before you rush into anything.
Smoke and mirrors come to mind

the hobbler said:

Peter is correct

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Words that I love to read but which I see so rarely, except from Tom.

the hobbler said:

I have found that the best way is to use the "Self-tapping steel inserts" from either "MEMFAST" or THE STEEL INSERT COMPANY".

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It sounds like MEMFAST, which I've never used, would be ideal in those situations where a tapped hole has been drilled significantly oversize. But given that in that repair system the M8 tapped hole for the exhaust elbow needs to be opened out to 12mm, if the original threads were just stripped in the normal way, I would be a bit concerned that I was taking out a lot of the original metal unnecessarily. Apparently a Helicoil insert of M8 needs to be drilled out only to 8.3mm. I've used a cheap, Chinese set of coiled, stainless-steel inserts in the same location with 100% success.

But I will concede the fact that the MEMFAST does seems to be much more straightforward system and doesn't rely on special tools and that what was good for Abarth or Radbourne ought to be good enough for anyone. But did inexpensive coiled insert kits exist back in the Dark Ages when those companies were current?

But we should heed Toshi 975 above; it might just be gaskets, so it's worth a try at a quick, in-situ fix.

I seem to remember that 'Helicoil' DID exist when I was a young mechanic, but as Peter infered,you need a special tool to insert them. The 'Self-tapping' steel inserts for a 8mm bolt/stud require a 11mm hole carefully (best to have a 2nd pair of eyes to help align the drill) drilled--the outer thread of the insert is 12mm, the 1mm difference is utilised in the'self-tapping'. Having repaired a few heads in my time, you would sometimes be surprised as to just how big the hole is when the bolt comes out with chunks of cylinder-head still in the thread. As I mentioned, we did this mod as a matter of course at Radbourne---saved a lot of hassles later in the car's life.
However, as 'Toshi' and Peter mentioned, if you can get the bolts out of the head cleanly, try just replacing the gasket; but try to clean up the contact surfaces on the head and the manifold best possible.

Thank you for the reply the concept of smoke and mirrors may work, using a mirror to find the exhaust fumes / smoke and see if it is a gasket and where. New fuel pipe fitted thank you.