My 425 has exactly the same problem as described by matt in his first post (except the 60mph thing). When accelerating or flat out life is rosy but when holding the throttle at any other position it stutters and coughs. It also does this on roundabouts or when cornering in general, the first application of the throttle when powering (LOL) out of corners it's hesitant, is this likely to be down to having the wrong jets or is it maybe that the float level is incorrect?

possibly
have you checked for induction leaks round the manifold?

I have, as far as I can see the only hole in it is the drain hole, I've squirted wd40 around it and it makes no difference, I do have a spare manifold on another engine, do you think it'd be worth swapping them over tomorrow lunchtime to see if it makes a difference?

if its not holed or not leaking why bother?

a squirt of wd 40 down the choke when its idling can sometimes make the smoke come out leaky bits on the exhaust. took me ages to work out why it had suddenly "gone off" changed the crossbox twice before it spluttered to a stop while i was peering at it in disgust - and spotted a puff of white vapour from the balance pipe.

Yes I'll be replacing that manifold as soon as the parcel of bits arrives from uncle roy.

Not looking forward to removing corroded headstuds

As Russ has a 425 the manifold is a tad different with no balance pipe for those who've not seen one. The exhaust pipe from the right hand side comes over to the left via the base of the carb to join the left hand exhaust in a Vee and then the exhaust goes forward and round under the left hand barrel.

In some cases russ the vee join can fail so check carefully for holes and cracks.

If yours is a 63 russ I think you'll have the "GT" 425cc unit with 18hp and a 4 stud manifold?

That should be a Solex 28 on that I think, I'll look in the book at home and see what jets you have (If my book goes that far back). It might be well worth your while getting the gasket kit for it from the usual sources.

On a separate topic are there any manuals on the older engines 375 and 425cc? 1948 - 1970's?