Apologies folks.
Got side tracked just as about to type!
Recently had a rough running engine but have eventually hopefully tracked down the problem.
Never had the original inlet manifold drain tube but just a bolt screwed in to it.Turns out this had slackened off and I can only assume was drawing too much air. However was a good opportunity to drill a half mm hole through the bolt and loctite the bolt back in.
Also gave the Solex a strip down and all appears good again.
John White