Aye norm,
Welcome to the site.
Agree with SteveC#222, trying to run at 30mph in 3rd or 4th gear may cause this snatchiness. 'Thumpers' they may be, but long stroke, British singles, they aren't!
Treat them as such and they will knock out the big end, gearbox, chain and sprockets!
Give it some 'wellie and it'll clear its throat!
Don't forget the GB500NZ method of blowing your orifices through with liquid butane lighter fuel, works a treat!
All the best, Bill.