Engine driving us mad

We are nearing the end of our build , or so we thought...the engine is a 1641cc , twinport, running a scat c35 cam, solex pict34 ....an 009 dizzy, pertronix ignition, Bosch coil...we set the engine up and ran the cam in...all was ok until the clutch went...we changed that...got it running, idling between 850-950 rpm...my son adjusted the clutch to the point it stalled as he tried to drive off...re-adjusted he drove it down the road no problems...on the way back the engine suddenly revved to 3000 rpm almost as if it had stuck open...he got back and it stalled...tried to start it and it turned over really fast but would not fire...I turned it over by hand and it went tight and then loose , tight etc etc..so it seemed to have compression (haven't got a compression tester but have one on the way)....before all this it always seemed to me to turn over "tight" if that makes sense and it was as if something had suddenly freed itself allowing it to turn over a lot faster...the starter is brand new, so is the battery...we have spark, fuel and air....anybody have any idea what it could be ...there is no rattling as if something is broken internally...this car is fighting us!