Well I got the new motors and have 2 installed. I tested the car just before I put them in and still had the same problem. I have narrowed it down (I think ) to the exhaust mod I did. The stinger is too big and not giving enogh pressure to the tank at high revs and so the engine goes lean. If I richen it uo then at low revs it bogs down and doesnt work.
I have changed the stingers from 10mm to 7.5mm and made them longer. Initial tests seem ok but I need a day at the track to really try it out.