rev limit problem

after many months restoring the body of my mk1 i then started to do the motor. it was rebuilt about 2 years ago and converted to 1900 cc on the original injection, of which i removed and fitted double weber dcnf carbs. i have now removed this engine and replaced with a 2.0 agg block with a 1900 deisel crank and the motor pulls really well but with a problem. it appears to hit a rev limit at about 4800 rpm. so i changed the limiter and the leads but the problem persists. the carbs were re-adjusted again to suit the now 2042 cc motor but the problem is still there. i think this must be electrical but not really sure.any ideas    thanks for any help.