k100 non starter?

i own a 1986 k100 which will not start, it seems as if its over fuelling, if i keep it cranking over and disconnect the fuel pump, it tries to start, then backfires and all sorts, when its trying to fire up i keep flashing power on off the fuel pump which either stops it firing all together or if i get it right seems to make it want to start up even more, which it has once or twice, and when warm starts no probs, although runs very rich. i have changed plugs, all filters, checked pressure at fuel rail and pressure reg valve operation, what can i check next?