1987 k5 blazer cranks a long time before starting
 

I have a 1987 K5 blazer with a throttle body injected 350. Ninety percent of the time I have to engage the starter for around ten or fifteen seconds total before it will fire up. After that it runs fine but always requires an extended cranking procedure to fire up. Occasionally but not very often it will fire right up like it's supposed to. It does not seem to matter if it is cold or hot. I can see that fuel is not coming out of the injectors until just before it fires up. I have replaced the fuel pump and also replaced the fuel pressure regulator in the throttle body unit. Any ideas on a remedy for this would be greatly appreciated. :confused: