I have a 1990 Chevrolet Silverado c1500 I struggle to turn it on butt after a while I manage to do it. I have also notice that if I add a little bit of gas to the carb it turns on right away. I am trying to sell it but I want to fix this problem because I know it can be a dissapointment when people come and test it.

 

With out seeing it and hearing it in person, it's hard to give it a proper diagnosis
Could be a weak battery.
Bad fuel pump on it's way out.
An old plugged fuel filter.
Bad throttle body gasket

 

Ok, if you have a 1990 Chev full size, you don't have a carb, you have TBI (Throttle Body Injection)...it looks a lot like a carb, and follows a number of the same rules, primarily the ones that have to do with manifold vacuum.

What is sounds like to me is that either you have a disconnected/broken/cracked vacuum hose or your intake manifold gaskets are shot, which is typical of a small block of that era. Also, the block(gasket) between the TBI unit and the manifold. There are three bolts that hold the TBI down; make sure they are tightened evenly.

Your engine is getting too much unmetered air through a leak somewhere, it sounds like to me. Holding your foot on the pedal changes the position of the throttle, so the Throttle Position Switch reads this and shoots more juice, which is why it starts.

 

Dear, this problem really act as a hurdle for selling your car. In silverado, this type of problem is vary rare, I don't know how it comes in your way. I think you should check the alignment of wheels, may be it was due to that one. Or this problem come when you got enough speed. Tell in detail when this happen.