Nissan Repair: 1983 720 pickup, throttle plate, venturis

Question
I recently bought an 83' 720 pickup. It idles very smooth and it runs very smooth at higher rpms as well, but if you barely push and hold the throttle, it starts to cut out and run very rough. It will do this for as long as you hold the throttle in that position. Once you let off or open the throttle more, it runs fine again. Do you know what might be causing this? I just replaced the accelerator pump and did not fix.

Thanks,
Phil

Answer
Philip,

There is a blockage in the carburetor that is causing the problem.  There is a transition from the idle circuit to off-idle and when the fuel starts to transition to this circuit the path is partially blocked.  You could probably take the carburetor off and clean the plates and fuel circuit with carburetor cleaner and reinstall the carburetor and solve the problem.  If it is really gummed up you might be due for a rebuild.  The accelerator pump is to aid in transition when the throttle plates open up gas it pumped in to allow the fuel feed from the venturis to catch up.  At steady state the carburetor feeds from a given circuit and not the accelerator pump.  It could be just a blocked port at the throttle plate.  Things tend to plug up under the plates and if you look where they start to open, on the side where the plate moves up there should be small holes, slits, where the fuel transitions from idle to off-idle.  Use cleaner there.  Also, make sure the float is not soaked with fuel.  Old foam floats get saturated with gas and need to be replaced.  If yours is brass make sure it does not have a leak.  If your float is fine be sure to make sure it is set properly.  If the float is too low the fuel tends not to flow properly after you step on the gas slightly.  Once the velocity picks up the gas flows fine.  But, just off-idle there will be a stumble at a steady state.