My 78 Bronco 351M (stock 2150 carb), will sometimes idle very slow or stall after coasting for an extended time. I can pump the accellerator once and the idle will pick back up to normal.

The bowl vent actuator rod does not touch the vent plunger at idle; it has a gap about the width of a sheet of paper, and the vent does not open unless the throttle is opened slightly. My thoughts are that the closed bowl vent (at idle) is causing my stalling......Is the bowl vent supposed to be open at idle?

Thanks.
Don

 

The front bowl vent should open only when the accelerator pump discharges fuel. (The bowl vent lever is actuated by the accelerator pump lever.)

Your bowl vent lever is adjusted correctly. It is not causing the problem. Remember, there are two, much larger bowl vents on the front of the choke tower.

You might need to re-adjust the carb's idle settings (idle speed set screw and idle mixture needles).

One feature of the accelerator pump system is a bleed orifice between the pump chamber and the fuel bowl. When fuel warms up inside the pump chamber, the bleed orifice keeps pressure from building up in the chamber, which would push fuel through the system and produce unintended discharge into the venturis. The bleed orifice also acts as a calibrated relief to reduce the effective discharge volume of the accelerator pump by bleeding some of the fuel in the pump chamber back into the bowl when the pump diaphragm pushes on the fuel in the chamber.

Because the main bowl vents open to the air inside the air filter element, there is always a slight negative pressure (vacuum) on the main bowl vents when the engine is running. The idle and main metering/demand enrichment systems are calibrated (jets, air bleeds, etc.) to accommodate that slight vacuum in the fuel bowl during steady-state operation. However, the accelerator pump bleed orifice requires atmospheric pressure in the fuel bowl to work properly, so the additional bowl vent opens to relieve the slight vacuum in the fuel bowl during accelerator pump operation.

On late model 2150s used with evaporative emissions controls (EVAP), the front bowl vent is also a valve that shuts off the EVAP bowl vent when it is opened by the vent lever. That cuts off the residual vacuum in the EVAP system, which also contributes to a slight negative pressure inside the fuel bowl.

 

Dave, thanks for the very good explanation of how that venting system works. Somehow I figured that simply bending the vent lever would be too easy a fix for the stalling. At least that eliminates one possible cause.

I'll tinker with the idle setting and mixture some more when the weather warms up a bit. I have found out that I can turn the mixture screws out to 4 1/4 turns and it dosen't stall, but that seems a little too rich. I have them at 2 1/2 turns now. I may just bump the curb idle up slightly and forget it......

Thanks again.