OK - Here is the whole thing with the explanation from Lars Grimsrud.
Since this issue involves several problems and topics, a general understanding of the systems and issues is important rather than simply addressing a single question on fuel pump check valves. I’m going to throw out a few facts and issues here so you can see and understand the multiple problems with the fuel disappearing issue.
The Q-Jet carb is a bit unique in its design in that the needle and seat assembly is located towards the bottom of the float bowl rather than in the top of the bowl or in the airhorn (as is the case with Holley and Carter carbs). In fact, the needle/seat assembly is located on a raised ledge at the same “elevation” as the accelerator pump inlet slot. The key about this fact is that if the fuel level in the float bowl is allowed to drop below this point, the accelerator pump will not fill with fuel, and the accel pump will not function, even if there is fuel in the float bowl below this level.
Holley carbs, as you know, have the accel pump located in a well hanging below the float bowl. If the float bowl has any fuel in it at all, the accel pump will fill and operate.
Although Q-Jets are equipped with chokes, an engine will generally not cold-start on the choke alone: an engine needs a couple of squirts of fuel from the accel pump even if there is fuel in the bowl and the choke is closed. The accel pump shot is essential to engine starting, and the accel pump in a Q-Jet will not function more than 1 stroke if the float bowl fuel level drops more than about 1/8”.
The fuel level in a Q-Jet bowl can drop more than 1/8” over time from 3 separate causes and issues:
1. Although the problem is exaggerated, it is possible for the bowl well plugs to start leaking on a Q-Jet. When the factory drilled the main jet and secondary fuel transfer passages, they drilled them up from the bottom, through the bottom of the carb. Once the drilling was complete, they pressed lead plugs into the resultant holes in the bottom of the carb. It is possible for these plugs to start leaking. If they do, the fuel level in the bowl can easily and quickly not only drop below the accel pump inlet slot, but can actually empty the bowl completely. This will result in very hard cold-starting.
To check for this condition, simply remove the carb from the engine. Dry off any fuel from the bottom of the carb. Place the carb on 3 tall sockets on your workbench so you can see the bottom of the carb, and fill the bowl with fuel by pouring about 1-1/2 ounces of fuel down the vent tube. If the plugs are leaking, you will see immediate dripping or wetting on the bottom of the carb, and fuel will drip down onto your workbench. It’s very obvious. If this happens, you need to seal the plugs. This can be done by cleaning and abraiding the plug areas (the secondary plugs are not visible without removing the throttle plate) and applying a coat of JBWeld. This “fix” tends to last for a short while, because the JBWeld will crack and separate from the lead plugs. A permanent fix is to drill and tap a 10-32 hole into the primary well plugs and install a 10-32 countersunk socket head screw with JBWeld applied to the threads prior to installation. This permanently fixes the problem. The leaky secondary wells can be fixed by using the secondary well plug seal found in good quality carb rebuild kits.
If fuel does not drip out the bottom of your carb during the leak test, you do not have leaky well plugs, and you have one of the other 2 problems.
2. Fuel evaporation is a problem on a Q-Jet due to the fact that the bowl is vented, and you don’t have to lose very much fuel before the level drops enough to cause a problem. The fact that our modern fuels contain ethanol (15% in many locations) increases evaporation rate and exaggerates the problem. Evaporation rate is higher in hot weather conditions (summer). Other carbs, such as Holleys, are not affected by some fuel evaporation, since the accel pump inlet is in the bottom of the bowl. Remember – the Q-Jet float level only has to drop a slight amount in order for problems to occur, and evaporation will induce this situation (although it will normally not occur in an over-night 12-hour period).
3. The only other place the fuel can go on a Q-Jet is out the needle/seat, and back down the fuel line. Since the seat is located towards the bottom of the bowl, and since the carb is at a higher elevation than the fuel tank, you can see the obvious possibility that the fuel will simply siphon out of the bowl and return to the tank if allowed to do so.
Early Q-Jet fuel systems, generally, did not use fuel return lines. Although return lines have been used by GM in some performance and air conditioned applications since the mid-60s, the common use of return lines to reduce vapor lock issues did not become prevalent until the mid-70s. In the early fuel systems used with the Q-Jet, the non-return fuel pumps relied on the fuel pump’s internal check valve to prevent the column of fuel between the pump and the carb from draining back through the pump. As you can imagine, if the column of fuel between the pump and the carb is allowed to drop down, there will be a suction created at the carb by the retreating fuel in the line. The needle/seat, although tight-fitting, is not designed to positively stop fuel flow in the opposite direction. So the fuel draining back through the leaking fuel pump check valve can actually draw the fuel out of the float bowl down to the level of the needle/seat elevation. This drop in fuel level is enough to prevent operation of the accel pump, and the car will not cold start easily. On cars without return lines, a good quality AC/Delco pump, with a check valve intended to eliminate fuel drainback, is essential to good cold-start characteristics. Most aftermarket pumps do not have check valves that will prevent drainback over a period of time.
With the advent of fuel return lines on later Q-Jet systems, other safety issues were also addressed in the GM fuel systems. The most notable of these is the “check valve inlet filter,” often thought to be a fuel return checkvalve. This is not the case: it is coincidental.
The check valve inlet filter is actually a roll-over fuel spillage prevention device. GM found that in the case of a vehicle rolling over, the fuel tank ends up being higher than the carb. Depending on the attitude of the tank and the fuel level in the tank, it is possible for fuel to flow from the tank, through the lines, through the pump, and right through the carb. You can imagine the possible consequences of being trapped in a rolled-over vehicle with fuel pouring out of the carb and onto the ground… So GM installed a filter in the carb with a spring-loaded check ball. This check ball requires a certain pressure to unseat and allow fuel into the carb – easy for a functional fuel pump to accomplish, but the pressure of gravity-fed fuel from a roll-over cannot unseat the valve and allow fuel flow. Roll-over problem solved.
But the roll-over valves also assisted in solving another problem, as you can now see. The return-style fuel pumps that came into use at the same time as the roll-over issue could not positively prevent the fuel from draining back from the carb due to the internal bleed orifice for the return fuel. The roll-over valves seemed to solve the problem, even though they were not designed to do so.
Yet problems persisted, because many people removed the “restrictive” check valve filters and installed the early-style filters without the valve. Also, if the filter pre-load spring is missing or weak, the check valve filter will not seal and prevent reverse fuel flow.
So what should you do about your hard-starting Q-Jet? Here’s the list:
Check for fuel leakage out the bottom of the carb and repair it if needed. My money is on that your carb won’t leak.
If you have a car without a return fuel line, use a good quality fuel pump with a good check valve. The pump should not allow fuel drainback.
And here is a problem that nothing to do with a fuel level drop. It has to do with accelerator pump compatibility with modern fuels. Many accelerator pumps in the older cars, and a bunch of pumps being sold in brand new rebuild kits, are not compatible with the ethanol in today’s fuel. The result is that the accelerator pump will swell in the pump bore and seize. The outer spring on the pump rod will allow the pump rod to move and appear to function, but the pump is not working. A hot engine will start without the pump working, but when cold, the engine will require excessive cranking to start. The engine has to have a pump shot to start cold. So make sure your accel pump is working and not seized in the bore. I see this commonly.
Use the roll-over valve inlet filters. NAPA part number for the early (short) filter is 23051. The long filters are 23052. Make sure your filter spring is installed.
If these measures fail, you can do 2 more things:
First you can use a needle/seat assembly that does not have the “windows” in the bottom of the seat. This will block the fuel from being able to enter the seat and flow back.
The other thing you can do is to remove the clip that attaches the needle to the float. This will allow the needle to drop down into the seat and seal off the seat any time there is no fuel pressure. Although it does not produce an absolutely positive seal, it does slow down the drainback enough to allow easy starting of the engine.