i've got this 84 z28 that has had the 305 pulled out and had machine work done it it and still retains the Q-jet...just got it out the other day and noticed that the fuel return line was NOT hooked up and didnt appear to have the hook up on the fuel pump??? whats up with this and was this hindering performance??

 

If you have an aftermarket fuel pump, it may have an internal bypass and not use the return line. If it's a stock-type pump, the return line must be used.

 

it looks as if it would be a stock one..but its chrome..?...i should probably go look at it..any pointers on where it would be on the pump?

 

Just put a "Tee" in the fuel pump fuel line that goes to the carb. You must use a much smaller return line then the 3/8" line going to the carb.

Auggie

 

If the pump is chrome then it's aftermarket. Aftermarket pumps are self-regulating via a spring inside the pump. Leave the return line like it is- you don't need it unless you are running a factory-replacement pump.

 

You know Damon, I have never heard of a self regulating fuel pump?? How does that work and how does the fuel return to the fuel tank?? Now I know that elect. fuel pumps have an internal bypass (pressure relief valve) for when the needle & seat closes in the carb to regulate pressure but my Holly red elect. pump doesn't have a return line. Anybody else have any ideas??

Auggie

 

In a mechanical fuel pump the lever arm (driven by a special lobe on the camshaft via a big fuel pump pushrod) works against a diaphragm in the pump to apply pressure to it (and therefore to the fuel on the other side of the diaphragm that's being pumped up to the carb). Now imagine that between the lever and the diaphragm there is a spring. The lever moves but it won't apply any more pressure on the diaphragm than the spring will allow. That's basically how it works. It builds pressure until the spring starts to compress and that's as much pressure as it can build. 2 sets of one-way valves assure that fuel can not back-flow to the tank when pressure builds in the pump, nor can it suck fuel back out of the carb when the arm retracts and creates suction to draw more fuel into the pump from the gas tank. It's not a constant flow- it pushes fuel up to the carb half the time and draws fresh fuel into the pump half the time. Since it's driven off a special lobe on the cam it pumps fuel for 360* of crankshaft rotation and reloads the other 360* of crankshft rotation (720* of engine rotation = 360* of camshaft rotation). A simple and effective device that dates back about a zillion years.

Works great for carbs that have a fuel bowl to draw fuel from between pumps, but it would be a complete impossiblity to use a mechanical fuel pump for fuel injection that requires a constant and steady supply of fuel and pressure under all conditions.

 

i dont know if this makes sense..but one of my friends said cause the return line isnt hooked up is why the exhaust smells so bad...its just coming off of the headers but i didnt think it should smell that bad??

 

Damon, your right. The fuel pump is driven by the camshaft lobe and the fuel pump rod but the reason that these mech. pumps need a return line is when the needle & seat in the carb closes and fuel stops flowing. This occers at idle when caught in heavy traffic. Perfect vapor lock conditions. When the N/S is closed the pump arm is still moving but the pump diaphram is not,because you can't compress a liqued (see pic in post "engine hard to start when hot"). As the heat starts to build in the fuel pump the liqued fuel turns into a vapor and as the car takes off this vapor is pumped into the carbs float bowl insted of liqued fuel and the engine stalls. This is where the return line comes in. If there is an outlet from the pump to the fuel tank, fuel is flowing all the time through the pump which keeps the pump much cooler and prevents vapor lock most of the time. When I bought my 1984 Z28 it did have a return line but when the conditions were right it still would vapor lock. The Dealer installed a low pressure (3 psi) elect. in-tank pump (kit #14085385). See GM Service Bulletin #84-174 Section 6c (VIm) Dated July 1984 and my problems were over. I live in MN and I can't even imagine what it must of been like in Arizona. If you have a fuel pump with out a return line you can just put a tee in the pump outlet fuel line to the carb and run a line (much smaller) back to the tank. Some aftermarket pumps can be taken apart and drilled for a return line fitting in the out chamber. I have seen pumps with the boss on the pump for a return line fitting but the hole is not drilled. The tee is the eaisest.


Auggie