Sort of a long post, but here goes...

I am trying to change from the mechanical fuel pump (which consistently gave 9-12 PSI) to an electric one. I have an Edelbrock 750 carb on a mildly worked 350. Edelbrock states 5.5 PSI is ideal, 6 PSI is ok.

First electric pump I tried was the Aeromotive Street/Strip pump (150GPH, and very loud) with an aeromotive 4 port with return regulator. This combination, no matter what I did would give me no less than 12 PSI. Not quite sure why, but guessed maybe becasue of flow rate and small return line diameter?

Second pump I tried was a carter street/strip (72GPH, much quieter) this pump looked good would put out a solid 4-6 PSI for about 5 minutes..and then fell to 0 PSI. After that pressure never came back. If I shut off the car and let it sit for 20 minutes would regain pressure to 6 PSI for another 5 minutes. Figured flow might not be high enough for some reason, bought the Carter HP street pump (100GPH, still quiet) and 6-8 PSI. Same thing..held 6PSI for about 8 minutes and then fell to 0 PSI same results and recovery as other Carter pump. Now here is what is interesting, while the guage was reading 0 PSI the engine never stopped running. Also I confirmed there was no fuel in the line by loosening some fittings (on the output side of the pump) and verifying that the line was dry. Tech at Jegs suggested running engine for a while to make sure no air in the line or cavitation. The pump is located on the frame rail near the starter. If the hose is removed from the inlet side of the pump the gas just runs out of the line, so a gravity feed is working.

Questions:

1. If the fuel is flowing in under a gravity feed to the engine why are the carter pumps losing pressure (and prime I would guess), yet the higher pressure and flow aeromotive pump doesn't lose either. It shouldn't be related to the fuel line diameter I wouldn't think.

2. If the issue is that the pump is running the line dry, weird again, but ok, would moving the pump back to the tank area resolve this? If so, is there room by the tank to mount the pump (car is '71 convertible). Also when I had some frame work done the hard fuel lines were replaced with originals, would hate to hack them up.

3. Any suggestions would be very helpful, I am just about going crazy now, and really don't want to buy any more pumps since they all seem to have the same sort of issues.

Thanks
-Kevin

well... i dont know specifically about the pumps you have used..

im running a holley blue in the same location.. constant 6 psi (demons like 6 with 7 ok) and have not had a problem. if i had to make a guess at why your losing pressure i would look at the rubber sections of fuel line in the rear and make sure they are in good shape.. a collapsing hose can cause this behavior.

also.. make sure you have the right gas cap. vented/unvented

The rubber lines were all replaced at the same time as the hard lines..about 18 months ago..and as far as the gas cap...it is the stock original (replacement), but the results are the same whether the cas cap is installed tightly, or the cap is removed completely.

then i would check to make sure the rubber pieces werent installed with a kink. beyond that.. you got me..

Ben, are you running the return line on yours?

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Someday!

nope... but i was planning to hook it up when i reassemble the car

Ok here is what I am guessing and you will need to verify this.

First off you talk about fuel pumps but no regulator or having a pressure gauge.

The pressure gauge should be mounted at the carb or on the supply side to the carb.

Most Edelbrock carbs require a regulator. There are different types of regulators. Some are low pressure(1-4 psi) others are high pressure (5-?). Some mfg are now making them where they will work from 1 to 25 psi, but require a different spring to vary the pressure range.

Your lose of fuel pressure is most likely caused by what is called dead heading the pump. When your pressure drops to 0 is the pump still running. If not then the pump is dead heading. That means the pressure in the line from the tank to the pump and the pump to the regulator has built up enough pressure to cause the pump motor to seize up.

Most likely the engine will continue to run for some time because you still have fuel in the carb fuel bowls.

Having the pump mounted by the starter means you most likely mounted by the exhaust. You could be causing it to also vapor lock the pump or over heat the fuel pump.

If you are running a fuel return to the tank then I would put a pressure gauge on the return line and watch the pressure on this line. It should read 0 and really no more then 1 psi on the return side. It could be the return line is not actually open to the tank. Return lines should return fuel to the top of the tank. not up through the bottom trying to force its way into a filled tank.

Mounting locations. Most gravity fed electric fuel pumps should be mounted by the tank due to the fact they need a good supply of fuel so they won't cavitate. The fuel inlet on the pump should be mounted no higher the bottom of the tank. If you have to mount it up front then make sure you have a good gravity feed.

Voltage supply should be a constant 12 volt source. There are several fuel pump relay kits on th market the direct 12volts directly from the battery only when the ignition is switched on.

I'm not sure why you chose electric, but the mechanical one should be fine if you should choose to go back to that setup most stock pumps designed for a q-jet only put out 4 to 5 psi anyway. If you go to a stronger mechanical fuel pump you would only need to add a fuel pressure regulator between the pump and carb and you won't need to even worry about the return line. Just cap it off.

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Well actually I mention the aeromotive regulator in the top of the post, and to get those readings I must be using a pressure gauge, or how would I know what the pressures are, and know when they fall? That aside, the pressure gauge is located right after the regulator, and within 2 ft of the carb inlet.

According to Edelbrock, they are looking for a steady state pressure around 5.5 psi, with a low of 2psi at WOT.

I don't think it is a pump "dead heading" issue as you called it. The pump still seems to be working, and when the pressure drops to 0 there is no fuel in the line, I have verified that by removing the line...it is dry. I agree it will run for a while from the fuel in the bowls, but most likely the engine will exhaust that reserver within 5 minutes or so at idle. There probably isn't much more than 2 or 3 cups of fuel (total) within the carb itself, so the engine should exhaust that within a realtively quick time. So there in lies another issue since while the gauge is reading 0 PSI there obviously must be fuel coming up the line since the engine will run for 45 minutes while the pressure gauge never comes off 0 that I can tell...but it is true that I wasn't watching the pressure gauge contniuously and even a couple seconds of 1 PSI fuel out of a -6AN line will most likely fill the bowls again.

While the pump is near the starter, it isn't so hot that you can't put your hand down there. Vapor lock could be a possibility, but this is happening within starting the car for the first time of the day (sitting all night) and the pressure drops within 4 or 5 minutes. While diagnosing this, the car is running at idle in the drive way with the hood open, and an ambient temperature of 65 degrees, so I can't imagine the vapor lock possiblity is any worse than sitting in traffic in the middle of summer with the hood closed and a stock mechanical pump. Not saying it couldn't be the cause, but I am not completely sold on that.

As far as I am aware the return line returns to the top of the corvette tanks, and I do know that the return line was replaced at the same time as the feed about 18months ago.

The pump cavitation theory sounds plausible, but I am not totally sold on that either because: the pump does have a gravity feed...actually pretty easy to achieve since the feed fitting is on the bottom of the tank, and the tank is located so high in the car relative to the engine and fuel lines. Secondly, if it was a cavitation problem where by the pump can't draw enough fuel to the inlet side and starts to cavitate, then I should have had that problem worse on the higher flow/higher pressure pump, but that was not the case, that pump consistently turned 12+psi and never dropped to 0. Also, there are people on the forums that have reported switching to a replacement fuel injection setup with no issues using the stock 3/8" feed line. So, while cavitation may be possible, I am not sure it is probable.

I am using a painless wiring fuel pump relay kit, the power feed for the pump comes straight from the starter hot lead.

The reason I am trying to switch to electric is that the stock q-jet mechanical pump (and a replacement pump) both consitently put out 9-12 psi. The edelbrock carb was unable to be tuned at this pressure and has consistenly run too rich. Switching to an electric pump looked like an easier swap, and would give a more consistent and efficient fuel supply.

I guess ideas are like bellybuttons, but have you verified that the return line is actually open all the way back? It seems to me that the mechanical pumps couldn't build that much pressure if it was. Try diconnecting at the regulator and blowing through it. Should hear air going into tank. That would also be consistent with the electric pump "bogging down" or deadheading with nowhere to go.

I still think you have a kink in the rubber portion of the line somewhere.. once you pull pressure its collapsing... are you sure the fuel strainer sock in the tank is clean ??

I will check the return line, I too think that the pressure of 9-12 PSI out of the mechanical pump is too high...

I know there isn't a kink up front, but that deosn't mean there isn't one forming in the rear..I will have to get back there and look...

As far as the sock, good question. The tank was replaced the same time te lines were, but I can't remember if the guy who did the work replaced the sender assembly, I am leaning towards he didn't. I would guess at that time (and with how much he charged me for doing all of the work) he would have made sure the sock was clean, but that doesn't mean it is now, although it has only ever had good quality super unleaded. Also, it doesn't mean the thing couldn't have fallen apart somewhat...but since the tank is 3/4 full right now, there isn't an easy way to check...