Fuel System: Must prime the carb to start - questions about siphoning, fuel lines and tank

[BruceD]

I have a '58 Silver Hawk that I need to prime the carb with gasoline each time I want to start it. I spoke with an experienced gentleman at a carb shop who recommended replacing the fuel lines before focusing on the carb, for a possible siphoning problem. While under the car checking the hose size I will need i order to replace the fuel lines, I noticed a seeping leak in the gas tank. Wondering if this could be enough to result in some siphoning back into the tank over the course of a day to result in needing to prime the carb again. Shouldn't the fuel pump prevent back flow through the line from the carb? Also wondering how difficult dropping the fuel tank out of a C/K body is, since I will need to address the fuel pump leak. Would a check valve downstream of the fuel tank help in the mean time - I don't know if that is a common practice or not.

[41 Frank]

I doubt very much the gas is syphoning back to the tank. Most likely you are experiencing the same problem as many of us with fuel evaporating in the carburetor if the vehicle sits for several days. Another reason myself and others have installed electric fuel pumps to prime the system before starting.Lots of reading on this subject here by doing a search. None of this applies if the vehicle only has to sit one day and still has to be primed. Checking for pin holes in your fuel lines would also be a good idea. You are correct that the fuel pump has a check valve to prevent syphoning.

[RadioRoy]

Most likely it is not siphoning, but rather it is evaporation. Modern fuels evaporate very quickly. There is a good discussion in the latest issue of Turning Wheels.

I mount an electric fuel pump on the frame by the gas tank, and power it through a switch from the accessory position of the ignition switch. That way, I can prime the whole system before attempting to start the car.

I forget what it is called, but this is the type of pump that the mechanical pump can pull gas through. The electric pump is only used for starting and for when the car vapor locks.

Check the seeping leak to see if it is from one of the rubber lines hooked up to it. If so, you may be able to replace them without dropping the tank.

Work safely and have a fire extinguisher as well as a garden hose handy.

[Mike Van Veghten]

Yep, that's all it is, evaporation, thanks partially to the added alcohol, which evapor. faster thAn gasoline.

Funny though, I removed the Edelbrock carb. from my Lark and reinstalled the original 2 barrel. With the 4 barrel on, the fuel would high enough to start the engine for about 6 days. Now that the stocker is back in place...it only lasts 2 days...before the engine needs to spin to get the fuel into the bowl.

Mike

P.S. No such thing as syphoning...! You need a direct connection between the fuel and the inlet line, and that doesn't happen in a carburetor... The float height keeps that from being so, along with the needle and seat..!

[BruceD]

Thanks. Is the electric pump in line with the mechanical pump? I can see where you would need to pull through the electric pump, but can the electric pump push through the mechanical one?

[E. Davis]

Indeed they do. An electric fuel pump will solve a lot of fuel delivery problems and they are easy to install and cheap to buy. Go to the advanced search and enter electric fuel pumps and it will answer all the things you wanted to know and were afraid to ask.

[Milaca]

Make sure that your choke is closing completely, as a partially open choke will make for difficult starting.

[christophe]

I had the same problem on my 1963 GT Six.
It took a while but I finally traced it to the spongy filter located in the pump bowl. It reduced the fuel pressure to the carb. Works like a charm since I removed it. I placed a little filter in the fuel line before the pump of course. In such cases, always hook a manometer at the exit of the pump to check the pressure.
Best of luck and nice week end to all.

[altair]

It is very possible there is a pin hole in the fuel line, I experienced a similar problem, if the pin hole is small enough the fuel pump will over come it once started however will not pick up the fuel on its own. I tested the fuel line by attaching a rubber hose at the fuel pump and blowing in the hose while spraying soapey water on the fuel line and looking for teltale signs of bubbles. I had no evidence of any fuel leak. Long and short of it replace the fuel line. Most have a connection in the area of the master cylinder, take that apart and the rest of it comes out quite easily. I replaced mine in about 1 hour. Dave British Columbia

[Dick Clemens]

I would advise you to check the accelerator pump. The new fuels dry thhem out.

studedick from the lower Ozarks

[BruceD]

Thanks, Roy. I think I will take this path. One question about the electric pump, did you put a filter upstream of the pump? I have read where this is a good idea. If I do that, then I'm thinking it might also be useful to put in a small valve so I could change the filter without draining the gas tank. Also wondering if you have a momentary action pushbutton or a toggle switch for activating the pump. I would think that a momenatary action would be safer to avoid it getting bumped and left on inadvertently.

Appreciate your help!

[RadioRoy]

Thanks, Roy. I think I will take this path. One question about the electric pump, did you put a filter upstream of the pump? I have read where this is a good idea. If I do that, then I'm thinking it might also be useful to put in a small valve so I could change the filter without draining the gas tank. Also wondering if you have a momentary action pushbutton or a toggle switch for activating the pump. I would think that a momenatary action would be safer to avoid it getting bumped and left on inadvertently.

Appreciate your help!

All of those are very good ideas, and I stamp them "Radioroy approved." That and two bucks might get you a cup of coffee.

[BruceD]

Thanks again, Roy. One more question - since I am only using this to prime the carb, do I need to be concerned with pressure regulation? I bought the Airtex E8012S pump I saw that you and a couple other guys recommended on another thread. It is good for 30 gph, and since I am using it only momentarily to crank the engine, I don't think I need to be concerned with pressure regulation. The rest of the time it will just having the fuel sucked through it by the mechanical pump.

[RadioRoy]

Your scenario is fine for priming to start the car, but there is another use for this auxiliary electric pump and that is to prevent/stop vapor lock.

The pumps generally come in two different pressure ranges. They are, and I am going from memory here, about 5-7 pounds for American cars and about 3-4 pounds for imports. Since our cars require about 5 pounds (except for supercharged cars) which is right in the middle of these two designs, I always use the import/low pressure pumps.

My theory is that I do not want to overload the carb inlet valve with too high pressure. I know that you can buy a pressure regulator, but I like to keep things simple. I use the electric pumps for both priming to start a car that has been sitting, and also to prevent/stop vapor lock in conditions where it has already occurred, or where from experience I know it is about to occur.

That's why I use a toggle switch. The momentary pushbutton switch is good for priming, but if you have to drive in traffic, then you need the pump to stay on. It seems like they never vapor lock while driving down the road, but only when idling, especially in traffic.

Many folks go even further and install automatic cut off switches driven by oil pressure so the pump will not run when the engine is not running. That keeps the pump from filling the car with gas if there is a wreck, or a rollover, or something like that.

Since I am the only one who drives my cars, and I am the only one who knows where the fuel pump switches are and how/when to use them, and since I have never had a wreck or rolled a car, I'm thinking that I'm fairly safe in that regard.

[altair]

fuel sustem

I have a '58 Silver Hawk that I need to prime the carb with gasoline each time I want to start it. I spoke with an experienced gentleman at a carb shop who recommended replacing the fuel lines before focusing on the carb, for a possible siphoning problem. While under the car checking the hose size I will need i order to replace the fuel lines, I noticed a seeping leak in the gas tank. Wondering if this could be enough to result in some siphoning back into the tank over the course of a day to result in needing to prime the carb again. Shouldn't the fuel pump prevent back flow through the line from the carb? Also wondering how difficult dropping the fuel tank out of a C/K body is, since I will need to address the fuel pump leak. Would a check valve downstream of the fuel tank help in the mean time - I don't know if that is a common practice or not.

If you plan to install an electric fuel pump and mount it close to the tank (as recommended) the fuel lines that were previouslly operating under vacuum are now operating as pressure lines. A pin hole operating under vacuum will not show any signs of leakage however under pressure a leak will be very visible. I strongly suggest that you confirm your fuel line is free of any holes prior to installing an electric fuel pump. Vacuum pumps have worked for thousands of vehicles and even if the bowl was empty two or three cycles should fill it up, except if the vacuum is lost. The electric fuel pump may only be masking the problem. Simply blow some air in to it and confirm. Dave British Columbia