No fuel at carb on startup

[Mike74Bronco]

I have a '74 302 with a mechanical fuel pump and I have a hard time starting it if it has been sitting for more than a couple of days. I can crank the thing until the battery dies and I still won't have any fuel at the carb. It always fires up the first crank if I pour some fuel into the bowl and it runs great after that so I think the pump is mostly working.

I tested the pump today by disconnecting the fuel line from the filter going into the carb and running the line to a jar. I cranked the engine for a good 20 revolutions and no gas made it into the jar. I then "primed" the pump by hooking everything back up and letting the engine run for a few minutes (had to pour some gas in the bowl to get it started). I then repeated the experiment and sure enough I got a nice squirt of gas every revolution.

How many turns should it take to get gas from the tank to the carb assuming that the fuel lines are starting out empty? I suppose this could be normal behavior and my battery is just too weak to crank the engine long enough. Any other ideas?

 

[helo-mech]

I thought pumps had one way check valve that kept the fuel in the lines when you turned the truck off. This would keep the fuel from following gravity back down thru the line and into the tank. That way when you tuned the key there would be fuel at the carb. I dont kow if you know what kind of mechanical pump you have , but if you do see if you can look online and see if it has a one way check valve.

I am not speaking from any type of experiance with this system. Just speaking from my knowledge of fuel systems.

 

[73stallion]

check your fuel lines, it could be sucking air from a tiny pin hole or crack in the line. might think about an electric pump to prime it also.

 

[Pa PITT]

Mine is hard to start also if it sets 4 days ..or more ..So I put a new battery on ..this helped just slightly, added time ...But if your a young-un that has grown up on efi stuff ...Then now that you got a carb ...heres how I start mine ....PUMP it about 5 to 8 times choke sets ...at this 1st point you want the choke closed ....now spin it over several times maybe 10 times if you know it want start .....stop spinning it ...stamp the pedel several times pump it to the floor completely about 10 times ..then turn it over about 10 more times ..choke should be completely set ....Don't pump while spinning it over this only causes the choke to not stay closed all the way ....when it is closed all the way it will suck more fuel forward ....either manual or auto choke closed needs to be closed ....if it hasn't acted like it want to start repeat ....repeat ....MIne takes 4 times (repeats) to start ......Hint while spinning it over dont push it to the floor ...IF you decide it has flooded then you hold it to the floor and dont pump hold to the floor adds more air ...pumping adds a squirt of fuel only pump if it needs more fuel ....EFI's dont react the same way..........

 

[telcolineman]

The gas will evaporate from the carb if it sets for a while, Crank it over a half dozen times and hit the gas a few times. It allways works for my bronco, And I start it once a month or so

Jim

 

[Highlander]

check your fuel lines, it could be sucking air from a tiny pin hole or crack in the line. might think about an electric pump to prime it also.

I second the part about checking the fuel lines. Mine was doing the same thing and that is what the problem was. I had a small hole/crack right at the first clamp back from the fuel pump. The line was very bridle do to heat I would guess from the header. The hole was above the fuel tank or fuel level so it was not leaking but it was sucking air. At the time it would not start very easy after a day or two, full chock in the middle of summer. Now it start with just a couple pumps of the accelerator.


Tim

 

[acbronco66]

I have a 351w so this may not help but when you start turning the motor over it should begin pumping. Their should not be a delay. I also know if you have installed a new pump maybe you got a bad one. I know firts hand this happens. What type of fuel lines do you have metal or rubber.

Ross Baker
Bama Gang

 

[knack]

I've always had the same problem with my '73. What I do now is turn the fuel tank selector switch to a position half way between MAIN and AUX when I park the truck for a long period of time. This seems to keep the fuel up in the carb when the truck sits - starts much easier.

Makes it a bit harder for someone to steal the truck too - hopefully the jerk would be sitting in the middle of the road trying to figure out how to turn the gas on.

 

[helo-mech]

I've always had the same problem with my '73. What I do now is turn the fuel tank selector switch to a position half way between MAIN and AUX when I park the truck for a long period of time. This seems to keep the fuel up in the carb when the truck sits - starts much easier.

Makes it a bit harder for someone to steal the truck too - hopefully the jerk would be sitting in the middle of the road trying to figure out how to turn the gas on.

I think doing this would be the same as having a oneway check valve that is working. It keeps enough fuel there to start so then the pump is creating the venturie effect. If you try this over the course of a week and it fixes it then I would look having a small hole in a line aft of the valve (or see if the pump you have has a check valve).

Do you have the same problem with both tanks?

 

[knack]

A check valve makes sense. I don't know if the stock mechanical fuel pump is supposed to have one or not. I had the same problem with the stock pump and an aftermarket electric pump.
Both tanks are affected equally. Mike74Bronco didn't say if he had dual tanks.
Any cool helicopter fuel check valves you can send us?

 

[Mike74Bronco]

I do have dual tanks but being that I am still working on getting the truck road worthy I have only been using the rear tank, so the aux tank is just sitting empty.

I will try checking my lines for leaks, or I may just go ahead and replace them all with as easy as it would be. All of my lines are rubber so it would be pretty cheap to do.

Does anyone know for sure if there is a check valve in the fuel pump? I could easily check for this by disconnecting the fuel line at the tank switching valve and run the line to a gas can, start up the truck, then turn it off and see if the gas drains back into the gas can.

 

[helo-mech]

Did you ever get this figured out? I was reading around and found this in another thread. It is not a check valve but it has the same effect.

You need to put the pump at or below the level of the bottom of the tank. Mounting it to the frame puts it above the fuel level, which lets it drain back into the tank. The pump is lubricated and cooled by the fuel running through it. If you put the pump at a level equal or lower than the fuel level, it will always have fuel in it and it won't need to prime. When you hear the pump running with the key on before you start it and it takes 10 seconds to prime, it is running dry and is burning itself up. It shouldn't need to prime itself ever.

Just wondering.

Just saw this as well.

I ordered a Carter 4070 from Summit along with a fuel check valve. I am going to mount the check valve as close to the fuel tank as I can. Hopefully that will keep the line charged as much as possible. The 4070 has a little higher output pressure than the Napa pump I was using. Hopefully this should help. According to my calculations you should only loose 1/2 - 1 psi through the hose and 2.5 ft height gain.

I believe this guys problem was about the same as you. He was loosing his fuel pressure and his pump was trying to re prime everytime he turned the key on. The difference was his would start, it was just burning pumps up like nobodies buisness.

 

[Mike74Bronco]

I won't get a chance to look at things until this weekend, but so far a hole in the line somewhere makes the most sense. I'll let you know what I find. Thanks to everyone for all their input!

 

[Mike74Bronco]

I just checked my fuel lines and I can't find any leaks. The rubber is a bit brittle so I think I am going to go ahead and replace it anyways though.

I am now suspicious of my fuel pump. I disconnected the fuel line at the rear gas tank and connected a vacuum gauge to the fuel line. I then cranked the engine and watched the vacuum in the fuel line. In would go up to about 1 in Hg, and then up to 1psi of pressure. It followed this pattern for each rotation of the engine. It seems to me that if it doesn't maintain a small amount of vacuum in the fuel lines then it won't be able to suck gas from the tank when the lines are dry. Anyone know if this is how the fuel pump is supposed to behave?

 

[Mike74Bronco]

I replaced the fuel pump today and now it works like it should! It took maybe 15 turns of the engine with the fuel lines completely empty and then it started right up!

I'm thinking that helo-mech was right about the check valve. The old fuel pump would create a small vacuum when the diaphragm went one way, but then when it went back the other way it would just blow air back through the fuel line because the check valve wasn't working. Thus it wasn't creating enough vacuum to suck the fuel through an empty hose. The new fuel pump on the other hand creates a nice 5-8 inHg of vacuum in the fuel line while cranking and it holds that vacuum after you stop cranking. Since it holds a vacuum now there should always be fuel at the input of the fuel pump (unless of course the tank runs dry).

Thanks for all of your input! It's going to be nice not having to take the carb apart to put fuel in the bowl anymore!