Fuel pump loosing prime?

[mriplaybass]

Now that the warm days are becoming few and far between, our 79 CJ7 is sitting for longer periods of time. There are, however, the occasional warm days, and we can't resist taking another ride before putting the Jeep to bed for the winter. Since we built this Jeep last spring, it has performed virtually flawlessly. But now, it sometimes sits for a week or more without being started. It recently sat for over 3 weeks following my knee surgery.

It appears as though the fuel pump is loosing prime when it sits for long periods of time. Everything in the fuel system is either new, or has been thoroughly cleaned. Rebuilt carb, new fuel filter, fuel pump, fuel tank sending unit and pick up tube. The fuel lines were all blown out, and the canister lines were replaced.

After sitting for awhile, all the cranking in the world won't start the engine. At least, I'm not going to crank until the battery goes dead . All it takes to get the engine started is a shot of ether through the air filter, or a dribble of gas down the carb throat. The engine fires up and runs great. As long as I start it every few days, I never have to resort to ether or gas down the carb throat, but let it sit a week or more, and I have to "persuade" it to start.

I do have the float set a bit low to prevent spilling when on steep hills, and I am still running 45 jets from our Colorado trip instead of the usual 49s, but I can't see where either of these items could cause the carb bowl to dry out and the fuel pump to loose prime.

Any ideas

 

[Hedgehog]

Sounds like she needs a little "get goin" drink to me. Your low float setting and jets could easily cause this to happen. Also, how is your accelerator pump? Even here in Arizona my CJ takes some persuading to start if she sits a while, like yours she fires right up if it's seen regular use. Mine doesn't need a drink though, just a little healthy mashing of the gas pedal to prime with the accelerator pump.

 

[Lumpy Grits]

Make sure ALL the fuel line fittings from the tank to the fuel pump are TIGHT and therefore not suck'n air.
Mechancial fuel pumps are 'sucker' type pumps.
Also-Change the fuel filter and see what happens.
LG

 

[Torxhead]

I have heard of issues with cracks in the rubber fuel lines when they get old and can just suck air. One line that is hard to keep an eye on is coming out of the fuel tank, which you did not mention replacing and there is also the return line. Also carbs can dry out thru the vent lines adding time for firing up after sitting awhile.

 

[Posi]

After sitting for awhile there should be plenty of fuel in the bowl to start the engine.
An easy way to check this is wait a few days and then open the air cleaner, poke your head in there, and manually push the throttle plates. Look for gas out of the accelerator pump. If you get some, the issue is not fuel leakage.

If you don't see fuel shooting out this means the bowl is empty. There is no way for the fuel to be picked up from anywhere below the needle and seat, this means its most likely leaking from either the metering block or the accelerator pump gasket not sealing properly. There is a weep hole on the bottom of the AP (front of carb).

 

[Hedgehog]

Or the fuel return line from the filter might move some fuel, but not empty the pump.

 

[Posi]

If it were me I would focus on the fuel bowl
You could remove the fuel pump, fuel tank, all the inlet and outlet fuel lines and plug the carb, if the bowl holds fuel it will start.
If the bowl has no fuel it's leaking.
Take a look in a fuel bowl, not many places it can leak from other then what was mentioned, or a possible crack but not likely.

 

[mriplaybass]

Hedge:
The carb was completely rebuilt including a new accelerator pump.

Lumpy: I will recheck the rubber hose clamps. I made sure they were tight when we did the initial build last spring, but they may have loosened up.

Torx: I did forget to mention that every hose was replaced last spring. The fuel filter is also new and in correct orientation.

Posi: I did forget to mention that the very first time that I had this problem, the first thing I did was to remove the air cleaner and pump the accelerator. No fuel squirting down the throat.

Obviously, the bowl is drying out for some reason after sitting for a while. Why?
Even with the lower fuel level setting on the float, (and this is only slightly lower than spec) the engine runs great. It just won't start after sitting several days. Could it be possible that the float needle is not seating completely allowing the fuel to drain back via the return line? Seems odd that the bowl could dry out completely through the float needle.
Also, why is the fuel pump not filling the bowl? I crank the engine for 10 to 15 seconds maybe 3 or 4 times before giving up and priming the carb manually to start it.
Sounds like I may have a double problem, leaking rubber hoses from the fuel tank to the fuel pump which is causing air to enter the pump supply line, and a possible float needle seat leak.

 

[Fastatv]

I'm experiencing similar with my CJ7 . Going to follow here for learning purposes.

 

[Torxhead]

Bass, how about trying some sort of heat isolator between the manifold and carb?

 

[mriplaybass]

This is not a heat issue. I can run all day long, and the engine will start right up every time. The problem is after it sits for several days, the carb runs dry, and the fuel pump will not refill the carb bowl by just cranking the engine. Oldjeeplady crawled under the Jeep today (I am still recuperating from knee surgery) and found a few hose clamps that were able to be tightened a little more, but it seems unlikely they were egregiously loose.
The next time it sits for several days, we just might bypass the mechanical fuel pump and use an electric pump to see if that makes a difference. If it does, that will raise a multitude of new questions.

 

[Lumpy Grits]

How old is the FP?
LG

 

[mriplaybass]

New in July, about 800 miles on it.

 

[Posi]

Could it be possible that the float needle is not seating completely allowing the fuel to drain back via the return line? Seems odd that the bowl could dry out completely through the float needle.

You answered your own question, and your correct.

When you manually add fuel the jeep fires up, when you repair the leak in the fuel bowl it will do the same.

 

[mriplaybass]

You answered your own question, and your correct.

When you manually add fuel the jeep fires up, when you repair the leak in the fuel bowl it will do the same.

So let me go further out on the limb of answering my own question. Is it possible that since I am intentionally running less fuel in the carb bowl, the float is not putting enough pressure on the needle to properly seat it?

 

[Posi]

This is possible, but that would add fuel to the bowl.
If it were me I would set the float to factory specs and inspect the accelerator pump/gasket.

 

[Flex BT]

I have a weber 38 with the same problem, perhaps not as pronounced. In mine, it will need a few extra cranks after about 3 days. After a week, it will need about 20 seconds of cranking to get fuel from the pump to the carb and flowing. I have a pressure gauge right before the carb and it will read zero until a few cranks before she fires up, leading me to believe both the bowl is empty and my lines are being drained. I was thinking about temporarily installing some kind of clear fuel line between the pump and the carb to see if I could track down the culprit. As with the OP, I have a newer pump and believe all my clamps are tight.

 

[Posi]

While your in there inspect the elastomer valve, kinda looks like a rubber tack that you pull into the bowl from the power valve side. Sometimes they break off causing a leak.

 

[Neuner]

Had an electric FP on my old AMC 258 i6 / 4.2l . Whether it sat overnight or for an extended amount of time, didn't make a difference in how it started since the fuel siphoned/drained back down the line. With the electric it was nice because it would kick on the pump with the click of the key, which I held in position for a few seconds, and start filling the fuel line before trying to start the engine. For some reason, pumping the pedal seemed to accelerate this process by maybe burping the air out of the line? Would start right away.

If I didn't let the pump prime the fuel line for a little bit, it took longer for the engine to start.

 

[mriplaybass]

Had an electric FP on my old AMC 258 i6 / 4.2l . Whether it sat overnight or for an extended amount of time, didn't make a difference in how it started since the fuel siphoned/drained back down the line. With the electric it was nice because it would kick on the pump with the click of the key, which I held in position for a few seconds, and start filling the fuel line before trying to start the engine. For some reason, pumping the pedal seemed to accelerate this process by maybe burping the air out of the line? Would start right away.

If I didn't let the pump prime the fuel line for a little bit, it took longer for the engine to start.

The reason pumping your accelerator quickens the starting process is that pumping the accelerator will squirt fuel into the carb throat. That is my problem, no fuel in the bowl for the accelerator pump to squirt. If suggestions posted earlier don't work, electric may be the route I will choose...............Or maybe I just need to drive our baby more often!!