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Unread 08-25-2009, 10:47 PM   #1
OldSkull
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1980 CJ7 
 
Join Date: Jun 2007
Location: Southern California
Posts: 170
fuel draining back out of fuel line ?

This may be really simple, but what could cause the fuel to run out of the line into the carb? I mean after several days the fuel line is dry at the carb inlet and I have to prime the carb to get it to run. Is this a sign of a bad fuel pump? I have stock 304 with a newly rebuilt MC2100. It usually stumbles on quick acceleration, but otherwise runs pretty good. Accelerator pump? I am just looking for things so I don't start looking in all the unlikely places.

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Unread 08-26-2009, 06:24 AM   #2
Mike Romain
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The most common reason is an upside down or incorrect fuel filter. The proper fuel filter has two outlets, the center one to the carb and the top one to the return line. If the return line isn't up top gas can siphon back to the tank when it sits. if the return line is missing, then it can vapor lock on a hot shutdown which blows the gas lines clear, which is why they use a return line.

Another reason can be a pinhole leak in the suction line between the pump and tank. This will just leave a stain on the line, but is enough to let air in when it sits, check the hoses at the ends of the steel line, the new mix they call gas will destroy the stock Jeep 'rubber' gas lines, they need an upgrade.

Then the drainback valve in the pump can also fail, though that is more uncommon. To check that, unhook the gas line on the suction side and see how much comes out, only a little dribble should, if the pump empties out, well....
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Unread 08-26-2009, 06:48 AM   #3
ET48
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Quote:
Originally Posted by Mike Romain View Post
The most common reason is an upside down or incorrect fuel filter. The proper fuel filter has two outlets, the center one to the carb and the top one to the return line. If the return line isn't up top gas can siphon back to the tank when it sits. if the return line is missing, then it can vapor lock on a hot shutdown which blows the gas lines clear, which is why they use a return line.

Another reason can be a pinhole leak in the suction line between the pump and tank. This will just leave a stain on the line, but is enough to let air in when it sits, check the hoses at the ends of the steel line, the new mix they call gas will destroy the stock Jeep 'rubber' gas lines, they need an upgrade.

Then the drainback valve in the pump can also fail, though that is more uncommon. To check that, unhook the gas line on the suction side and see how much comes out, only a little dribble should, if the pump empties out, well....
Nicely covered!
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Unread 08-26-2009, 08:34 AM   #4
JeepHammer
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Quote:
Originally Posted by OldSkull View Post
This may be really simple, but what could cause the fuel to run out of the line into the carb? I mean after several days the fuel line is dry at the carb inlet and I have to prime the carb to get it to run. Is this a sign of a bad fuel pump? I have stock 304 with a newly rebuilt MC2100. It usually stumbles on quick acceleration, but otherwise runs pretty good. Accelerator pump? I am just looking for things so I don't start looking in all the unlikely places.
The fuel line can drain with no effects on the vehicle starting/running since there is still fuel in the float bowl that will keep the engine running until the fuel reaches the carb again.

Return lines drain the line,
Valves in the fuel pump leak and let the fuel drain from the lines all the time, with no real bad effects on the system.

*IF*...
You have a carb that is leaking,
Along with the draining lines, then you will have problems with 'Hard Start' when you try and fire up the engine!

IF the fuel in the float bowl drains into your engine after shut off,
OR,
If the inlet valve (needle and seat valve) in the carb allows the fuel line to siphon the fuel back into the tank or vapor collector,
You will have hard start Issues...

Like was stated by Mike Roman,
Orientation of the return line can be an 'Issue',
So can be a worn fuel pump valving.

If the fuel is draining back to the tank, it's just a pain,

BUT,
If the fuel is getting through the carb and into the engine, it's thinning out your oil!

After running a while after oil change, pull the dip stick and smell it.
If it smells like fuel, time for a carb rebuild with fresh inlet valve!
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Unread 08-27-2009, 12:15 AM   #5
OldSkull
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Quote:
Originally Posted by Mike Romain View Post
The most common reason is an upside down or incorrect fuel filter. The proper fuel filter has two outlets, the center one to the carb and the top one to the return line. If the return line isn't up top gas can siphon back to the tank when it sits. if the return line is missing, then it can vapor lock on a hot shutdown which blows the gas lines clear, which is why they use a return line.

Another reason can be a pinhole leak in the suction line between the pump and tank. This will just leave a stain on the line, but is enough to let air in when it sits, check the hoses at the ends of the steel line, the new mix they call gas will destroy the stock Jeep 'rubber' gas lines, they need an upgrade.

Then the drainback valve in the pump can also fail, though that is more uncommon. To check that, unhook the gas line on the suction side and see how much comes out, only a little dribble should, if the pump empties out, well....
Quote:
Originally Posted by JeepHammer View Post
The fuel line can drain with no effects on the vehicle starting/running since there is still fuel in the float bowl that will keep the engine running until the fuel reaches the carb again.

Return lines drain the line,
Valves in the fuel pump leak and let the fuel drain from the lines all the time, with no real bad effects on the system.

*IF*...
You have a carb that is leaking,
Along with the draining lines, then you will have problems with 'Hard Start' when you try and fire up the engine!

IF the fuel in the float bowl drains into your engine after shut off,
OR,
If the inlet valve (needle and seat valve) in the carb allows the fuel line to siphon the fuel back into the tank or vapor collector,
You will have hard start Issues...

Like was stated by Mike Roman,
Orientation of the return line can be an 'Issue',
So can be a worn fuel pump valving.

If the fuel is draining back to the tank, it's just a pain,

BUT,
If the fuel is getting through the carb and into the engine, it's thinning out your oil!

After running a while after oil change, pull the dip stick and smell it.
If it smells like fuel, time for a carb rebuild with fresh inlet valve!
Thank you to both of you. I have read many of your posts over the last couple years. lots of good information and help.

I have looked at the filter and it is sideways, so I will fix that. Also, the 'stumble' I have is a big one. I just did the Team Rush upgrade (and replaced the original distributor) and it ran for about 3 seconds. Not enough time to get the fuel back up to the carb. I had to add fuel 3 times and then it ran fine. The oil does not smell like gas, but the 'stumble' seems bad enough that, eventhough the carb is a Pep Boys rebuilt carb, I may have something wrong with it.
It's been a long time since I have rebuilt a carb and I paid 270 bucks for the one I got (I needed it quick), so I just want to eliminate other things before I open it up ot take it back.
I have checked the fuel pressure, but can't remember what it was...somewhere around 5 or 6 psi, I think. I took the pump off too and it sounded and felt like it was working. Don't know if the drainback valve could have been bad though. I will check that per Mike's suggestion when I fix the filter.

Is the inlet valve problem likely to cause a major stumble on hard acceleration, but not gradual acceleration?
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Unread 08-27-2009, 01:00 AM   #6
sanddrag
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Now, I know you're a 304 and mine is a 258, but I had a similar problem on mine where if left sitting, I had to crank FOREVER to get it to start. Replaced the fuel pump and it mostly fixed that. Ran much better too. For how cheap and easy they are to replace, it's worth doing. I also rebuilt my carb, including replacing the needle seat and readjusting the float. These two things made for much quicker starts.
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Unread 08-27-2009, 06:20 AM   #7
jfwireless
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Location: Naples, FL
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Well I have a 304 and it was always a pain to start when it was carbureted and sat for a while, now a TBI engine. I simply added an electric fuel pump and problem solved, no more hard starts. I pulled the mechanical fuel pump and replaced it with a small block chevy fuel pump blocking plate, and added the electric fuel pump. Some off road warriors leave both pumps in place for redundancy. Adding a roll over switch to the electric feed to the fuel pump is a suggested safety device well worth the effort.

Jim
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Unread 08-27-2009, 08:58 AM   #8
JeepHammer
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I have dual fuel pumps on mine, but for a different reason...
I use the mechanical fuel pump for the engine engine operation, and I use the electric pump for sucking out of 'Jerry Cans', fuel transfer for washing parts, chain saw, ect. when I need fuel for something, the electric pump provides that,
Plus it's two valves and I can power the vehicle off the electric pump if the mechanical pump fails.

The Issue here is the time it's taking to get the fuel to the carb.
Three things can cause this,

One is the condition of the mechanical pump and it's support.
If the fuel pump eccentric inside the engine that makes the pump operate is worn down, the pump will take FOR EVER to get fuel to the engine!
I've run into this many times down through the years.

Second is the pump it's self getting weak.
If the pump internal pressure regulator bypass is old/weak, the fuel can be bypassing INSIDE the pump, and not getting to the fuel filter/carb.

If the intake and exhaust check valves in the pump are old, leaking, weak you might not be building pressure to start with.

Third is the fuel supply lines.
Like Mike Roman covered, if there is a even a pin hole in the line,
Or there is a fitting that is 'Sucking' air instead of drawing fuel between tank and pump,
It will take the pump WAY TOO LONG to get fuel to the carb.

I suggest a 'Clear' fuel filter so you can see if there are bubbles in the fuel,
Or see exactly how long it takes to get fuel to the carb,

I suggest you do a flow test on the fuel supply.
Stick the fuel line in a see through bottle, AFTER THE FILTER, (between carb and filter) to see how much fuel is pumped in 30 seconds of cranking the engine.

I also suggest a fuel pressure GAUGE inserted between filter and carb to see what the RUNNING FUEL PRESSURE IS...

This will tell you EXACTLY what condition your fuel tank/pump system is in (but not where it might be compromised at)

Flow should be about a pint or little more in 30 seconds of cranking the engine...
Pressure should be between 5 & 7 PSI in the line to the carb,
There shouldn't be any 'Bubbles' in the fuel as it fills the fuel filter.

-------------------------

Testing the VOLUME of the pump is pretty straight forward.
Connect a flexible line to the SUCTION side of the pump,
Pump out of a clear container so you can see the fuel it's moving, and try for the pint or more pumped in 30 seconds.

If you don't get very good flow through the pump,
You have a pump or engine fuel pump eccentric problem, and you will need a NEW fuel pump to establish which is the 'Issue'.

If you get pretty good fuel movement through the pump, then empty the container, and see how much runs BACKWARD though the pump back into the container when the engine stops cranking.
This will tell you if the one way check valves in the pump are working or not.

Since you do BOTH tests with the old pump in place, this will tell you if you have bad valving in the pump before you try a new fuel pump...
---------------------------------
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Unread 08-27-2009, 09:08 AM   #9
JeepHammer
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As for the 'Bog' or 'Hesitation' you have right now with the new carb...
That is probably a mis-adjusted accelerator pump linkage.

Might be something else, but since the carb is new, 9 chances out of 10 it's the accelerator pump not delivering enough fuel when you first open the throttle blades.

It's a simple adjustment, and I'm sure Mike Roman has something already up in print or pictures to make things easier...
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Unread 08-27-2009, 09:59 AM   #10
OldSkull
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Quote:
Originally Posted by JeepHammer View Post
As for the 'Bog' or 'Hesitation' you have right now with the new carb...
That is probably a mis-adjusted accelerator pump linkage.

Might be something else, but since the carb is new, 9 chances out of 10 it's the accelerator pump not delivering enough fuel when you first open the throttle blades.

It's a simple adjustment, and I'm sure Mike Roman has something already up in print or pictures to make things easier...
okay, I looked at the accelerator pump linkage and it is in the last hole on the forward end and the top hole on the rear end. Is it possible to blow the accelator pump with a backfire? When I first got the carb, it worked pretty good. I have had it for about 3 or 4 months. I did have an electrical short at the tach that caused some kill/start issues while running and it caused some backfiring.
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Last edited by OldSkull; 08-27-2009 at 10:00 AM.. Reason: typo
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Unread 08-27-2009, 05:26 PM   #11
JeepHammer
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Backfire is MURDER on a MC 2100 with power valve!

Overpressure in the intake/carb will blow out the diaphragm in the power valve and you are going to have problems with idle and fuel metering from that point.

One quick test to see if your power valve is cooked or not,
Turn the idle mixture screws IN, turn them together GENTLY, about 1/4 turn at a time until they bottom out.
DO NOT CRANK DOWN ON THEM ONCE THE BOTTOM!

If your engine is still running with the idle mixture screws all the way in,
the power valve is SHOT and you are siphoning fuel from the power valve enrichment circuit to keep the engine running.

If the engine ran well before you had the backfire, then it's probably something caught in the carb someplace.
Not uncommon to rubber or rust in the fuel lines to make it's way into needle and seat, passages, jets, ect.

That's why we are ALWAYS saying to use at LEAST two fuel filters!
I have a large metal one in the rear next to the tank,
An inline 'Clear' filter between pump and carb,
And there is a 'ROCK' (Compressed Brass Beads) filter in the inlet to the carb.

Two are disposable, and the carb 'Rock' filter can be cleaned with carb cleaner and reused,
And every time I take the top plate off my carb, there is STILL CRUD in the float bowls...
So you KNOW it's all through the carb!

BUT!
Not like this guy!


He complained about no 'Idle' below about 1,500 RPM and 'Running Rough' when it brought it in here and was complaining about the ignition upgrade I did on it 1 year ago (with HIS choice of parts)...





I put a plug wires set on it, (MSD, $70)
And a $13 rebuild kit in the carb and it ran like a top when he left!

One main jet completely clogged,
Both idle mixture circuits completely clogged,
And I put a fuel filter on this carb last year when I did the ignition!
This is all stuff that got passed the filter, or in the carb through the bowl vents!
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