View Full Version : fuel flow problems... 74 FJ40 F eng, weber

08-04-2010, 02:30 PM
ay carumba! the demon of fuel flow is visited upon me...

truck acts like it is running out of gas... getting worse. stalling.

new fuel filter, noticed not much gas in the line.
hooked up the guage and bounces between 1 and 2-3 lbs of pressure... not enough! shouldn't i be reading 7 lbs on this old tub?

Now, how do i know if the pump is bad? I've seen advice on "blowing out" the lines... I have a small compressor for tires, could hook up a regulator on that.

If I blow the line before the pump back to the tank, can I do damage with too much pressure?
Anyone "snake" these out with anything? what works best?

Are these old mechanical pumps servicable? Can I replace a diaphragm in there?
the overall problem gets' worse as it heats up. read where someone had a pinhole in the diaphragm that got worse with heat.

I am harbor freights new best friend. hope these chinese tools work a few times at least (pressure guage, timing light, rpm, etc.)


Air Randy
08-04-2010, 02:43 PM
If I blow the line before the pump back to the tank, can I do damage with too much pressure?

No, Ive done this before on mine using 100 psi, just make sure your gas cap is removed. Shoot the air in right before the fuel filter back towards the tank.

If there is an indication of a blockage that the air doesn't clear, disconnect the rubber portion of the fuel line between the metal lines and the tank. Hit it with air again at the same place, if it still acts blocked then the obstruction is in the short metal portion of the line. Check that for kinked or crushed spots or replace it. If the metal line is clear then just replace the rubber hose from that point to the tank. Hit it with air again, if its still blocked the obstruction is in the tank.

Chances are it's just a bad fuel pump. You can buy these new from NAPA and other parts places for about $60, probably easier to replace it than to rebuild the old one.

08-05-2010, 08:01 AM
thanks Randy, will try w my compressor. See you this weekend!

08-05-2010, 03:46 PM
blew out the lines, more importantly put in new fuel pump from Napa $74 and all is right with the world!!

Now to get the carb adjusted/ timing set up!

08-05-2010, 03:56 PM
Congrats on getting it resolved so quickly :thumb:

When you figure out your Weber tuning feel free to come by and do mine :D

08-09-2010, 11:47 AM
new fuel pump looked to solve the problem, but after 3 days of running good...
back to the same, rough running, stalling, no power, just gradually getting worse than before...

will run pressure check on new pump.

clean bowl.

could this ge in the ignition system? plugs look good, could use a new cap. electronic points.

fuel return line cause this maybe? if it's plugged? pvc valve?


08-09-2010, 12:18 PM
The PCV valve burned me once. The symptoms are running great then it just stops. Easy enough to prove out...plug the hose.

Is your Weber the 38 or the 32? I restrict the feul pressure to my Weber at 3.5psi. Corsair mentioned tuning the Weber so here is the data for that.

I bought all my jets from www.carbs.net (http://www.carbs.net/) a little spendy but I guess about average.

5280 elevation
Weber 38/38

135 Main------------->73801.135
190 or 200 Air------->77201.200 Either will work but depends on altitude if lean or rich.
50 Idle Big----------->74409.050
50 Idle Small-------->74403.050
70 Pump------------->76215.070 Weber came with the 70 pump

I have the 38 DGEV on my 78. I went through tons of $5.00 jets to get it right. The most important thing to check is the adapter that comes with most Webers. It is usually two plates and the screws are too long. You will never be able to cure the vacuum leaks on a Weber unless you grind off the last two threads on both sets of screws. I did this and no longer have any vacuum leaks or that, oh so nice, whistling noise under the hood. A good rule for adjusting the jets is one stop for every 3000 feet in elevation. Use the main jets for major changes and the air for fine tuning.

08-09-2010, 02:36 PM
Awesome info Mike :thumb:

I think I only understand 1/2 of it but that is a lot more than I understood/knew 5 mins ago :)

08-09-2010, 03:58 PM
I can not find any i.d. on this carb.

any advice on id'ing the carb? I can find a serial # 10312 on one of the pumps. that's it.

It doesn't look like any carb i can find on the web. came from JT outfitters. None have the same fuel bowl I do.

I think I may need to do some tuning, but I can't seem to get gas to the damn thing.

I blew out the lines from pump to tank. a little resistance then "puff" with 10-15lbs of pressure, then line blew clean.
blew from the carb into the return line, same thing built up to 10 lbs of pressure, then "pop" and blows free.

I can not register more than 1-2 lbs of pressure after the pump and only about that much vacuum before the pump.
Carb bowl has less than 1/2 inch of fuel. Is perfectly clean, float seems okay, no crap in bowl or inlet.

I either got a bad pump, or something in my tank is obstructing flow on and off. Could that have "burned out" the new pump.

08-09-2010, 04:18 PM

a 34/34 dgec according to Jt outfitters. cable adapted to rod linkage.

Air Randy
08-09-2010, 06:30 PM
The 10 psi pressure build and pop you're feeling is probably just from pushing the fuel in the lines back in to the tank. Once it's all out of the lines it blows freely, thats normal.

Where is your fuel filter at? Have you replaced it? It's not likely that you got a bad fuel pump or that it was damaged if the fuel line to it from the tank got restricted again.

I had the same exact situation with my 40 2 years ago at Moab. Idled OK but acted like it was running out of gas at higher rpm. I had replaced the fuel tank sending unit a few weeks before and had used black silicone to seal around it but apparently I didn't use the gas resistant type. Some of it melted into the tank and plugged up the fuel filter which was right before the mechanical pump. So the pump was working really hard to get what little fuel it could. I replaced the filter and the pump worked fine with the float bowl filling correctly.

08-11-2010, 09:35 AM
thanks Randy

and great info Mike! I think that will be my next project, rejetting and setting up the carb for altitude.


It's been a while, am I advancing my timing when I turn the distributor clockwise or counterclockwise?

I finally found the marks behind the windo near the starter.

And: the bad "stumble" upon acceleration, is it a carb jetting problem, timing or both?


Air Randy
08-11-2010, 09:49 AM
I think you should focus on the fuel problem for now. Point is, the truck was running fine then started having problems. You blew out the fuel line and replaced the fuel pump and it ran fine again for 3 days until the problem returned. Now the same symptoms have returned. You may have some rust or crud in your fuel tank blocking the pickup tube. Trying blowing the line to the tank again then test driving it. If it runs OK again even for a short time that is a key indicator. The intermittent symptoms don't seem to point to it being a carb problem, hold off on the rejetting and such for now.

My point in sticking to one issue is if you start changing other things at the same time you may never know what caused the original problem.

If you are able to rule out it being a fuel problem, then I would check the distributor. If the timing advance in the distributor is getting flakey it could cause the intermittent symptoms you describe.

Once you know everything else is good and it is running as good as it used to, then re-jet and adjust the carb.