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Thread: Fuel Problem

  1. #1

    Default Fuel Problem

    I am having some problems and need some advice. My truck has been running pretty poorly the last 2 months. It started off stalling when I came to a stop, then it would start right up again. It also started sputtering when the engine was loaded and having some black smoke when cold (fouling out my spark plugs pretty bad).

    Replaced the fuel pump and rebuilt the carb. Ran great for 2 days, now it's doing it again. I checked timing, points, coil etc...its getting plenty of spark. What did I miss?
    1969 M715 WO/W

  2. #2


    When you rebuilt the carb, did you set the needle valves back to where they were? What are they at right now? Mine would idle "OK", but stall at a stop. I adjusted my A/F ratio and she idles great now, and no more stalling problems. I also got more power from the engine during climbs and what not from getting a better A/F ratio. Have you checked or replaced your fuel filter? I'm not sure if the above stated is your problem, but just some ideas.
    Last edited by rpgdeity; April 29th, 2007 at 12:59 AM. Reason: Spelling
    68 M-715
    67 M-725

  3. #3
    Join Date
    Feb 2004
    Silver Lake Sand Dunes MI


    Also check to make sure that you are getting good flow from the tank and that the pick up is not clogged up. There is a little screen at the end and they clog and deteriorate over time. Some members have had good luck blowing back throught the line with air to nock the screen off the pick up and restore full flow back to the fuel pump.

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  4. #4


    Quote Originally Posted by rpgdeity View Post
    When you rebuilt the carb, did you set the needle valves back to where they were? What are they at right now? Mine would idle "OK", but stall at a stop.
    I set the needle valve like the manual says, one full turn, but it runs very rich there. The other problem getting this right for me is the valve seat on the idle mixture is cracked, but I got it running fine the first time I rebuilt the carb. It runs great with the idle mixture barely open (i guess because of the crack in the seat). I'm going to try and set it back to super lean and see if that helps. It does act like the choke is smothering the engine.
    1969 M715 WO/W

  5. #5


    Quote Originally Posted by Beast View Post
    Also check to make sure that you are getting good flow from the tank and that the pick up is not clogged up.
    I got gas, I wonder though if maybe too much. The replacement pump puts out the average 8oz in 15 seconds in standard volume test. That amount is recommended for most cars with mechanical fuel pumps, but is it OK for the M715? The old pump only put out 2.5-3oz in the same time, but it still worked, sort of.

    However, when I replaced the pump, it ran great 2 days, then began messing up again (stalling at stops and running rough).
    1969 M715 WO/W

  6. #6
    Join Date
    Dec 2003
    SF Bay Area, CA


    The cracking around the idle mixture needle seat is somewhat common on these old carbs. Not much you can do about that. I think the carb is still the problem. Maybe the float is sticking, the float needle and seat are not sealing because of debriss, or the float level is to high. The fuel level in the float bowl can be checked with the carb on the engine by removing the "power valve" from the top of the carb (three screws holding down a cap thingy). Measure from the top of that surface to the fuel (think dip stick). There's a spec for that somewhere. Maybe someone knows off hand.
    Concerning the fuel pump, from the maintenance manual:
    b. Fuel Pump Pressure Test.
    (1) Disconnect fuel inlet line from carburetor inlet pipe.
    (2) Attach a fuel pump pressure test gage to the fuel line.
    (3) Run engine at idle speed (600 rpm) on gasoline in carburetor bowl and observe indication on pressure gage.
    (4) If fuel pump is operating properly, the pressure will be 3-1/2 to 5-1/2 psi and will remain constant. If the pressure is not within 3-1/2 to 5-1/2 psi, the fuel pump should be replaced.

  7. #7
    Join Date
    Dec 2003
    SF Bay Area, CA


    Wet float level, from the -34 maintenance manual:
    (5) Checking wet fuel level.
    (a) With engine running and vehicle on a level floor, fuel level can be checked or measured through the economizer diaphragm opening.
    (b) Using a 6-inch scale with a depth gage, measure the distance from the machined surface. (Fig 4-12) The measurement should be 3/4-inch. If not correct,
    readjust float setting by removing fuel bowl and adjusting float tab using needle nose pliers.

  8. #8


    I'll give it a try. I think the carb is the problem too.
    1969 M715 WO/W

  9. #9


    I checked everything. Fuel flow from tank is OK. Fuel Pump is operating normal. Carb float is perfect. I took off the economizer and the fuel level checked out as per the manual. I went ahead and re-measured the float setting and its still at 3/16. Its getting all the gas it needs at the carb and pump.

    The problematic symptoms gradually started up over 2 months of driving. I still think this is a fuel problem, but darned if I know where.

    Maybe the carb has had it? Hmmm...what's next? M715's are more stubborn than I am!
    1969 M715 WO/W

  10. #10
    Join Date
    Dec 2003
    SF Bay Area, CA


    looking down the carb venturi (using a mirror and safety glasses incase of a backfire), you should not see gas dribbling out of the accelerator or main metering outlets at IDLE. The throttle plate and venturi should look dry at idle. Just thinking about the rich mixture you discribe.

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