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HELP - 72 FJ40 Running rough at full throttle

Discussion in '40- & 55-Series Tech' started by BB, Feb 16, 2004.

  1. BB

    BB

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    Hopefully someone can shed some light on this problem. I have a 72 with the original F motor (~200K) and DUI dist. Carb was rebuilt 2 years ago by Jim C, new OEM fuel pump within the last year, and the rig has been running fine until a few weeks ago. While driving home from work on the freeway at 60-65 at 2800-3000 rpm, I got on it to pass someone, and the truck lost all power. It would run fine until you gave it full throttle, then it would feel like the brakes were being applied. The motor would stumble and it would backfire thru the tailpipe. I made it home and parked it. Today I had a chance to look at it - opened up the top of the carb to check for debris in the float bowl, checked gas flow to the carb, fuel filter, accelerator pump, etc. All OK. Sprayed some carb cleaner in the carb and took the truck out for a ride. Same problem at freeway speed, but ran great around town, even with accelerating from 35 to 60 MPH in 3rd. Anyone have any idea what is going on? This problem came on suddenly, which leads me to think it may be something like junk in the fuel tank clogging the fuel line or something like that. All else seems OK. Have not checked ignition, but assume since it runs fine at all but freeway speed, it may be fuel related, just because of how it behaves. Thanks in advance for your help.

    Brian
     
  2. Bryan-71

    Bryan-71

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    65mph in a '72 wow, I am confident my '71 would explode at anything over 50mph...haha, no advice, just impressed you can get that kind of speed out of it.
     
  3. dd113

    dd113

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    Replace the fuel filter and check output on the fuel pump. Also, check and see if you have any wobble in the dist shaft.
     
  4. Jukelemon

    Jukelemon

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    I would guess it is a fuel delivery issue. Tuning/timing issues typically do not have the sudden loss of power symptom you describe when throttle is fully applied. I am assuming that you have the original carb? Not sure about the original carb but I imagine it is most like others in that you have a main jet. Possibly it is clogged or possibly your butterfly is not opening enough to account for the fuel delivery in full throttle mode. Float could be sticking as well not allowing full fuel delivery into the main jet. I would tear down the carb and thoroughly clean it/inspect it.
     
  5. Jukelemon

    Jukelemon

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    Ditto to dd as well. I had a harley that would run great up until 50 mph and then it would die. Turned out to be a bad pet cock i.e. not allowing enough fuel into the float to keep up with the needed supply for the main jet. Anything below 50 was beautiful though.
     
  6. toddslater

    toddslater

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    Yep, sounds like a fuel starvation problem at the top end. Worth replacing the fuel filter and check the strainer (little brass bastage accessed directly above the float needle/seat). These are cheap and easy..if they don't work..maybe we look into the fuel pump....maybe getting weak. Do the cheap easy first.