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F head miss under load

Discussion in 'Early CJ5 and CJ6 Tech' started by jasonjp62, Apr 4, 2014.

  1. Apr 4, 2014
    jasonjp62

    jasonjp62 Member

    Pennsboro WV.
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    Ok guys I bounce something off of you F head guys?
    I have a bad miss and loss of power in my F head when you are at cruising speed and engine is up to operating temp. I can run around all day long at 30 in town and it runs great, off road it performs great. just when you get it up in the RPM range is where the issue shows itself. And it seems to be getting worse. It holds great oil pressure.
    So far I have changed the coil, plug wires, cap/button, points/condenser, the plugs all looked good and was at proper gap. I also have changed the fuel pump, carb, and filter. I have checked the gap on the valves, and timing. and it has fresh gas in it.

    Could I have a burnt valve or a cam that getting flat? would a flat cam miss at all speeds? and would the oil pressure be as good as it is? And my experience with burnt valves is that it will smoke, and it does not smoke at all.......I'm at a loss. Any Ideas?


    Thanks guys!
     
  2. Apr 4, 2014
    jpflat2a

    jpflat2a what's that noise?

    Hermosa, SD
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    no smoke for burnt valve, but it would be spitting out the exhaust at idle.
    sounds more like a spark plug breaking down
    I'd change em out unless you can isolate the affected cylinder
     
  3. Apr 4, 2014
    Walt Couch

    Walt Couch sidehill Cordele, Ga. 2024 Sponsor 2023 Sponsor 2022 Sponsor

    cordele, Ga.
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    Runs fine about 30 mph but fails at higher rpm may mean the spark advance system is not doing it's thing.
     
  4. Apr 4, 2014
    PeteL

    PeteL If it wasn't for physics, and law enforcement... 2024 Sponsor 2023 Sponsor 2022 Sponsor

    Hills of NH
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    "a bad miss"

    Is it clearly one cylinder, or all of them generally rough running?

    If it's a single cylinder I'd be wondering about a sticky valve, or something that specific to that cylinder, such as a spark plug. If all four, check the timing and advance like Walt said.

    When you say "under load," do you mean like lugging uphill? Or do you mean speed?
     
  5. Apr 5, 2014
    jasonjp62

    jasonjp62 Member

    Pennsboro WV.
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    "When you say "under load," do you mean like lugging uphill? Or do you mean speed?"

    Pete it does it at speed, I have a long hill just past my house and it will lug up it at 35 in 3rd gear no issues, it seems to really show itself once you get up to operating temp.
     
  6. Apr 5, 2014
    PeteL

    PeteL If it wasn't for physics, and law enforcement... 2024 Sponsor 2023 Sponsor 2022 Sponsor

    Hills of NH
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    Then look at ignition timing, distributor advance function, points (clean and adjust).
     
  7. Apr 5, 2014
    Southtowns27

    Southtowns27 Custom Title

    The Backhills of...
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    Check for a vacuum leak. Also double check the point gap. Couldn't hurt to do a compression test either
     
  8. Apr 5, 2014
    jasonjp62

    jasonjp62 Member

    Pennsboro WV.
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    Pete is on to it here. I haven't gotten it figured out completely yet but It is something in the timing, or mechanical advance. gonna mess with it Monday.
     
  9. Apr 5, 2014
    1960willyscj5

    1960willyscj5 Well-Known Member

    Mesa, Arizona
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    Could also be worn shaft bearing surfaces for the distributor
    .
     
  10. Apr 5, 2014
    1960willyscj5

    1960willyscj5 Well-Known Member

    Mesa, Arizona
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    Or your electrical system might be marginal. Battery close to the end of it's life, Generator (or alternator, depending) needs new brushes. That sorta thing.
     
  11. Apr 7, 2014
    jasonjp62

    jasonjp62 Member

    Pennsboro WV.
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    Thank you guys for the ideas, Walt and Pete pretty much nailed what it was, I tore the distributor apart cleaned everything up adjusted the points and timing and she is running like a top. even better than before the issue started.

    Thanks again for all the help!!
     
  12. Apr 7, 2014
    Alan28

    Alan28 Well-Known Member 2022 Sponsor

    Châtillon en...
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    Same for me. I cleaned the carb, changed one spark plug, make a better positioning of timing and my Hurricane is like a new one.
    In fact they are very strong engines but what is around is to be change sometimes. These machines are not young.
    I have a direct oil pressure gauge and water temp, a good carb, but to check the electric parts is more difficult.
    I understood that my ground connexion was bad (with ohmmeter) so now all works well.
     
  13. Apr 7, 2014
    Walt Couch

    Walt Couch sidehill Cordele, Ga. 2024 Sponsor 2023 Sponsor 2022 Sponsor

    cordele, Ga.
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    Well, well, well. What a successful day all around. Enjoy.
     
  14. Apr 8, 2014
    jasonjp62

    jasonjp62 Member

    Pennsboro WV.
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    Ok it's back, coming home this afternoon it started this back up. I was really listening to it and it is popping thru the carb. It would hardly run as long has I had my foot on the gas. I pulled off the road and it idled great. sat there for a couple minutes at idle. Then pulled back out and got on it and it ran great again. It's like it was running out of gas. It has a new fuel pump, new lines, new filter. I did notice my inline filter was dry when I got home.

    Gonna run it tomorrow if it does it I'm going to take the top of the carb off and see if the bowl is empty. This is the second fuel pump I have put on this thing since it started this.

    What do you guys think?
     
  15. Apr 8, 2014
    Walt Couch

    Walt Couch sidehill Cordele, Ga. 2024 Sponsor 2023 Sponsor 2022 Sponsor

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    Do you have a sealed fuel cap? The carb fuel bowl won't have any gas in it if there is not any in the filter. May have restriction in the fuel line from the tank.
     
  16. Apr 8, 2014
    PeteL

    PeteL If it wasn't for physics, and law enforcement... 2024 Sponsor 2023 Sponsor 2022 Sponsor

    Hills of NH
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    "May have restriction in the fuel line from the tank."

    Try back-pressure on the line to clear any crud at the tank end.

    Also be sure there is no air (vacuum) leak in the line from the tank to the pump
     
  17. Apr 8, 2014
    Southtowns27

    Southtowns27 Custom Title

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    Given the new info, you likely have a restriction between the pump and tank. Happened to me twice actually. Take the filler cap off the tank, and take the line from the tank off the pump. Blow air back through the line into the tank. Reassemble and test.
     
  18. Apr 9, 2014
    jeepcj

    jeepcj Member

    cincy, Ohio
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    agree with the restriction from tank to pump. I just went through something similar to this. Mine would run great, then out of know where it would start sputtering, if I kept fluxuating the gas pedal, sometimes it would smooth out, but most of the time I had to pull over and as soon as I pushed the clutch in, it would die, wait a min with pump on and it would start and drive again for a while. Turned out to be fuel filters, I had one before the pump and two after, I changed the two after the pump and problem was still there,, I changed the one between the tank and pump and no more problem..
     
  19. Apr 14, 2014
    FinoCJ

    FinoCJ 1970 CJ5 Staff Member

    Bozeman, MT
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    Jasonjp62 - any update? Was wondering if you found a solution.

    I have been having a similar problem with a V6 - intermittently seems to get fuel starved at highway speeds in 3rd gear + OD (and most notable when going up slight incline). Changed the inline filter (between tank and pump) and also blew air back through line into tank. I checked fuel pump pressure a couple months back and had the proper 3-4 psi and did all the timing advance stuff as well. Runs great in town up through 3rd gear, but sometimes seems to sputter on the highway with OD - other times it runs beautifully.

    Wondering about checking float level in carb - another opportunity to do and learn something new I guess.
     
    Last edited: Apr 14, 2014
  20. Apr 15, 2014
    tymbom

    tymbom Member

    Siskiyou Co.
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    My brother in law had a problem like this once with his cj5. Turns out there was some crud in the tank that was too big to get in the fuel line, but it would plug the pick up. He cleaned out all the fuel lines and changed filters and pumps before pulling the tank one day. He dumped out the tank and strained the fuel and found the crud. Worked fine after that...
     
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