High speed miss -UPDATE- we have a winner.

 I agree Lee it sounds like a spring. That was one of the reasons I went into the valves earlier. I really checked them over because I suspected a spring might be causing my rough running issues. What I found were just very tight valve lifter clearances.     Got that adjusted and she runs perfect….. until it doesn’t.  

    I’m going to say the valve springs were checked , rechecked the next day and not found wanting.  

    They might be weak but they are not broken.    

    Thanks.   

  I’m thinking that this Jeep went out of service as a daily driver due to what would appear to be fuel issues.  

  I make this determination because of the actions of the previous owner.      It has a new steel fuel tank , in-line fuel filter , new electric pump mounted within a foot and below the tank and a new repop carter carb.  All this is plumbed with new fuel hose. I am running a fuel pressure gauge on hose into the cab so I can monitor at speed.   It now holds solid between 3 and four pounds and pumps more than adequate  volume.     

   The carb was so far out of adjustment that one wonders if the previous owner might have made his mechanic mad at some point.      I ordered the carter tools and made the needed adjustments.  

   Float level , idle mix screw, accelerator pump stroke and high speed needle.      It starts and idles perfect. No flat spots on acceleration either slow or fast.   If I never tried to run faster than fifty you would say it runs perfect. 

 I have a temporary Fuel pressure gauge connected at the carb inlet and  mounted in the cab so I can monitor pressure at speed. Pressure never drops below 3.5 pounds ever. I am sure the issue is not a starvation in supply into the carb.    For a while I did have an issue , that’s why the I installed the gauge.  The carb seems to be doing it’s job as well as I can tell. 

  Early when I was convinced I had a fuel issue I tried grabbing a handful of choke during the stumbling period.    My thought was that if there was fuel available in the bowl it would cause an over rich situating and perhaps smooth out indicating a carb problem.  Pulled the choke and nothing changed.    I thought I was onto something.    Next test was to confirm that the test was a valid test.   I tried to prove this by pulling the choke when the engine was running good at high speed.     Nothing happened, no over rich ,  no stumble,  Nothing.     Invalid test.    

    I’m thinking that the high air velocity sucks the spring loaded plate open even though the cable is pulled. 

  The cap that came with the Jeep had aluminum contacts. I never like the use of aluminum for electrical connections or devices subject to corrosion.     I replaced it with one having brass contacts.  I have no idea if it is the correct one.   It was more of a matching up the existing cap with a new one.   

     When you tell the kid behind the counter that the vehicle is a 1943 GPW he then asks what the heck that is.     Well , it’s a Jeep built by Ford for the war department during your grandaddy’s war. 

    The eyes glaze over and out come the old books.     

  I’ve not had much luck.      It’s an IGC 4705  built mid 1943. 

      I should offer it in trade to the GPW guys in trade for a more common one

  I’m sorry I didn’t make myself clear, the miss doesn’t go away when I stomp on the gas.    What I meant was that after I slow down a bit and the miss just goes away that I can then stomp on the gas and it will fire on all four and pull hard , running very well at full open throttle until again getting up to around 55 mph. 

 It runs good at about 50.   Push it to 55 and a single cylinder stops firing.    Only one.   It’s not random but a constant dead hole.  It will continue this running on only three cylinders cruising along and I think it would maintain almost 55 for a long time.    The fourth cylinder never fires like this.  Then if I back off the throttle just a bit, let the speed drift back down to about 50  the miss goes away.    When this correction takes place it needs to be running at this lower speed for ten seconds or so.    It doesn’t correct itself immediately at the lower speed.      THEN  after it has slowed and is running just below the speed where it cuts out it runs very well

    If at this fringe RPM when it’s running very well I can then stomp on it and the engine reacts with a slow burst of speed accelerating nicely under full load for quite some time until I once again cross the threshold where it cuts back to three cylinders.   

    I understand that points or valves floating is constant with RPM and not load dependent.  My concern is that I can not identify a miss or stumble unless the engine is loaded.    

    I ran a twin injected small block boat offshore for years that I knew had a dead hole miss and got compliments all the time on how my engines sounded.  Not once did someone other than myself notice a problem.    It sounded good at idle and under load the miss got lost in everything else happening.   

   Unless you knew how to read the engine monitoring systems  you would never know.   

    The prime monitoring system in my Jeep now is my ear and it ain’t as good as it was.   

      I might be running a little lean perhaps. I’ve thought the spark plug color looked good but ….      I was fighting an overfueling condition early. At that time I was carbon fouling plugs.   In my mind anything other than black was good.  It might be showing a little lean. I’ll give that a closer look.    It’s easy enough to reset the metering rod now that I have the tools I just might try opening it a tiny bit more.    That’s a good thought.   Thanks 

Why could you not post that here in this post? Why make us go back to the original post?

Spoiler: It was the metering rod adjustment.

How hard was that?

I am glad that you found the problem & were able to fix it fairly easily. Good show!

 In another forum that I am active in its customary to change the title to include “update” when there has been a resolution. it clues the watchers in that there has been an update AND a reader just needs to scroll down to the end of the original post. It’s really easier to simply scroll the original post to read update.   Adding a new response gets the update possibly buried in pages of responses.        Anybody wanting an update , it’s one page one.     

    I did this and then realized this forum might operate differently so I quickly dropped a clue.    

       Sorry if I offended.  Not my intent.       I did want to give back to this forum what info I could.        

      Thanks again