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    1. #1
      Join Date
      Aug 2004
      Location
      Fenton, MO
      Posts
      80

      dieseling problem

      Ok after replacing the busted valvespring, after resetting the valves, resetting the timing, ect.... the car still diesel when you shut it off. Now the car seems to run fine, doesnt run hot. But will diesel every time you shut it off. I am at a loss as to why shes still running lean and need suggestions.



      Compression on that cylinder is good so I assume I dont have a bad valve. What else can I check? Possibly a pushrod? I checked it on a straight edge and it looked ok. When setting the valve im going while running back off till they tick, tighten till they stop and then 3/4 turn. Possibly still a valve out of adjustment? Timing is @ 12 initial also. HELP guys im stumped.


    2. #2
      Join Date
      May 2002
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      Northern California
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      10,715
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      how high is your idle speed? check to see if your throttle blades are open past the transfer slots, also if your secondaries are open as well.
      MrQuick ΜΟΛ'ΩΝ ΛΑΒ'Ε

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    3. #3
      Join Date
      Apr 2001
      Location
      The City of Fountains
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      Do you have an MSD box?

      Andrew
      1970 GTO Version 3.0
      1967 Cougar build
      GM High-Tech Performance feature
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    4. #4
      Join Date
      Aug 2004
      Location
      Fenton, MO
      Posts
      80
      Quote Originally Posted by MrQuick
      how high is your idle speed? check to see if your throttle blades are open past the transfer slots, also if your secondaries are open as well.
      1000 or so at idle, 800 or so in gear. Note nothing has changed on the car since before the valve spring broke but i have double checked all the settings.

    5. #5
      Join Date
      Aug 2004
      Location
      Fenton, MO
      Posts
      80
      Quote Originally Posted by andrewb70
      Do you have an MSD box?

      Andrew
      Yes MSD Digital 6. I am going to try and back the valves off 1/2 turn or so to make sure im not hanging a valve open. but I wonder if I damaged the valve when the spring broke possibly? The cylinder has good pressure 210psi so I dont think anything is bent. I am going to pull and inspect all of the plugs and see what they look like.

    6. #6
      Join Date
      Apr 2001
      Location
      The City of Fountains
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      15,971
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      If you don't have a diode in the MSD wiring it will cause it to keep running even after you shut the key off.

      Andrew
      1970 GTO Version 3.0
      1967 Cougar build
      GM High-Tech Performance feature
      My YouTube Channel Please Subscribe!
      Instagram @projectgattago
      Dr. EFI
      I deliver what EFI promises.
      Remote Holley EFI tuning.
      Please get in touch if I can be of service.

      "You were the gun, your voice was the trigger, your bravery was the barrel, your eyes were the bullets." ~ Her

    7. #7
      Join Date
      Aug 2004
      Location
      Fenton, MO
      Posts
      80
      Quote Originally Posted by andrewb70
      If you don't have a diode in the MSD wiring it will cause it to keep running even after you shut the key off.

      Andrew
      Well ive run this box for 2 years now and this is the first time its ever deisled. Once again the car never deisled until the valve spring broke.

    8. #8
      Join Date
      Aug 2004
      Posts
      2,413
      I too have to agree that your throttle blades are open to far at Idle. Was my problem for a while also.

      Maybe before/during your spring issue you needed a differant carb setting but now that everything is good you need to do more tunning.

      Does your mixture screws have a big effect on your idle quality? If not

      Adjust your throttle screw down till it is running very slow then play with the Idle mixture screws.
      Nothing says "I built this" better than tool marks and dykem blue..

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    9. #9
      Join Date
      Aug 2003
      Location
      Orlando, FL
      Posts
      8,745
      Two things, if you throttle blades are to far open, you are idling off the jet instead of the idle circuit, which can cause this and a hard to adjust fat idle. Also, see if the fuel tank is holding pressure, make sure the tank is vented. If it is not vented properly, it will be under pressure it will push fuel after you trun it off causing it to diesel and make it hard "floaded" start.

    10. #10
      Join Date
      Aug 2004
      Location
      Fenton, MO
      Posts
      80
      Ill check the throtle blades tonight and see what they are set at. The tank is vented so no issues there.

    11. #11
      Join Date
      Sep 2004
      Location
      Chicago - western burbs
      Posts
      680
      I had a similar problem. it started all of a sudden too. Turned out there was a vaccuum leak in my brake booster.

    12. #12
      Join Date
      Aug 2004
      Location
      Fenton, MO
      Posts
      80
      Well I checked my carb settings, lowered my idle RPM to 900, backed the #7 valves off to 1/4 turn past clicking and still the same thing. So i pulled the plugs and found this.....

      http://www.odiesgarage.com/Nova/images/G05Break/plugs

      Check out 5 and 8 whats up with that?

    13. #13
      Join Date
      Mar 2002
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      Redwood City, CA
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      1,895,413,640
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      #5 and #8 are more what your plugs should look like normally. Have you thought about why the valve spring broke? Have you looked at your timing chain at all? I'm just wondering if it hasn't stretched and is causing that.
      Allen Ortega
      Meanstreets Performance Fabrication

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    14. #14
      Join Date
      Mar 2003
      Location
      Boringville
      Posts
      1,987
      yes 5/8 look good, the rest look like it is running too rich. Definetly put some new ones in. I don't think the timing chain being stretched would cause dieseling, especially if it runs fine. You need to take the carb off, and look at the bottom of it, there should barely be any of the transfer slots showing, they should look almost square. Adjust the idle screw to get them correct, that should get you the correct idle, also the idle mixture screws should be about 1 1/2 turns out. Also advance your timing to about 15 degrees and then turn down the idle, advancing the timing make ths idle go up, so then you can turn the idle down on the carb, this exposes less transfer slot. Do all that and let us know

    15. #15
      Join Date
      Aug 2004
      Location
      Fenton, MO
      Posts
      80
      The timing chain maybe has 10k miles on it if that. The spring had almost 40K with loads of drag passes.

      I will check the Idle and carb, first im going to do a leak check on the intake. I assume if it was just an Idle issue the car wouldent read lean while on the road.

    16. #16
      Join Date
      Aug 2004
      Location
      Fenton, MO
      Posts
      80
      Quote Originally Posted by yody
      yes 5/8 look good, the rest look like it is running too rich. Definetly put some new ones in. I don't think the timing chain being stretched would cause dieseling, especially if it runs fine. You need to take the carb off, and look at the bottom of it, there should barely be any of the transfer slots showing, they should look almost square. Adjust the idle screw to get them correct, that should get you the correct idle, also the idle mixture screws should be about 1 1/2 turns out. Also advance your timing to about 15 degrees and then turn down the idle, advancing the timing make ths idle go up, so then you can turn the idle down on the carb, this exposes less transfer slot. Do all that and let us know

      Done.... the dieseling seems to be gone but still a lean to almost off the scale a.f reading. I am going to put some more miles on it tomorrow and see how the plugs look.

    17. #17
      Join Date
      Mar 2003
      Location
      Boringville
      Posts
      1,987
      well it might be time to play wiht the jetting, glad that got rid of the dieseling. I just had a similar problem, and doing what i said pretty much got rid of it also. Altough I would of liked to had my idle at 950(manual car) than the 875-900 now so my alternator would charge just a little more at idle. What kind of carb is it, and what jets are in it? Also AF guages can lead you wrong, if your plugs are that black, i don't see how it could be lean, unless you were running way to rich before at idle, definetly put some miles on it and check the plugs and let us know, also when it is lean? at idle or crusing

    18. #18
      Join Date
      Aug 2004
      Location
      Fenton, MO
      Posts
      80
      Quote Originally Posted by yody
      well it might be time to play wiht the jetting, glad that got rid of the dieseling. I just had a similar problem, and doing what i said pretty much got rid of it also. Altough I would of liked to had my idle at 950(manual car) than the 875-900 now so my alternator would charge just a little more at idle. What kind of carb is it, and what jets are in it? Also AF guages can lead you wrong, if your plugs are that black, i don't see how it could be lean, unless you were running way to rich before at idle, definetly put some miles on it and check the plugs and let us know, also when it is lean? at idle or crusing
      When you fire it up it will read rich until the O2 sensor heats up ( a few seconds) then it will begin to slowly creep to lean. and stay there Idle , cruise, WOT you name it. The plugs will tell if the 02 sensor is bad after a few more miles.

      It has a 750 DP with 69's in front and an adjust a jet in the rear which I have no idea where its set but that would only effect WOT right? Is it possible that a weak exhaust spring could have been giving me a false Rich reading? then as it got worse the reading went lean?

      Is there any way to test the O2 sensor? Pull a wire or two off the plugs on that side, should givef a rich reading with dead plugs right?

    19. #19
      Join Date
      Mar 2003
      Location
      Boringville
      Posts
      1,987
      No idea on testing the o2 or whether a weak exhaust spring would throw it off. However i think 69's are pretty lean for a 750dp. I don't know off the top of my head but i think stock they are 72's or something. You are correct about the secondaries, that would be only if your foot was in the pedal more than about half way. Forget about everything you had before, you need to start new and start reading those plugs, BTW the AF guage is only usefull at part throttle and idle, and even then they aren't that reliable. I think you miht of been either
      1. idling rich before and lean when crusing
      2.you were too lean in the primaries, but when you went to WOT the jetting in the rear was too much and would run too rich making the plugs black. I would find out what was stock in the front of that carb and put it back to that, then find a way to adjust the rear jets to whatever stock would be. Go from there, I bet you are too lean in the primaries with those 69's

    20. #20
      Join Date
      Sep 2004
      Location
      Chicago - western burbs
      Posts
      680
      Quote Originally Posted by ChevyIIMan
      ... should givef a rich reading with dead plugs right?
      I may be all wet here, but Ill take a stab at that. I would have thought so too, but the O2 sensor is measuring the amount of Oxygen in the exhaust and comparing it to the amount of oxygen in the free air (not comparing air molecules to fuel molecules). If you never fire the plug, all the O2 that went in comes right back out again, and should indiacate LEAN, since O2 should be used up in the combustion process.

      I dont know, makes sence to me, and yet it doesent

      Rubes

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