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Altitude changes and pre-ignition(pinging) issues

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Can Can, Oct 12, 2003.

  1. Can Can

    Can Can Pusher Man Staff Member Super Moderator

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    This issue is driving me crazy!!!!! For the last year Ihave been travelling to my rec property which is about 1000 ft lower in elevation than where I live. Every time I get about 2/3rds of the way there, the truck starts pinging pretty bad.

    This last trip I tried something new and disconnected my vacuum advance line to limit the timing advance and to myt delight the pinging issues almost disappeared at the lower altitude. Which leads me to the following observations and questions:

    1. How many of you have issues like this where you begin to have timing issues with around 700 ft. of elevation loss? Is there a trick that you guys have to avoid these issues?

    2. My intial timing is set at 4'(factory setting). I have a recurve kit installed in the distributor, so maybe that's bumping up my mechanical advance a hair. Is there a chance that the recurve kit is too strong when combined with the vacuum advance and is contributing to these problems I'm having?

    3. Is there a chance that a 305 with 180k miles could have enough stretch in the timing chain to give a false reading with a timing light? Maybe 4' is actually 5 or 6 degrees if the chain is stretched a wee bit? How many degrees can cause a relatively stock 305 to enter preignition territory?

    Sorry for the long convoluted post, but I need a little help here before I blow a hole in the top of one of my pistons............ /forums/images/graemlins/shocked.gif /forums/images/graemlins/tongue.gif
     
  2. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    Ever try a mid grade fuel or premium for the trip there? I know as altitude increases you commonly see 85 octane as the base fuel, whereas down lower 87 is the base octane fuel. I'd try a run out there on premium and see if you still have the same problems.

    A thousand feet of elevation shouldn't make much difference though...i never noticed much of a problem until I got about 5000 feet above what my 350 was tuned for, then I just bumped the timing a bit to compensate. The 6.2 doesn't really care, just doesn't have quite the same power as it does at sea level.

    Rene
     
  3. Can Can

    Can Can Pusher Man Staff Member Super Moderator

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    A buddy of mine just told me that I could be running a little lean and that could be a source of pinging. Is there a chance that a lean running motor could be more obvious at lower elevations?
     
  4. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    If you're running a little lean at your regular altitude then it'll just get worse the lower you go. There is more air, which means the mixture will be getting leaner. You could prolly go up a jet size so that it'll do OK at both altitudes.

    Playing with the timing is basically just a crutch to get rid of pinging for the short term.

    Hopefully someone with more carb savvy will chime in.

    Rene
     
  5. justhorsinaround

    justhorsinaround 3/4 ton status

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    What Rusty said. I believe that if you do bump a size or two in the jets that should help ya out. I'm lookin at the same thing as I'm about to move 4,000 feet lower. Maybe try the gas idea first though. Certainly a lot easier! Good luck to ya.

    Allan
     

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