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carb backfire

Discussion in '1969-1972 K5 Blazer | Truck | Suburban' started by TED, Oct 21, 2003.

  1. TED

    TED Registered Member

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    I pulled out of the driveway and on to the road tonight and it hiccuped out of the carb. I assume it is timing since the last thing I did was screw around with the weights and springs in my HEI. Any recommendations?
    That leads me to the next issue- advance curves. I did some research and found a few recommended curves for 350's with HEI distributors but all the recommended curves were very different. I remember reading that most motors like to run at 34-36 degrees total advance, I'm at 45 degrees total advance at 2500 rpm after checking tonight starting out at 7 degrees at idle with vacuum advance disconnected. I think I might need one of those adjustable vacuum advance units since I am at about 20 degrees mechanical advance at 2500 rpm's. I had to buy a new spring / weight kit since the bushings wore out on the stock set and the weights got stuck all the way advanced. The lightest springs seem to create the best HP but low end is affected and the mid range springs rob HP but gave me low end. And for some reason I got that carb hiccup with the middle springs. Any experts want to take me to school on this one?
    Ted
     
  2. Greg72

    Greg72 "Might As Well..." Staff Member Super Moderator

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    I'm no expert......but 45* advance sounds like WAY too much.

    You should definitely be getting detonation at with that much advance, so it's not a good thing.

    I've also heard that 32 - 36* is the "magic number" for total advance. I played around a little with advance weights on my MSD, but I had the opposite problem....no matter what the RPM was I could never get more than about 22* total advance.

    I swapped to a softer set of springs and was able to finally get total advance of 34* (or something close)..... basically, I set up the timing for the total advance that I want, and let the advance number (at idle) fall wherever it may.
     
  3. 72beater

    72beater 1/2 ton status

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    45 seems extreme. bust out the haynes or the chiltons, they both have the specs but I do not have them with me right now.
     
  4. mudjunkie 82

    mudjunkie 82 1 ton status GMOTM Winner

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    45 degrees seems like way too much, /forums/images/graemlins/thinking.gif /forums/images/graemlins/thinking.gif Back it down to 38 max
     
  5. Topdown

    Topdown 1/2 ton status

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    from what I have learned recently (I have similar issues as you) you will want something along the lines of:
    10* BTDC and 32* - 34* total advance at 4K RPM.

    I have never seen anything for a street application with more than 36* recommended advance.

    those seem to be the "magic numbers" for a general performance arc. Now, if I can just find someone to teach me how the hell to get there... I would be a happy guy!
     
  6. TED

    TED Registered Member

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    Thanks guys, I agree that 45* is way too much. What I'm really interested in is setting the curve. I have researched curves and came up with quite a few "recommended" curves for 350's with an HEI mostly stock build but they seem to be very different in terms of degrees of advance at different RPM's. Some advance very quickly and others recommend a steady advance. Anyway, I just got an adjustable vacuum advance unit so I'll mess with that and the different spring combinations I have until I end up at around 34* total advance.
     
  7. Topdown

    Topdown 1/2 ton status

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    hey,

    when you figure out how to get there... let me know, I am on the same path only much slower and apparently retarded... /forums/images/graemlins/crazy.gif

    /forums/images/graemlins/thumb.gif
     

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