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HELP!!! 350 not running under load

Discussion in 'The Garage' started by SHINTON, Jan 30, 2004.


    SHINTON 1/2 ton status Premium Member

    Dec 26, 2001
    Likes Received:
    Triad area of NC
    Ok, my GMC Jimmy either HATES me or LOVES me too much, I have just about had it sold twice, and this time a guy drives almost 7 hours one way and she won't run right.

    Here is a short version, 2 months ago, crank crank crank, not even trying to start (no spark) we replace distrib cap and rotor and all works till this last weekend.

    Sunday, snowing like mad in NC, I go outside to crank it up, the guy is on the way to buy and crank crank crank, not even trying. I get some advice and decide to replace the ignition coil on top of my distrib cap. The Jimmy DOES crank after a while (wet electric issues from snow blowing into engine bay while replacing)

    It finally starts, and I let it run/idle for 20+ minutes hoping all will dry out. I try to take it for a test drive and as soon as gas is given under load it stumbles and wants to die, let off and ok (ie idle no load)

    I can race the engine in neutral/park, and is fine. (If I goose the gas real hard the carb/engine is backfiring some, carb makes a pistol shot sound and spits up a bit of gas vapor)

    Based on some advice, I take it a mechanic who know more than me, but we are still having issues.

    In theory, he has checked the timing and it is fine, he replaced the module (under the rotor) and checked the pickup coil which he said is ok.

    You still cannot put a load on it, or it dies, I havent asked him about the backfire.

    Some MISC info, the stupid exhaust manifold on drivers side IS cracked and is part of the backfire issue I am sure, you can hear the leak.

    His next suggestion is he is going to swap the carb off his Blazer onto mine to see if it is the carb causing the issue.

    If not, boy I am really scratching my head on this one, what do I look for? This has to be either electric or fuel right? Since I "had" an electric issue that I fixed and this problem started right after, doesnt that mean I still have an electric issue?


    Sam Hinton
  2. ZooMad75

    ZooMad75 1/2 ton status Premium Member GMOTM Winner

    Apr 5, 2003
    Likes Received:
    Pueblo, CO
    Just a thought, but it sounds like its running out of fuel. It may be that the mechanical fuel pump is on its way out or you got a plugged fuel filter in the carb. That would explain in my mind why it runs at idle and revving no load because the high volume of fuel isn't really needed and can run off of the fuel in the carb for a short time. Once the volume is needed (ie, under load, in gear) and the pump is weak or filter plugged (or both) you can't replenish the fuel in the carb to keep it running.

    I've been through that same senario on my Dad's '57. With the glass filter bowl is was easy to see the crap in the fuel system. Would not run past 2,000 rpm. New pump and a total fuel system flush including the tank and it was back to its normal self.
  3. 99firehawk

    99firehawk 1/2 ton status

    Dec 15, 2002
    Likes Received:
    Well I have to 2nd the lack of fuel, I also have to say that a cracked manifold is going to cuase piss poor performance and will lead to burned valves and blown head gaskets. I would also be checking around for a big ass vacum leak.
  4. 4xrick

    4xrick 1/2 ton status

    Sep 14, 2003
    Likes Received:
    kirkland Wa
    i must agree also sounds like fuel pump is going out or the fuel filter is cloged I had the same problem on my 78 k5 I put in a new filter and bam it was good to go and even got better gas milage (imagine that) /forums/images/graemlins/blush.gif

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