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Thread: 1974 304 backfiring thru carb, need help

  1. #1
    Join Date
    Jan 2013
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    3

    Default 1974 304 backfiring thru carb, need help

    Hello all, I'm new here...want some advice.

    We have two loadstar trucks for hauling grain, and each has the apparently common backfiring problem. One is a 1963 1800 with a 345ci, and the other is a 1974 1600 with 304ci. For this tho, let me focus on the 304.

    We bought it used in very good shape 5 years ago, had a head gasket go out, which forced us to do a major rebuild (new crank, cam, pistons, replaced cracked head, valves, seats....everything but the carb I guess). That was in '11 and it has ran flawlessly up till two weeks ago. On last trip to town, start popping and farting, until at last it would just limp home unloaded. Seems to idle fine, and if you can smash the pedal it'll cough but speed through to high rpm. It's the running under load that's causes the backfire.

    So, we've checked the timing (good), took valve covers off (all rockers moving good distance, push-rods spin freely & straight), inspected points/condenser (look good), coil can jump a spark about 1/2" on new plug wires, so we're thinking that's good.

    Right now we're running out of obvious solutions, last thing I can think of is vacuum leak? Is the advance at play here? It doesn't seem like fuel delivery, but we will certainly look to that before taking off the heads/valley pan.

    Any thoughts watsoever appreciated here.

    John Schmeling

  2. #2
    Join Date
    Jan 2013
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    3

    Default

    Dad added this in later today:

    Just got home..... read your description and it's all correct except to mention that it occurs only at partial throttle in the run up in a gear. If you get some speed up, then mash the throttle down completely it quits the backfiring, but doesn't seem to have the power it once had.


    I'm thinking about putting it back together, so it will run again, but before that check the valve springs like Steve suggested. I'm also suspicious of that PCV vent in the back of the valley pan. At full throttle there is not much vacuum in the manifold to draw in unwanted air. Only restriction would be the venturi in the carb. Half throttle would still create a good vacuum and consequently a lean mix with unwanted air. The Good idle is still a ? in that theory tho

  3. #3
    Join Date
    Feb 2009
    Location
    Aiken, SC
    Posts
    2,423

    Default

    Sounds like you have a hole in the vacuum advance diaphram. Try blocking off the port to the carb and see if the problem goes away.
    Allan E.
    Curmudgeon Extraordinaire
    Charter Member, Old Hippie IH Club
    Old fashioned binder freak

  4. #4
    Join Date
    Jan 2013
    Posts
    4

    Default thanks Allen E.

    Quote Originally Posted by Allan E. View Post
    Sounds like you have a hole in the vacuum advance diaphram. Try blocking off the port to the carb and see if the problem goes away.
    304 Backfiring (semnjohn)
    Vacuum advance was bad. None in the country so ordered a rebuilt one.

    So - on to the 345. Has (I think) a 2300 Holley carb. Has a governor on the carb. Good runner, always strong on power. Problem is some times it will decide to idle on four cylinders and will produce a backfire and very poor power coming out of idle. The start of the run up in a gear is very unpredictable, some times after a backfire it will even die.

    When this engine is running right it is so smooth-- I swear it must be balanced and blueprinted!

    Any Ideas on this one ? ?

  5. #5
    Join Date
    Jan 2013
    Posts
    4

    Default thanks Allen E.

    Quote Originally Posted by Allan E. View Post
    Sounds like you have a hole in the vacuum advance diaphram. Try blocking off the port to the carb and see if the problem goes away.
    304 Backfiring (semnjohn)
    Vacuum advance was bad. None in the country so ordered a rebuilt one.

    So - on to the 345. Has (I think) a 2300 Holley carb. Has a governor on the carb. Good runner, always strong on power. Problem is some times it will decide to idle on four cylinders and will produce a backfire and very poor power coming out of idle. The start of the run up in a gear is very unpredictable, some times after a backfire it will even die.

    When this engine is running right it is so smooth-- I swear it must be balanced and blueprinted!

    Any Ideas on this one ? ?

  6. #6
    Join Date
    Aug 2019
    Location
    Syria
    Posts
    13

    Default 1974 304 backfiring thru carb need help

    Thank you for reaching out Im new to this camping repair stuff and need all the help I can get. My control panel has a fan speed slide switch, fan auto button,, fan dome up-down button and air flow button in-out. Would the switch you reference work?
    Thks in advance, Ken

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