455 Backfiring Through Carb

Discussion in 'Street/strip 400/430/455' started by srich1207, Nov 16, 2017.

  1. srich1207

    srich1207 New Member

    OK guys I have something that is just kicking my butt. Let me start off by saying I have a 1972 GS 455 Stage 1, and about 3-4 months ago I started noticing a large clunk every so often when I would crank the car. So I determined that possibly something was going on with the main bearings although I still had great oil pressure, and a note that will come in play later on the car ran great and had tons of power although it is a pretty much stock rebuild to 1970 Stage 1 specs. So to make a long story short pulled the engine found that believe it or not the main cap bolts were bottoming out on a couple and were not pulling the cap all the way down. So I shortened the bolts by about .060" and that fixed the problem. Put new bearings in, Plasti-gauged everything for peace of mind all looked good so re-sealed engine, (intake, front cover, oil pan, new water pump, new balancer as old one looked cracked etc.) and put back together. Got engine back in car hooked everything back up, primed engine, reset timing put in distributor and after I got fuel it fired right up no problem and had great oil pressure. OK I dialed in the carb and idled great. But now when I drive the car feels down on power and if I just accelerate normal will drive just fine but once I start to give it more gas from any speed or from a stop it spits and backfires back through the carb. I know that this is a lean condition of some sort so installed a different carb and same problem. Please refer back to my previous statement about how good and how much power this car had ran like a top and had tons of power. I have tried advancing timing until it pings and it does get a little better but still spits and backfires. I have an old distributor machine that I restored and have run the distributor on it and everything is working just as it should and let me say I haven't changed anything from before I took it all out. The only things I can think of is maybe an intake leak????? Maybe the new harmonic balancer's mark is not in time with the keyway???? I am at my wits end. Again idles perfect with 16 inches of vacuum and only parts I changed that would effect anything were harmonic balancer, fuel pump, and re-sealed intake. Any suggestions.
     
  2. dan zepnick

    dan zepnick Well-Known Member

    Sounds like a retarded timing issue to me.16 inches of vacuum seems low.recheck balancer,timing mark.timing chain alignment marks correct?
     
    john.schaefer77 likes this.
  3. john.schaefer77

    john.schaefer77 Well-Known Member

    The balancer 0* line should line up with the keyway, just it it to one side slightly. Can' remember clockwise or counter.
     
  4. TrunkMonkey

    TrunkMonkey Well-Known Member

    Just tossing out a guess that you might have timing gear issues. If it was a plastic/nylon tooth gear and has broken down, that can throw cam/crank timing, and result is backfire and inability to get the timing right moving the distributor.

    Cap bolt stretch is another issue that asks for further inspection of rotating assembly.
     
  5. Stevem

    Stevem Well-Known Member

    Advance the Dizzy timing 6 degrees and if the motor responds better and does not ping at part throttle or knock lightly at full throttle then I would say it's timing chain and gears time, or atleast time to do a simple slop test to determine the amount of play you have!
     
  6. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    16” of vacuum with what cam? Have you checked your fuel pressure? What carburetor?
     
  7. Schurkey

    Schurkey Silver Level contributor

    Backfiring through carb
    A. Burned intake valve
    B. Restricted exhaust including exhaust valve not opening
    C. Lean fuel/air mix

    What is the compression of all eight, and what is your fuel pressure when it backfires? Any crushed exhaust pipes? Rocker arms all move the same amount?
     
  8. vanilla bean

    vanilla bean Well-Known Member

    Rocker arms button probably broke it happened to me lol
     
  9. 8ad-f85

    8ad-f85 Well-Known Member

    If you are advancing the timing until it pings and there isn't a major vacuum leak then your intake valves aren't seating or don't seal.
    Of course it's always possible there's a mismatch of cam and compression leaving the max timing you can dial in still too far retarded.
    Obviously there's been no mention of initial settings or what the actual timing curve is.
    We would assume these things are checked with a timing light if you used a distributor machine.
     
  10. mikec

    mikec Well-Known Member

    Look for a gap between the bottom of the distributor shaft and the top of the distributor gear. Should be about .020" to allow for thermal expansion. As the engine revs and the gap is large, it will retard the timing as distributor gear walks up the cam gear. Moroso makes a shim kit.
     
  11. Schurkey

    Schurkey Silver Level contributor

    I'd have said "distributor housing" instead of "distributor shaft". To me, the "distributor shaft" is the main shaft the gear is pinned to.

    Oil pump drag acting through the helix of the gears should force the distributor gear against the distributor housing . There would also be rubbing-block drag if it's a points-style distributor.

    Anyway, if you have reasonable oil pressure, I'd expect no timing retard at any rpm from the helix angle/vertical distributor shaft movement, because it's already at it's limit of travel. Am I wrong?

    Shimming the main shaft to reduce clearance between gear and housing makes the distributor "longer", be sure you have end-play or you'll score the oil pump bottom plate.
     

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