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backfiring trough carb

Discussion in 'The Garage' started by muscle, Aug 17, 2005.

  1. muscle

    muscle 1/2 ton status

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    hey guys my 400 sm blk started backfiring through the carb recently. this happens when you wick the throttle to WOT and the motor rpm will start to rise, hesitate, backfire and then quickly throttle up. my timing is at 12 BTC at 800 rpm using a holley 3310 750 cfm 4bbl with vac secondaries.

    i do know that the carb needs jetting from 72 main down to 68 or so to start as i am at 6000 ft elev. also plugs are fairly black. If i advance the timing to 16 BTC the backfiring goes away. Also it diminishes greatly when at operating temp.

    my thinking was that i should not trust the timing plate on the motor. I do not want to run it too advanced either for fear of damaging piston skirts on start up of a far too advanced motor.

    what do you guys think? timing or carb or both? also the motor has about 30k miles and built up to approx 375 hp.
     
  2. diesel4me

    diesel4me 1 ton status Premium Member

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    It could be the timing,if the backfire ceases when you advance it..I'd be leery of a blown power valve on any older holley 4 bbl carb--that can cause hesitation,and backfiring is what usually kills one in the first place,so even if it didn't cause the backfiring,its likely blown now!..(it can cause poor idle and flooding at low speed--might be why your plugs are sooty)...

    Check your vacuum advance and the weights inside the distributor,make sure they are free and not rusted and stuck in one position..the vacuum advance should work if the diaphram is good,some engines prefer manifold vacuum rather than ported vacuum...try both and see if it helps..you might need different springs on the weights to change the advance curve--high altitude makes a big difference in settings,they dont run the same as they do at sea level.. :crazy:
     
  3. muscle

    muscle 1/2 ton status

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    Distributor is new, carb is new also.

    I just finished jetting main jets from 70 down to 66 and its running alot cleaner now. will jet the secondaries from 72 down to 68 here soon. I also checked valve lash on all 8 cyl and everything is adjusted. timing now shows 16 BTC and it runs good. no sounds of piston skirts slapping cyl walls at hot start up. slight hesitation just after idle on the throttle but i think this will be some accel pump adjustment.

    My machinist was fairly adamant of not going more advanced past 12 BTC but it runs like sh!t there. so im back to thinking the timing tab is lying to me...

    :confused:
     
  4. MTMike

    MTMike 1/2 ton status

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    Very possible. The outer "ring" on the balancer may have slipped or shifted throwing your mark off.

    Your best bet would be to retime it from #1TDC. Find TDC of the compression stroke on the #1 cyl and then set your marks on the timing dial, and then go from there.

    Mike
     

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