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Thread: problems with backfiring

  1. #1

    problems with backfiring

    i had a 421 sbc built about a year ago. i had it built turnkey including dyno time for tuning and breaking it in. well on the dyno everything was fine and a couple days ago i finally completed my truck so put the engine in and attempted to fire it. im using a msd digital 7 part number 7531. i have not hooked a computer up to the digital 7 yet so im using all defualt setting. im also using no cam sync.
    my problem is when the motor attempts to fire it will backfire and flame up through the carb. it does it everytime. what could be wrong. ive check the firing order a dozen times and made shure it corresponds to the plugs. the cap was marked my my engine builder with a #1 so i assumed that was #1 plug. my timing on my dyno sheets says that total timign was 36*. the defualt from the digital 7 start retard is 10*. is this taking out too much timing? does the msd have to be programmed prior or will the defualt settings work? please help

  2. #2
    Senior Member RACING JUNKIE
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    I don't know anything about digital ignitions,too serfistikated for me.I don't believe timing too far retarded will cause that.To backfire thru the carb usually means too far advanced timing.The spark plug is lighting the fuel mixture before the intake valve has closed completely and the flame travels all the way up the intake port and out the carb.But I wouldn't adjust the timing just yet.My car is REAL cold natured and alot of times will backfire thru the carb when first starting it.It comes from a cold intake that condenses the atomized fuel mixture back to a liquid.Then you end up with a lean-out condition that can cause small backfires.I just pump plenty of extra gas into the motor before I start it,then go real easy on the pedal.Might even try increasing the pump shot a little.Just my two cents and brief experience.Hope this helps
    "The problem with quotes on the Internet is that you cannot confirm their validity"- Abraham Lincoln

  3. #3
    Senior Member RACING JUNKIE jmarksdragster's Avatar
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    The cam sync is only for timing control on individual cylinders. What distributor and do you use a crank trigger? Are you using a 4/7 swap cam? Take the time to hook up and check the settings on the 7. 10 degrees start retard is no problem, I run 20 on mine.
    Mark Whitener
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  4. #4
    Quote Originally Posted by jmarksdragster
    The cam sync is only for timing control on individual cylinders. What distributor and do you use a crank trigger? Are you using a 4/7 swap cam? Take the time to hook up and check the settings on the 7. 10 degrees start retard is no problem, I run 20 on mine.
    well i got it running it is just cold natured as hell. if you pump the gas 2 times it will flame up through thte carb if you pump it 4 times it will fire up everytime. now i have a problem with it idleing i cant keep it running under 3000 rpm or it will cut off. the engine was dynoed and hasnt been touched since so the carb and timing was set then. we ran it for over a minute and it still wont come down to a idle. im not shure if when it flamed up maybe it did sone damage to the carb. or maybe becuase after i had the motor built it sat in my shop for a year maybe something is sticking in the carb. also i have a issue wuth my fuel pump making a fluttwering sound. its a magnafuel 300 and im using magnafuel regulators. the pump is pumping fuel but will start to vibrate really bad, the pressue is showing its fine at 7psi . could this be a probelm with the pump?

  5. #5
    Senior Member RACING JUNKIE
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    Check and make sure your powervalve is OK.I know the new ones are supposed to be "protected".I still lose one occassionally on my cold natured rattletrap
    "The problem with quotes on the Internet is that you cannot confirm their validity"- Abraham Lincoln

  6. #6
    Quote Originally Posted by olds48
    Check and make sure your powervalve is OK.I know the new ones are supposed to be "protected".I still lose one occassionally on my cold natured rattletrap
    probelm solved. to loose carb bolts weher i forgot to tighten the throttle return bracket down. got those bolts tight and now she idles perfect and runs awesome

  7. #7
    Senior Member DYNO TECH
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    :lol: Seems like it's always the simple things that bite you in the back side! :lol:
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  8. #8
    Senior Member RACING JUNKIE
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    Quote Originally Posted by blittle
    :lol: Seems like it's always the simple things that bite you in the back side! :lol:
    Aint that the truth !! I been pissing with the front end on my digger for a week now cant get the rack to turn very good. Ended up being the tierod ends have a left and a right lol I've put several together and never knew that, guess I just got lucky... boy did I feel stupid :lol:
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  9. #9
    Senior Member RACING JUNKIE
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    :lol: glad it was something simple,and glad I'm not the only one to do stuff like that
    "The problem with quotes on the Internet is that you cannot confirm their validity"- Abraham Lincoln

  10. #10
    Senior Member RACING JUNKIE jmarksdragster's Avatar
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    A big vacuum leak will do it. ops: I asked about the distributor because if you run an HEI make sure you get the low resistance center conductor for the cap. Make sure you have real good wires, MSD 8.5 or Accel 8.8 and keep your plug gap at or under .035. The Digital 7 lights a hefty fire. And no resistor plugs.
    Mark Whitener
    http://racingfuelsystems.myfunforum.org
    __________

    Good work isn't cheap, and cheap work can't be good.


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