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Backfire

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    Senior Member 74Bonneville's Avatar
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    Default Backfire

    BBC 468 Procharger set up, MSD Pro Billett distributor, 6M-2L box, not using rev limiter.

    Distributor - 2 light blue springs, stop bushing stock blue 21 degrees

    I attempted timing on the water, but wasn't getting good readings so I went by feel. Last time I felt I had a good timing reading it was at a total of 34 degrees. Rotated until it backfires, go other way till engine rpm comes up and smooths out, then just a little more for safety (I think this is for safety)

    My backfire happens when coming off throttle at the point where I'm feathering the throttle. Always a single pop that worries me. This set up has never idled very well for me so I have to feather the throttle. I'm able to keep the backfire from happening all the time, but it does happen and I want to stop it. If I run at a steady 2000 rpm no issue. Same at a steady 3000 rpm. Wide open runs very well.

    I was thinking my advance curve may not be ideal. Should I go with both light silver springs for quickest advance? How about the stop bushing, if I allow more advance curve then at low rpm the timing will be later than it is now. I think I'm thinking in the right direction. If I'm going the wrong way straighten me out.

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    mo balls than $cents$ IMPATIENT 1's Avatar
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    How much boost are you runnin, what was timing at your max boost and how are you removing tíming as boost comes in?

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    Kinda scary there, running a boost motor and not knowing exactly were the timing is.The fact that you can't set it with a light could be the cause of the backfire.
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    Senior Member 74Bonneville's Avatar
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    Quote Originally Posted by IMPATIENT 1 View Post
    How much boost are you runnin, what was timing at your max boost and how are you removing tíming as boost comes in?
    9 psi, 34 degrees, I'm not

    The single backfire comes in between the advance curve. I don't know how else to explain it. It only happens when I'm between 1500 & 2000 rpm when I have to feather the throttle.

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    Quote Originally Posted by SoldHondaBoughtHondo View Post
    Kinda scary there, running a boost motor and not knowing exactly were the timing is.The fact that you can't set it with a light could be the cause of the backfire.
    I have marks on my distributor & intake that I made when I originally set the timing at 34 degrees. I'm right there. Not that scary for old school engine guys. Lots of them could turn away and hear it better than we could set it.

    I disagree with your last sentence, although I do appreciate you input

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    Ain't Right Racin piston in the wind's Avatar
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    It could be going lean while the boost is still in the intake pipes & your off the throttle. Just 2 cents
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    I have to agree with piston in the wind, I wold look for a lean condition.
    If you are running a carburetor, back the idle air/mixture screw out just a slight amount (1/8 to 1/4 turn depending on where the screws are now) to richen the idle.

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    Senior Member 74Bonneville's Avatar
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    Quote Originally Posted by piston in the wind View Post
    It could be going lean while the boost is still in the intake pipes & your off the throttle. Just 2 cents
    If I simply let off the throttle it doesn't backfire, it's when I give it a little gas to keep it idling is when it happens. Got any more pennies?

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    Senior Member 74Bonneville's Avatar
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    Quote Originally Posted by jimsplace View Post
    I have to agree with piston in the wind, I wold look for a lean condition.
    If you are running a carburetor, back the idle air/mixture screw out just a slight amount (1/8 to 1/4 turn depending on where the screws are now) to richen the idle.
    I'll give that a try. Thanks!

    QFT blow through carb

    I may be missing something here, but the explosion is happening with the intake valve open right? How does a lean condition cause a backfire through the intake?

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    Do you have a fuel pressure gauge ?

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    gn7
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    Quote Originally Posted by 74Bonneville View Post
    I'll give that a try. Thanks!

    QFT blow through carb

    I may be missing something here, but the explosion is happening with the intake valve open right? How does a lean condition cause a backfire through the intake?
    You were advised to send that carb off to C&S in your previous thread. I am going to guess you haven't done that. YOU WILL!

    QF has NO business being in the blow thru business. I could say the same about the E85 business as well, but I won't.

    You fighting the engine to run because its lean in the idle and transition circuits.

    QF has a shitty habit of locating the idle feed restrictor high in the idle fuel well, which makes the fuel supply to the idle port and the transition slot erratic. Its a simple fix to relocate it down to the bottom of the idle well, but in truthfull reality, you should box the POS up and send it off to somebody that KNOWS what a blow thru is, not some company that saw one once.

    BTW, there is a little event in every camshaft design called the overlap period. Its is what allows the thing to bang thru the carb when the thing is lean, and/or the timing is late.


    I stopped timing engines by ear in the 60s, never once timed a boosted engine by ear, and always knew exactly where max timing was and when it occurred on every boosted engine I ever tuned.

    I don't think the idea of running a timing curve on a centrif boosted jet would ever enter my mind. I simply cannot think of a single reason to do it.

    JMO



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    Last edited by gn7; 07-08-2013 at 09:59 PM.

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    mo balls than $cents$ IMPATIENT 1's Avatar
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    Quote Originally Posted by 74Bonneville View Post
    9 psi, 34 degrees, I'm not

    The single backfire comes in between the advance curve. I don't know how else to explain it. It only happens when I'm between 1500 & 2000 rpm when I have to feather the throttle.
    34 Is wayyyy too much timing and you shlould have a way to remove a degree per lb of boost.either need a 6btm box or a boost timing master from msd.i have the same qft bt carb and its not aplug and play deal.do you even have a wideband in this deal?

    When i pulled my qft outta the box the tslots were set wrong, iab's were to small,pvcr'S too small.you have to setup the carb with a wideband setup, its pointless without one.at 9lbs of boost you should be in the 25-28* range..
    Last edited by IMPATIENT 1; 07-09-2013 at 06:44 AM.

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    Senior Member 74Bonneville's Avatar
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    Quote Originally Posted by gn7 View Post
    You were advised to send that carb off to C&S in your previous thread. I am going to guess you haven't done that. YOU WILL!

    QF has NO business being in the blow thru business. I could say the same about the E85 business as well, but I won't.

    You fighting the engine to run because its lean in the idle and transition circuits.

    QF has a shitty habit of locating the idle feed restrictor high in the idle fuel well, which makes the fuel supply to the idle port and the transition slot erratic. Its a simple fix to relocate it down to the bottom of the idle well, but in truthfull reality, you should box the POS up and send it off to somebody that KNOWS what a blow thru is, not some company that saw one once.

    BTW, there is a little event in every camshaft design called the overlap period. Its is what allows the thing to bang thru the carb when the thing is lean, and/or the timing is late.


    I stopped timing engines by ear in the 60s, never once timed a boosted engine by ear, and always knew exactly where max timing was and when it occurred on every boosted engine I ever tuned.

    I don't think the idea of running a timing curve on a centrif boosted jet would ever enter my mind. I simply cannot think of a single reason to do it.

    JMO
    You're right about sending carb to C&S. This is a process of priority due to available funds. I just did a pump shaft & inducer $$$$ Had too, bent the stock one just a little
    This is next on the table.
    Now I'm understanding why and where the bang is coming from, I see the light

    You wouldn't run a timing curve, I haven't learned any other way, yet, please enlighten my brain to what you're talking about. I've heard about a distributor you plug your lap top in and program what you want it to do. Any thoughts? Although my funds won't cover this one for a while so if I could set up my curve better I will.

    Thanks!

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    Senior Member 74Bonneville's Avatar
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    Quote Originally Posted by IMPATIENT 1 View Post
    34 Is wayyyy too much timing and you shlould have a way to remove a degree per lb of boost.either need a 6btm box or a boost timing master from msd.i have the same qft bt carb and its not aplug and play deal.do you even have a wideband in this deal?

    When i pulled my qft outta the box the tslots were set wrong, iab's were to small,pvcr'S too small.you have to setup the carb with a wideband setup, its pointless without one.at 9lbs of boost you should be in the 25-28* range..
    This is the first real issue I've had with this. Other than primary and secondary jetting I haven't done anything else.

    Thanks for the input

    I'm thinking carb first then when funds allow ignition changes. With my current ignition if I set 28* at full advance with the current stop idle timing would be 7*. If I go with black stop then idle would be 10* Any suggestions

    Before I run again I will hook boat up to water hose to time it. Much easier than having my 16 year old nephew trying to hold an rpm for me in the river with the early morning water skiers cruising by.

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