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BRINGING IT ALL TOGETHER.... A's, G's & E's


4 posters

    Backfire through the Carb.

    Scott
    Scott
    Scott


    Number of posts : 1651
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    Registration date : 2008-05-20
    • Post n°1

    Backfire through the Carb. Empty Backfire through the Carb.

    Post by Scott Mon Apr 30, 2012 4:22 pm

    I know my timing is off, but I'm not sure where to go with it. Once when it backfired through the carburetor I figured the engine was still a little cold.

    Well it's done it a couple of times since, once today while I was cruising at 55, steady gas. I know it also has to do with the timing curve, but it happens at different speeds.

    It has an aggressive cam in it, enough were I was told it will break valve springs if I romp on it a lot.

    It set at about 12 degrees Before TDC.


    I'm just looking for a little direction here.
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    jrinaman
    jrinaman
    jrinaman


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    • Post n°2

    Backfire through the Carb. Empty Re: Backfire through the Carb.

    Post by jrinaman Mon Apr 30, 2012 7:50 pm

    how far advanced at higher rpm? mine is set for 38 at full advance and is all in by 2000 rpm. that is with a 219 @.50 cam with .513 lift. your cam may require more or less but your problems will occur when advanced,setting at idle is standard practice but if advance doesnt work right, its meaningless. besides, recurving made a huge difference. not more power, just runs better.
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    Scott
    Scott
    Scott


    Number of posts : 1651
    Location : Anoka, MN
    Age : 54
    Registration date : 2008-05-20
    • Post n°3

    Backfire through the Carb. Empty Re: Backfire through the Carb.

    Post by Scott Tue May 01, 2012 8:04 am

    I seem to be getting backfire when it's running about 1500 RPM, but maybe once in an hour of driving. I'll check that timing at different RPMs.

    Here is my CAM Specs,
    Backfire through the Carb. Screen72
    How do I decipher this?


    When I got the engine this card was in the paperwork, and he told me he had rebuilt the distributor, but I don't know what all has been changed.
    Backfire through the Carb. Screen19
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    jrinaman
    jrinaman
    jrinaman


    Number of posts : 890
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    Age : 60
    Registration date : 2011-03-10
    • Post n°4

    Backfire through the Carb. Empty Re: Backfire through the Carb.

    Post by jrinaman Tue May 01, 2012 8:47 am

    your cam isnt as agresive as mine. .194 @ .50 and .465 lift. the advance you show is adjustable, you put a small allen wrench in where hose connects. i am not 100% sure its timing, but thats where i would look first. here are different cam specs.http://www.inliners.org/
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    jrinaman
    jrinaman
    jrinaman


    Number of posts : 890
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    • Post n°5

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    Post by jrinaman Tue May 01, 2012 8:58 am

    sorry, link went to home page. click on tech tips, then 250 cam specs. yours is close to schneiders first cam. it is a little more duration but short enough to keep power band low and a fair increase in lift. on our 6's, we want power band low so we need short duration and power increases are gained with more lift. sounds like an excelent cam for a daily driver. any more lift would need longer duration so any power gains would be a trade off for low end torque.
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    Scott
    Scott
    Scott


    Number of posts : 1651
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    • Post n°6

    Backfire through the Carb. Empty Re: Backfire through the Carb.

    Post by Scott Tue May 01, 2012 3:58 pm

    Being that this is more of a daily driver, even with the few mods I've got. Do I need the performance stuff in my distributor?

    Would a stock one due, or will this modified one do a better job?
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    jrinaman
    jrinaman
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    • Post n°7

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    Post by jrinaman Tue May 01, 2012 4:36 pm

    you dont need the distributor mods, but it will run so much better. first verify that its not advancing to much. if it never goes above 38 degrees, your fine. i will walk you threw recurving it later. if it goes past 38, thats your problem. if not, maybe jetted too high and when you get on it a little the fuel builds up and backfires when you let off. spark could be weak too. do you have full 12 volts to ignition? points had less voltage and you need to run a new wire from ignition. if you have a voltmeter, check at battery, should be 13-13.5 volts, then check neg of battery (or ground) and pos. at coil. it should be the same. i think points was around 11 volts but not sure. cheap voltmeters are less than $10.00 and ive seen them for 5.
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    Scott
    Scott
    Scott


    Number of posts : 1651
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    • Post n°8

    Backfire through the Carb. Empty Re: Backfire through the Carb.

    Post by Scott Tue May 01, 2012 9:32 pm

    Well, I got sidetracked on a brake problem tonight, and didn't get to the timing. It's now on tomorrows to-do list.

    Thanks for the info on testing the volts going to the coil. I think I'm good there, but I never put my multimeter on it to be sure.
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    Scott
    Scott
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    • Post n°9

    Backfire through the Carb. Empty Re: Backfire through the Carb.

    Post by Scott Thu May 31, 2012 7:04 pm

    OK, I'm building a list of things to check on the VAN, and starting with what you listed. Something else I want to check is the Spark-plug gap. What should it be with an HEI distributor?
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    panelmanrd
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    • Post n°10

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    Post by panelmanrd Fri Jun 01, 2012 7:25 pm

    scott I would not run any more than .045 gap on the
    plugs, anymore makes the rest of the ignition sys work
    to hard, and elecricity will find the path of least resistance to ground and usually finds its way through the
    center of the rotor.
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    Scott
    Scott
    Scott


    Number of posts : 1651
    Location : Anoka, MN
    Age : 54
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    • Post n°11

    Backfire through the Carb. Empty Re: Backfire through the Carb.

    Post by Scott Sat Jun 02, 2012 3:21 pm


    Thanks everyone for the help, I'm a little over my head on timing curves.

    Here's what I found,

    Gap is currently .035, which is stock gap for a 1980 250 w/HEI. Let me know if you think it should be different.

    Engine off the coil is getting 12v, same as battery.

    Engine idle at 800rpm is 13.5v, same as battery.

    Timing is set at 10.2 Before TDC at 800rpm idle w/o vacuum.

    Timing is goes to 18.3 Before TDC at 2000rpm w/o vacuum.

    Timing goes to 18.5 Before TDC at 2000rpm with vacuum.

    I plugged the PCV line and tried it again, thinking maybe a little more vacuum would help.

    Now, with vacuum 19 Before TDC @ 2000rpm
    and, with vacuum 20.2 Before TDC @ 2500rpm

    Is this something I can adjust with that vacuum canister on the distributor? Or, could those numbers be close to right on mine even though you are running closer to 38 Before TDC @ 2000rpm?

    Also, I have noticed that usually when I get the backfire it's when I let off the gas, and then hit it again. Like slowing for a corner, and then powering out of it. Or letting off because I think I may be cut off, and then hitting the gas again when they don't. It's gotten to where it only happens like every hundred miles, so its not like it happens all the time.

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    jrinaman
    jrinaman
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    • Post n°12

    Backfire through the Carb. Empty Re: Backfire through the Carb.

    Post by jrinaman Sat Jun 02, 2012 5:14 pm

    38 degrees would be with vacume connected. yes, an adjustable vacume canister can be set for that. could also be loading up, jetted a little high. verify timing first, if its o.k. then try 1 size lower on your metering rods. long time since i messed with an edelbrock, so call summit for advice on jetting.
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    donivan65
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    • Post n°13

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    Post by donivan65 Sat Jun 02, 2012 5:23 pm

    Well the basics are,,,,,,timing is around 4 degree before TDC,,,,centrifugal advance ADDS about 18 degrees and vacuum advance ADDS 20 degrees,,,,,and the HEI came with the tiny 5/8" wide gap plugs,,,,you can't just open up the gap on the bigger 13/16" smaller gapped points type spark plugs,,,,,
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