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  1. #1

    Default Wot hesitation and spark knock or backfire


    Ive got a WOT hesitation with backfire/ spark knock that I can't figure out.



    It's a TBI. I've checked the fuel pressure. Ran off portable tank. Both fuel filters are new. Distributor cap, rotor and plugs new last year. Plug wires new. Checked the timing.
    I've got a EFI live data scanner, nothin obvious showing
    Plugs show rich mixture.
    Compression's good
    Map sensor seems to be right.
    Throttle position sensor voltage may be off at wot. I read it should be 4.5. Ive got 4.0 at wot. But not sure if this is normal range

  2. #2

    Default UPDATE: Wot hesitation and spark knock or backfire

    Quote Originally Posted by Maraian View Post

    Ive got a WOT hesitation with backfire/ spark knock that I can't figure out.



    It's a TBI. I've checked the fuel pressure. Ran off portable tank. Both fuel filters are new. Distributor cap, rotor and plugs new last year. Plug wires new. Checked the timing.
    I've got a EFI live data scanner, nothin obvious showing
    Plugs show rich mixture.
    Compression's good
    Map sensor seems to be right.
    Throttle position sensor voltage may be off at wot. I read it should be 4.5. Ive got 4.0 at wot. But not sure if this is normal range
    Interesting update. When running WOT with the bow trimmed all the way down there is no hesitation. The rpms are just a little lower at WOT. Then as I trim the bow up, the hesitation starts, as I continue trimming up, hesitation gets worse and spark knock / backfire starts.
    I'm starting to think it might be the ECM or the pick up coil, ignition control module as my efi scanner shows all the sensors do what they are supposed to.

  3. #3
    Join Date
    Dec 2006
    Location
    Chaumont Bay, N.y., USA
    Posts
    5,218

    Default Re: Wot hesitation and spark knock or backfire

    Ayuh,.... The title's symptoms are classic fuel starvation,....

    Yer diagnosis says otherwise,....

    Bein' an electronic motor, I've got no good answers for ya,....
    Any Grease is Better,... Than No grease at All....

  4. #4

    Default Re: Wot hesitation and spark knock or backfire

    Quote Originally Posted by bondo View Post
    Ayuh,.... The title's symptoms are classic fuel starvation,....

    Yer diagnosis says otherwise,....

    Bein' an electronic motor, I've got no good answers for ya,....
    Thanks actually, replaced the throttle positions sensor, because the data was a bit off, it didn't clear up the symptoms they seem a bit less intense. I took a good look at the idle air control valve and it seemed to be moving in and out. So I cleaned it up and that seems to have taken care of the WOT spark knock.
    But still have the WOT hesitation. It looks like the timing is being retarded as if a rev limiter has been kicked in or the knock sensor is kicking on, but the knock sensor is not showing any feedback voltage on the scanner. So brings me back to the ECM.

  5. #5

    Default Re: Wot hesitation and spark knock or backfire

    ***UPDATE***
    problem solved!
    turned out to be, someone put the wrong fuel injectors in this TBI!
    turns out, 5.7L marine engines have the 7.4L TBI and injectors in them.
    The injectors that were in there were aftermarket ( marked : RIN-1001) and are for a 5.7L automotive engine!
    napa part number 218207 are a replacement.

  6. #6
    Join Date
    Dec 2006
    Location
    Chaumont Bay, N.y., USA
    Posts
    5,218

    Default Re: Wot hesitation and spark knock or backfire

    So, it was in fact a lean condition,....??....
    Any Grease is Better,... Than No grease at All....

  7. #7

    Default Re: Wot hesitation and spark knock or backfire

    Quote Originally Posted by bondo View Post
    So, it was in fact a lean condition,....??....
    Yes it was weird because it was rich condition at idle and elsewhere. But lean at WOT.
    I was running both lean and rich, which added to the dilemma.
    It turns out the injectors that where in there are designed to run at 12 psi, my system is 30 psi, the fuel was just dumping in, except at higher RPM WOT, when it was starving. Actually before I got the prop cut down, the Wot hesitation symptom did not included lean spark knock. After cutting down the prop 1/2" diameter and 1/2" pitch, the rpm went up and the wot hesitation symptom was with popping! Higher rpm requires more fuel ��

  8. #8
    Join Date
    Dec 2015
    Location
    maplevalley WA
    Posts
    3,541

    Default Re: Wot hesitation and spark knock or backfire

    WOT rpm is used to determine proper prop for your setup. There are a few factors when determining engine performance. Is the engine running close to 100%? Is the hull and prop clean in good shape? Trim angle good and dry double bottom. Size the prop to achieve WOT rpm. For best fuel economy and extend engine life determine engine rpm where progressive timing advance reaches its limit and keep the rpm there for normal operation. If your on-a schedule just leave a little earlier in general 3/4 throttle gives you best fuel economy and extends engine service life.
    It takes a nickle to go first class!
    If it aint broke dont fix it!!

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