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  1. #1

    Unhappy mercruiser 5.7 back fire through carb

    Can anyone help me please or point me in the direction of someone who can please.I have a 1996 boat with a 5.7 mercruiser electronic ign which has a problem with back firing through the carb on fast pull off,as soon as the revs get to 1500 under load the engine back fires through the carb.The engine will rev well in neutral. I have replaced the plugs. dizzey cap,leads and check the timing and cleaned the carb out,I dont think the auto choke is working but i do allow the engine to warm up well before testing.We where taking the boat away this weekend for the first time but,it's not looking good. thanks

  2. #2
    Kawasaki's Avatar
    Kawasaki is offline Registered User
    Location : Anglesey Wales
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    Wiil the boat perform ok if you don't do a 'fast pull of'?
    Plus
    Where are you?

  3. #3

    Default mercruiser missfire

    Thanks for the reply,the boat will not perform well at all, over 1500 rpm.
    we live nr kidderminster thanks

  4. #4
    Kawasaki's Avatar
    Kawasaki is offline Registered User
    Location : Anglesey Wales
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    OK , too far away from Me
    Was just a thought

    If she ticks over ok that usually means there are no probs with the 'low tension' side of the Ignition system.
    The 'high tension' side
    IE
    Coils or the CDI/ ECU side of things could be a prob.

    I know it sounds daft
    But if you have been changing plugs and stuff
    Make sure you have the leads going to the correct places!!

    Although She revs well in neutral.

    How did the prob first start?

  5. #5
    Join Date
    Feb 2005
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    If you want specific help on your engine I suggest you try this US forum:

    http://forums.iboats.com/forumdisplay.php?f=21

    No disrespect to anyone on here, but they are real Mercruiser experts.

  6. #6

    Default mercruiser back fire through carb

    Morning, I forgot to say that I have replace the coil as well. I Recently brought the boat so decided to service it(I am what caused the problem),when I first tested the boat after servicing it it misfired and seamed to lack fuel hence replacing the ign parts one by one a week ago I tried the boat and it was much better but still not prefect so I decided to fit a new electric fuel pump and clean the carb out i noticed the the coke flap had sized so I ordered a new repair kit from key parts (now their best customer of the month) fitted a new jet pump and freed off the choke that is when I noticed that the electric choke is not working. we now have a new problem back firing,I have stripped the carb and cannot see anything wrong??

  7. #7
    Join Date
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    First go through every single plug lead from the distributor to the plug and double check its the right one.
    If they are correct then I would suspect the timing is wrong, does it have a knock sensor (350 Magnum motor the sensor is low down on the block in front of the starter motor with a blue/white wire) if so how did you check/adjust the timing as there is a special procedure, you have to put it into 'service mode' first. There is a purple and white wire near the distributor which you have to earth out with a jumper wire before you can adjust the timing or idle speed, then when the adjustments are done you remove the jumper lead and it locks on to the new settings.

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