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  1. #1
    Petty Officer 3rd Class conk3416's Avatar
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    Default 302 Mercruiser, Popping/backfire through Carb?

    I have a mid 70's 302 V8 Ford Mercruiser motor. The engine runs fine at idle and also if you slowly give it gas. But when you hit the gas hard or are on a load in the water it will pop and backfire through carb. Also sometimes will stall after running in water for a while. I have changed the distributor to an HEI instead of points, and have also changed the cap, rotor, coil, checked spark plugs and wires and seem to be fine. I am running out of things it could be. If anyone has any ideas please respond.

    Thanks Andrew

  2. #2

    Default Re: 302 Mercruiser, Popping/backfire through Carb?

    I'm guessing, lol, .................. possible worn Timing chain ?????

  3. #3
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    Default Re: 302 Mercruiser, Popping/backfire through Carb?

    Quote Originally Posted by cajun32 View Post
    I'm guessing, lol, .................. possible worn Timing chain ?????
    Always a possibility but IMHO not likely in a relatively low hour 302. You could be one tooth off in the installation of your new dizzy or you could have wired it for the std 302 firing order. Any Merc 888 used the 351w firing order.

  4. #4
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    Default Re: 302 Mercruiser, Popping/backfire through Carb?

    did you check/set the timing??

    fuel filters??

  5. #5
    Petty Officer 3rd Class conk3416's Avatar
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    Default Re: 302 Mercruiser, Popping/backfire through Carb?

    Cajun32, i will check that in the morning. But it seems like it would always pop and run bad if it was the timing chain. It only backfires when its under a load or if i hit the gas real hard. If you have any other ideas please let me know.

    Thanks Andrew

  6. #6
    Petty Officer 3rd Class conk3416's Avatar
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    Default Re: 302 Mercruiser, Popping/backfire through Carb?

    wizeone, I do have the merc 888, do you know what the firing order is for the 351w? I believe i do have it wired for the 302. Thanks Andrew


    dubs283, Yes i did set the timing, it is at 8deg. And i could not get the water fuel separator off so did not change that, but i did put an inline fuel filter on it. Thanks Andrew

  7. #7

    Default Re: 302 Mercruiser, Popping/backfire through Carb?

    Have you gone through the carb yet? If the accelerator pump plunger or aluminum (on the Rochester 2bbl) check valve ball (below the plunger) aren't working right, the engine goes lean when you try to spool up the revs quickly. Same with float height - if the fuel bowl level is a little low, it will lean out and stall when the bow rises or demand allows air into the main jets. I would pull the top (air horn side) screws and take a look. You may have some junk down in the fuel bowl that is interfering with fuel delivery.

  8. #8
    Petty Officer 3rd Class conk3416's Avatar
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    Default Re: 302 Mercruiser, Popping/backfire through Carb?

    Surfdancer, The carb is a holley 2bbl. I have a friend who is very familiar with carbs and he is going to go through the carb today and check to see if anything is wrong with it. Thanks for the advice, I will let you know if it works.

    Thanks Andrew

  9. #9
    Petty Officer 3rd Class conk3416's Avatar
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    Default Re: 302 Mercruiser, Popping/backfire through Carb?

    ok i am getting the carb rebuilt. That is going to take a couple of days. But if anyone else has anymore possible causes for this problem please tell me.

    Thanks Andrew

  10. #10
    Lieutenant Commander proshadetree's Avatar
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    Default Re: 302 Mercruiser, Popping/backfire through Carb?

    Check the intake for a fireing order.Did it act up before you changed the dizzy or after?

  11. #11
    Commander bifflefan's Avatar
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    Default Re: 302 Mercruiser, Popping/backfire through Carb?

    351w firing order
    1-3-7-2-6-5-4-8
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  12. #12
    Petty Officer 3rd Class conk3416's Avatar
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    Default Re: 302 Mercruiser, Popping/backfire through Carb?

    yes it was acting up before i changed the dizzy, i was hoping that the hei style would make it run better. Thanks bifflefan, i will check to make sure my firing order is correct.

    Thanks Andrew

  13. #13
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    Default Re: 302 Mercruiser, Popping/backfire through Carb?

    Classic syptoms of a lean fuel mixture. Accelerator pump not working. Rebuild the carb, set the choke proper and look for squirts of fuel in the throat of the carb when the trottle is pushed forward. If it squirts, the accel pump is working.

  14. #14
    Petty Officer 1st Class fuzzybob's Avatar
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    Default Re: 302 Mercruiser, Popping/backfire through Carb?

    My WAG is going with the carb fuel prob. You have a couple days waiting for the carb rebuild, plenty of time to get the fuel/water sep off and replaced along with any other filter. Maybe pull the line out of the tank and check the pick-up screen if it has one and the anti-siphon valve. You can also check the tank vent while you're there to make sure your not putting a vacume in the tank. New build on the carb it would be a shame to pull a bunch of water and other crap into it. Good luck.
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  15. #15
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    Default Re: 302 Mercruiser, Popping/backfire through Carb?

    The Merc 888 Ford 302 runs at 10 deg btdc. The 898 Chebby 305 runs at 8 deg btdc. I doubt the 2 deg's difference is you problem though. Check the firing order and the fuel delivery and it wouldn't hurt to pull the plugs and take a reading. If you are running lean, the plugs will show it.

  16. #16
    Petty Officer 3rd Class conk3416's Avatar
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    Default Re: 302 Mercruiser, Popping/backfire through Carb?

    fuzzybob, i will check those things tomorrow. I have just put in an inline fuel filter but i will change the fuel water separator. Is the fuel/water separator just a glorified fuel filter or does it actually work better? I will have both but just wondering.


    Thanks Andrew

  17. #17
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    Default Re: 302 Mercruiser, Popping/backfire through Carb?

    Quote Originally Posted by conk3416 View Post
    fuzzybob, i will check those things tomorrow. I have just put in an inline fuel filter but i will change the fuel water separator. Is the fuel/water separator just a glorified fuel filter or does it actually work better? I will have both but just wondering.


    Thanks Andrew
    IMHO typical marine spin on fuel/water filter cartridges are not world class water seperators but they do work. I think that they are easily overloaded so they should be changed regularly. They do, however, do a bang up job of filtering out particles which makes any other kind of additional particle filter redundant.

    'World class' F/W seperators have the ability to drain the seperated water off the bottom of the filter device. Your typical marine style spin ons do not.

  18. #18
    Petty Officer 3rd Class conk3416's Avatar
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    Default Re: 302 Mercruiser, Popping/backfire through Carb?

    thank you wizeone. That was exactly the answer i was looking for on the fuel/water separator. I will be picking up a new one in the morning.

  19. #19
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    Default Re: 302 Mercruiser, Popping/backfire through Carb?

    Quote Originally Posted by conk3416 View Post
    thank you wizeone. That was exactly the answer i was looking for on the fuel/water separator. I will be picking up a new one in the morning.
    ...and if you should happen to have an inordinate amount of water in the gas tank, sometimes unexplainable, the new one will get overwhelmed shortly as well. If that happens, you'll want to suck as much gas as you can out of the gas tank and try to find the reason for it.

  20. #20
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    Default Re: 302 Mercruiser, Popping/backfire through Carb?

    its the firing order

  21. #21
    Petty Officer 3rd Class conk3416's Avatar
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    Default Re: 302 Mercruiser, Popping/backfire through Carb?

    i have siphoned out a couple of glass fulls of gas out of the tank and dont seem to see any water in the fuel. How do you know if you water/fuel sep. is full or needs replaced?
    or is that something that should be replaced once a year or sooner?

    Thanks Andrew

  22. #22
    Petty Officer 3rd Class conk3416's Avatar
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    Default Re: 302 Mercruiser, Popping/backfire through Carb?

    Ok i have just checked the firing order on my boat and it is 1-3-7-2-5-6-4-8 instead of 1-3-7-2-6-5-4-8. Well that should solve my problems but right now the carb is out getting rebuilt. So i will let everyone know when i get the carb back but i would like to thanks everyone right now for all of the help,

    Thanks Andrew

  23. #23
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    Default Re: 302 Mercruiser, Popping/backfire through Carb?

    Quote Originally Posted by conk3416 View Post
    i have siphoned out a couple of glass fulls of gas out of the tank and dont seem to see any water in the fuel. How do you know if you water/fuel sep. is full or needs replaced?
    or is that something that should be replaced once a year or sooner?

    Thanks Andrew
    If you know that you siphoned gas out from the bottom of the tank and have no noticable amount of water in the bottom, when it settles, then I would just change the element every year. If you do not know when the last time it was change, I would do it now to give you a baseline.

    10% ethanol gas will suck water out of the air, so there is no doubt that the element will have a job to do even if minor. I've siphoned gas out of tanks where I have had a whole cup full of water settle out in the bottom, but you have to be sure to get to the far lowest corner of the tank. I made a semi rigid dip tube that will reach the far lowest corner of my tank, then I just crank up the bow of the boat so that all the water will go to that far lowest corner.

    Yep, that messed up firing order will throw a wrench into the whole works. Good you checked it.

  24. #24
    Petty Officer 3rd Class conk3416's Avatar
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    Default Re: 302 Mercruiser, Popping/backfire through Carb?

    ok i put the rebuilt carb on and also got the firing order right and engine runs GREAT. But it is overheating on me. A problem i thought i had figured out but guess i dont. I started a new thread for the overheating problem. But i just wanted to thank everyone for there help on the running issue. I really appreciate it.

    Thanks Again, Andrew

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