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302 Mercruiser, Popping/backfire through Carb?

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

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

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

      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.
      For the time will come when men will not put up with sound doctrine. Instead, to suit their own desires, they will gather around them a great number of teachers to say what their itching ears want to hear. They will turn their ears away from the truth and turn aside to myths.-2 Timothy 4:3-4:

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

        did you check/set the timing??

        fuel filters??
        Sometimes when people try to save a penny, they end up spending a dollar

        It is best to fully understand how to properly operate, inspect, troubleshoot and repair your equipment

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        • #5
          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

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          • #6
            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

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            • #7
              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.

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              • #8
                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

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                • #9
                  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

                  Comment



                  • #10
                    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?

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                    • #11
                      Re: 302 Mercruiser, Popping/backfire through Carb?

                      351w firing order
                      1-3-7-2-6-5-4-8
                      2011 Triton 190 Escape SE 150 Mercury Pro XS

                      I never learned much from winning, but always learn from failure.
                      Semper Fi

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                      • #12
                        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

                        Comment



                        • #13
                          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.

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                          • #14
                            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.
                            93 Larson 190 All American, 4.3 LX 4 brl Weber, Sn OF 073363, Alpha 1 gen 2 SN OF 723905, GR 1.94 R, transom SN OF 090473

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                            • #15
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                              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.
                              For the time will come when men will not put up with sound doctrine. Instead, to suit their own desires, they will gather around them a great number of teachers to say what their itching ears want to hear. They will turn their ears away from the truth and turn aside to myths.-2 Timothy 4:3-4:

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