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Backfire during deceleration

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  • Backfire during deceleration

    Chilly sunset putt tonight with wife and pup. Beautiful clear evening. Cruise about 3k to 3400 mixed with moments around 2k. Pulling throttle back quickly from the higher rpms I'm getting exhaust backfire. I've noticed it a couple other times out but not as pronounced as tonight. A slow throttle down results in no backfire.

    Here's the stats. 1990, 26 open with a 2004, 5.7 MPI with about 970 hours.
    New iridium plugs, wires, cap, rotor, coil...about a month ago (10 hours). Noticed the slight backfire before all that was put in.

    I've read its caused by unburnt fuel in exhaust...but, again I'm no mechanic. Any one experience this before or point me in a direction?

    Thanks

  • #2
    Check for an exhaust leak at the head, the center ports on SBCs run hot and cause the manifold to warp. Exhaust backfire happens when hot fuel meets oxygen. Exhaust leaks flow both ways, we hear the high pressure outflow but not the low pressure that draws air in.
    Steve
    1998 260 Mackinaw Sea Maxx 5.7 HO TBI 12' Klamath w/ 20hp Tohatsu EFI

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    • #3
      Thanks tuner. I'm planning on pulling manifolds and replacing them when the boat pulled. But I guess to check for a leak prior to removing manifolds I should listen at the manifold-head mating area? I haven't heard any prior to now but I'll check next trip.

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      • #4
        I use a length of vacuum hose. Put one end in your ear and run the other end around the exhaust flange. A short piece of soft copper tubing in the rubber hose can be very helpful in hard to reach areas, just bend as needed.
        Steve
        1998 260 Mackinaw Sea Maxx 5.7 HO TBI 12' Klamath w/ 20hp Tohatsu EFI

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      • #5
        Click image for larger version

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Views:	39
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ID:	29743 Well, the manifolds came off without too much of a struggle...I was surprised. The hardest part was separating the elbow from the manifolds. I ended up using a angle grinder and a cut off wheel to remove the portions of the elbow that the bolts pass through . After it was cut a BFH did the final separation.

        The question now is do I attempt to salvage the manifolds? They are FWC but seem to be showing their age. The elbows definitely will be replaced.

        This brings up another question. What brand manifolds and gaskets should I go back with? OEM or off brand? ​ ​​​​​​​

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        • #6
          If the manifolds are FWC, they should be good forever. Whatever riser you get, test them before installing and retighten 3xs after install. Coat the entire bolt with never seize.
          1988 26' cuddy
          1975 20' open
          16' Sandpiper
          14' Certified Fiberglass

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          • #7
            Originally posted by Quik Fix View Post
            Coat the entire bolt with never seize.
            I just bought a new PCM set for a Ford. In their instructions they tell you not to use anti seize. I think they do that to keep manifold sales up.

            As to clean up can you use a rubbermaid type container and carefully soak them in muratic acid? Or this method- https://antique-engines.com/electrol.asp

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            • #8
              Wow, thats a pretty interesting way to clean manifold, sort of a plating process.

              Honestly all I was thinking about doing is using my angle grinder with a wire wheel on it to remove the flaking and surface rust. Then I was going to wrap a piece of 2 by 4 with Emory cloth and hit the flat faced surfaces.

              Engine has over 900 hours. I'm trying to squeak out another 500. Hopefully these manifolds will not show any damage and be able to make it the additional 500.

              While they are off I'm going to check shaft alignment since the front mounts will be easier to get to.
              Also clean the trans oil cooler and the heat exchanger.
              Maybe pull injectors and send them to Brocoto, replace fuel pump filter and fresh water pump..

              Nothing is actually giving me trouble, just trying to be preemptive.

              I'll put it all together and it won't run, just watch.
              Probably wish I'd have stopped at manifolds.

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              • #9
                Gary, I like simple. Muratic acid is my friend. Use carefully. I think that you are not supposed to use never seize on the bolts is for torque purposes. If you retighten after the first few cycles, I don't see a problem. Been doing it that way for a long time. The bolts seize in the manifold, not the threads.
                1988 26' cuddy
                1975 20' open
                16' Sandpiper
                14' Certified Fiberglass

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                • #10
                  The more I think about the electrolysis cleaning the more it makes sense since all the internal passages will be cleaned as well.
                  Maybe a brushing and a soaking. We'll see.

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                  • #11
                    When I say muratic acid, I fill the salt water passages only.
                    1988 26' cuddy
                    1975 20' open
                    16' Sandpiper
                    14' Certified Fiberglass

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                    • #12
                      When dad brought home the Shamrock I have now I remember him using the muratic acid but I do not remember how they looked when he took them out. He made a box lined it with plastic and submerged them for a time. Don't think he left them in long,they worked and looked fine for 19 years in fresh water. A year after he passed I ran it to the ramp and took all summer to fix and paint it up. Ran it in the drive and got a milk shake. Got some used ones to tide me over,kind of a last minute deal just before I took it south.
                      TSP is available at Home Depot and works great in an electrolysis tank but electrolysis will not do the insides of the manifold because the electrode needs to be in a direct line of the derusting . You will have to wire brush it when done but your not taking rust off your just removing left on gunk. You also need the old style battery charger these new computer chip sensored ones won't allow it to turn on.

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                      • #13
                        Take the manifolds to a machine shop that has a large belt sander for exhaust manifold flanges.I use Felpro marine exhaust manifold exhaust gaskets. Be sure to check the torque on the bolts after the first 2-3 heat cycles. Do the first re-torque after running it on the the hose for 10-15 minutes.
                        Steve
                        1998 260 Mackinaw Sea Maxx 5.7 HO TBI 12' Klamath w/ 20hp Tohatsu EFI

                        Comment


                        • #14
                          Will do. Probably won't happen for a week or 2.

                          Anti seize question....do I anti seize the threads also or just the shank portion? My concern about putting it on threads is that they'll loosen on their own.

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