1983 Shamrock CC 302 Holley Backfires Under Load

Discussion in 'Mechanic's Corner' started by Jeepinator, Sep 13, 2021.

  1. Jeepinator

    Jeepinator Member

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    We installed a temporary automotive carb to troubleshoot some other issues. The boat will not get on plane due to backfiring through the carb and low power at higher RPM. Under 2300 it runs great. Checked distributor, plugs; no obvious issues. I'm hearing it maybe due to the secondaries starving for fuel and we may need a carb that has sight glass to allow us to adjust the carb for bow-up to plane? Someone else mentioned a "wedge" plate.
     
  2. crhodes

    crhodes Well-Known Member Supporting Captain

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    Ignition system issues can cause this. Don't know what you have (points & condenser, electronic, etc.) for an ignition system so hard to say. Bad condensers, improperly set points, or weak coils can cause this. There are so many things that could cause this. On my boat before I changed to fuel injection could lift the engine hatch underway and with a removed flame arrestor look down the barrels to see if the secondaries were flowing gas. (There is some risk with this.) Not knowing if "CC" stands for Cuddy Cabin or Center Console not sure if this is practical on your boat. Would have a friend along to hold engine cover if practical.

    There are major top-end issues (weak valve springs, etc.) that could cause this but premature to go there.

    Questions, were "other issues" engine related and did you have this problem with the previous carb?
     
  3. Jeepinator

    Jeepinator Member

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    No points and the rotor and cap looked good. The old carb was full of a yellow gel-type goop especially the secondaries. It never ran.

    We had water in the fuel to the point where we had to completely drain the tanks.

    it’s a cuddy cabin so easy access to engine.
     
  4. redneck

    redneck Well-Known Member

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    Does the distributor have advance weights that are sticking?
     
  5. Jeepinator

    Jeepinator Member

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    That I do not know. We are still sorting out how this mechanical advance works. I had a few people tell me that if time wasn't advancing then it would backfire through exhaust. I'll take a look at the weights though.

    Tnx!!
     
  6. redneck

    redneck Well-Known Member

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    I had this same problem on a Merc 470 it was stuck flyweights in the distributor. You should be able to take the cap off and see either fly weights or some sort of of advance mechanism. Try turning the rotor by hand you should see the advance mechanism work.
     

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