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Thread: High RMP Backfire

  1. #1
    Senior Member propless's Avatar
    Join Date
    Feb 2009
    After enjoying most of the summer problem free boating, this morning reminded me of the joys of boating – problems 20 miles from the boat ramp. I have an Indmar 454 w/Berkley jet drive. After running for about 45 minutes at 3800-4000 RPM, my engine began backfiring, very sporadic, and only at 3900 rpms and above.
    I shut down in the lake, couldn't find anything loose, distributor tight & vacuum. Started it back up and again, only at high rmps would it begin, and again only sporadic.
    If I attempted to take off (full throttle), it backfired badly so I backed down and slowly accelerated up to a smooth ride. As long as I kept under 3900 rpm's no problems.
    Any idea & thanks up front and appreciate your thoughts!

  2. #2
    Seth Livzz
    Sounds to me like either your ignition timing is off or you're running lean. I wonder if possibly you're getting crossfire between the plug wires.

  3. #3
    Sangster 79
    Sounds like a possible lean condition....

  4. #4
    Senior Member propless's Avatar
    Join Date
    Feb 2009
    Showing my ignorance here, so bear with me:
    I just checked my timing, 34 degrees - OK.
    If my wires are crossed, would I not how this problem throughout my rpm range, or is it there and I do not recognize it at rpms lower than 3900, but greater than 4000 rpm's - I'm confused.
    Running lean, I run and have always ran 93-octane fuel. Should I look for something on the plugs or look elsewhere???
    As always, appreciate the ideas & thoughts! I love boating and want to remain boating!
    (Any idea what is like being a zillion miles away from the dock, with a wife and a great son and a friend of my son’s? They all want fun and no problems and daddy's boat act's up. My wife lowers her glass's and looks at me - come on, you know that look, my son and his friend go, is this it??)
    Ah, the joys of boating when it is in the 90's and not a cloud in sight. Oh yes, the humidity is also 90 - welcome to Louisiana

  5. #5
    I had a similar problem and it turned out to be the plug wires were bad and the spark was transfering through the wires under load.
    might be worth a look.

  6. #6
    spectras only
    Check for excessive rotor play and for hairline crack in the cap near the plugwires.Check the condition of the advance springs under the breaker plate .Next: carburation.

  7. #7
    are u running a points distributor?--could be it finally took a dump--pull it and have it run checked on a distributor machine---is it vaccum advanced?--check it out---cap, rotor, wire replacement would be good insurance if not done recently-- check the coil for cracks--check the primary wiring for loose connections---be safe and watch out for the other guy

  8. #8
    comin' unscrewed
    Is it backfiring through the exhaust or carburator?
    Kind of a way worst case senario but perhaps the cam is going flat? When an exhaust lobe goes flat the cylinder still pulls in a charge and burns it, but since the exhaust valve doesn't open the exhaust exits through the intake valve and pops or backfires through the carburator. You probably won't notice the pop until the RPM range you are talking about.
    Hopefully your problem is ignition related but you might want to pull a valve cover and check for sparkley oil just in case.
    [This message has been edited by comin' unscrewed (edited July 07, 2002).]

  9. #9
    Pull the plugs and check for a burnt off electrode. One of your plugs may have worked loose- this will overheat the plug and burn off the electrode. Also if your plug wires are more than 5 years old replace them with a new set of spiralcore wires.

  10. #10
    Senior Member propless's Avatar
    Join Date
    Feb 2009
    The backfire is coming from the carb. So do I now assume it is the cam or valves?
    Thanks again for the comments!

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