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  1. #1
    Join Date
    Oct 2010
    Location
    Indiana
    Posts
    5

    Unhappy HELP!!! 1980 351 motor dies after warm up

    I have a 1980 Master Craft w/ 351 pleasurecraft I replaced practically everything but the motor itself and alternator...rebuilt carb,new fuel pump and filter, new pertronix ignition with 40,000 volt coil (which I bypassed the resistor because the coil has an internal resistor) So now it starts great and runs great for awhile...after it warms up the motor dies while idling...I restart giving it gas and stays running only with more throttle...throttle down to idle and it dies. It did this before the new pertronix in the distribtor...was out boating for a while then after 20 minutes it started acting up when slowing down to idle....was hard to keep running while trying to trailer the boat. Last summer and the beginning of this summer it worked great. Do you think the alternator has something to do with it? The next step is to have it tested to help rule it out or in as a problem.

  2. #2
    Join Date
    Sep 2008
    Location
    Maryland - USA
    Posts
    8,182

    Default Re: HELP!!! 1980 351 motor dies after warm up

    Not to "beat you up" but that's why many will tell you "throwing parts" at a problem is usually an expensive method.

    Assuming the boat has been cared for reasonablly well, I'd suspect you have a fuel delivery issue. If you want to bet the fuel pump is okay, I'd say look for a restriction between the pump and the tank. Suspects would be crud in the tank, aged hose, broken clamp, or malfunctioning anti-syphon valve. A vacuum gauge can help doing this. Another proven technique is to temporarily hookup an outboard can to supply the fuel pump. If you want to be thorough, you should also verify the fuel pump delivers sufficient fuel (volume) with adequate pressure (fuel pressure gauge).

  3. #3
    Join Date
    Oct 2010
    Location
    Indiana
    Posts
    5

    Default Re: HELP!!! 1980 351 motor dies after warm up

    [QUOTE]
    Not to "beat you up" but that's why many will tell you "throwing parts" at a problem is usually an expensive method.

    Boat: A hole in the water surrounded by wood into which one pours money.

    I'll have to check the lines....I just thought it wouldn't be the lines due to the fact it operates very good after giving it more throttle just dies out when the motor runs for about 20 minutes when I put it back to idle. Any second thoughts?

  4. #4
    Join Date
    Sep 2008
    Location
    Maryland - USA
    Posts
    8,182

    Default Re: HELP!!! 1980 351 motor dies after warm up

    I'm thinking crud in the tank gets stirred up and 20 minutes later it has covered the intake screen at the bottom of the pickup or made its way to the first restriction - the anti-syphon valve.

    The only way to be sure is to get out those instruments and make some measurements.

  5. #5
    Join Date
    Oct 2010
    Location
    Indiana
    Posts
    5

    Default Re: HELP!!! 1980 351 motor dies after warm up

    I checked the fuel pressure and it was right on the money so I took out the 20gallon tank to check to see about the anti-siphon valve....There's one problem...I didn't see anything like an anti-siphone valve. The "L" fitting just has a hose fitting on it and then goes straight down into the tank and the other end goes directly to the fuel pump. I took off the fuel sending unit and looked inside and the tank is really clean. I blew thru the fuel line and nothing was clogging it either. So believe the motor is getting a good amount of gas. So I believe I'm back to square one....anymore thoughts would be greatly appreciated.

  6. #6
    Join Date
    Sep 2008
    Location
    Maryland - USA
    Posts
    8,182

    Default Re: HELP!!! 1980 351 motor dies after warm up

    Fuel pressure was monitored until the issue reappeared (nominal 20 minutes) and was over 4 psi?

  7. #7
    Join Date
    Oct 2010
    Location
    Indiana
    Posts
    5

    Default Re: HELP!!! 1980 351 motor dies after warm up

    After finding out the alternator was bad and putting in a new one this saturday morning (would of been sooner if wasn't for the rain instead it snowed last night) the boat ran for a very long time without it dieing. So the problem is fixed so far. I still have to get it out on a lake and run it out there to see if all problems are solved. Maybe tomorrow morning?!? Thanks Mark for any possiblities but it wasn't in the fuel system but just a bad alternator.

  8. #8
    Join Date
    Oct 2010
    Location
    Indiana
    Posts
    5

    Default Re: HELP!!! 1980 351 motor dies after warm up

    When I installed the Pertronix ignition and Flame Thrower coil (which told me no need for the ballast resistor) I believe I found the problem and it was done and over with. Nope! After long time online trying to find the problem It came down to the Ballast Resistor. The new coil was rated at 1.5 ohms which is only good for the strip and short time runs because the boat ran fine until the coil got to hot to work properly thus the motor dies out. I replaced the ballast resistor and so far it ran good for a long time but I still need to let it run for a much long time. You definately need that resistor to get the voltage down before it get's into the coil and using a resistor of an additional 1.5 ohms seems to be doing the trick. Now for the others with the same problem of a hot coil make sure you test your ballast resistor to make sure it doesn't need to be replaced (12-13 volts on one side - 9-10 volts on the other side of the resistor)

  9. #9

    Default Re: HELP!!! 1980 351 motor dies after warm up

    Quote Originally Posted by skiingMike View Post
    When I installed the Pertronix ignition and Flame Thrower coil (which told me no need for the ballast resistor) I believe I found the problem and it was done and over with. Nope! After long time online trying to find the problem It came down to the Ballast Resistor. The new coil was rated at 1.5 ohms which is only good for the strip and short time runs because the boat ran fine until the coil got to hot to work properly thus the motor dies out. I replaced the ballast resistor and so far it ran good for a long time but I still need to let it run for a much long time. You definately need that resistor to get the voltage down before it get's into the coil and using a resistor of an additional 1.5 ohms seems to be doing the trick. Now for the others with the same problem of a hot coil make sure you test your ballast resistor to make sure it doesn't need to be replaced (12-13 volts on one side - 9-10 volts on the other side of the resistor)
    Im having the exact same issue on my 1982 MasterCraft with the same motor. Did the ballast resister fix your problem?? It seems from your post that the alternator seemed to have fixed it but after further testing it ended up just being the lack of ballast resister was causing the coil to get too hot? Looking for help on my same issue. This is the best thread Iíve found so far. Thanks in advance for your reply!

  10. #10
    Join Date
    Dec 2015
    Location
    maplevalley WA
    Posts
    5,049

    Default Re: HELP!!! 1980 351 motor dies after warm up

    Just be sure and provide battery voltage to the module as low voltage is fatal to the pertronix and hot spark ignitions. Just power the module from the ignition switch side of the ballast.

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