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  1. #1

    Default 85 PCM 351 losing spark as rpm's increase


    I'm working on a Nautique that idles great and motors smoothly out of the marina. As soon as you begin to put her under a load the engine begins to stumble and not pick up speed. Increasing throttle causes the motor to lose power.

    The only thing that I have noticed occurring is when I hold the coil wire just out of the center of the distributor cap while the engine is idling I can see a nice, strong, blue spark jumping the air gap I've created. There is an audible "snap" every time the spark makes the jump. As I begin to increase rpm's, the spark begins to fade and the sound begins to diminish to the point where I have to push the coil wire further down into the cap in order to keep the engine running. It appears as though something may be wrong with the ignition system because I'm pretty sure the spark should stay strong and blue throughout the rpm range.

    Here's what I've done -

    Compression test good
    Engine vacuum test good
    Coil Resistor
    Ran 12v to the coil direct
    Made sure no other wires were connected to the coil like the tach
    Added an extra ground to the distributor and the motor
    Removed the distributor plate to look for a broken braided ground strap, none
    Checked operation of the distributor weights, good
    Added a squirt of starting fluid to the carb while the engine was missing in case of a carb issue, no change
    Advanced and retarded the distributor by hand while engine was missing, no change

    Short of upgrading to an electronic ignition I can't think of anything else to try. Thanks in advance for any help you can give. I'm so over it right now.

  2. #2
    Join Date
    Sep 2008
    Maryland - USA

    Default Re: 85 PCM 351 losing spark as rpm's increase

    ignition switch isn't on your list....and it may be easier to execute a trial with a fused jumper straight to the ballast....

    I would also make sure the timing advance is functioning correctly my measuring the curve....
    'operation of weights' != correction function provided...

  3. #3

    Default Re: 85 PCM 351 losing spark as rpm's increase

    Thanks for your reply makomark.

    I ran a 12v lead from battery positive to both sides of the resistor and directly to the coil with no change. I went ahead and used a timing light today and timing is advancing. I hadn't used a timing light before because I assumed advancing the timing by hand at 2k rpm's with the engine missing was esentially doing the same thing. No change in pattern at all.

  4. #4
    Join Date
    Sep 2008
    Maryland - USA

    Default Re: 85 PCM 351 losing spark as rpm's increase

    eliminates the switch being an issue...

    did you set the points w/ a dwell meter or feeler gauges?

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