Results 1 to 2 of 2
  1. #1
    Click avatar for tech links/info, donation request K447's Avatar
    Join Date
    Jul 2007
    near Toronto, Canada

    Arrow How does the Fuji (650,750,780) ignition coil system actually work?

    The blue Fuji engines (but not the Pro 785) fire all three spark plugs together, every time any cylinder needs spark. All three ignition coils are connected together in series, and the CDI simply pulses all three coils together as a set. This is known as a 'wasted spark' ignition system.

    Note: The two cylinder red domestic engine with Gen III (Update Kit) ignition is also a wasted spark design. As far as I know the other Polaris engines and versions are all 'sequential spark' ignition.

    All three coils fire all three spark plugs at the same time. The cylinder that happens to be near TDC actually ignites, while the other two cylinders also spark but there is no combustible fuel in the chamber so no ignition occurs. Then the next cylinder approaches TDC and the CDI fires all three spark plugs again. And so on...

    At 6,000 RPM the CDI is firing all three spark plugs 18,000 times per minute. Each spark plug is sparking three times per crank revolution. You can see how important it is that the entire ignition system be in good shape.

    Resistor spark plugs and decent plug wires are critical to controlling this onslaught of ignition pulses.

    Also important is the ground return path. Every time the CDI pulses the ignition coils, each coil delivers a high voltage spike down the plug wire and into the connected spark plug. But the high voltage pulses don't stop there!

    Electricity always travels in a loop. The high voltage pulses that go through the spark plugs still need to return back to the ignition coils. That means the pulses from all three spark plugs go through the engine head and cases, and continue on through the Black 'ground' wire back into the electrical box. Yes, the high voltage 'spark' pulses actually travel back through that thin Black wire from the magneto stator.

    You can see how important that Black wire 'ground' connection between stator and electrical box really is. Within the electrical box the voltage pulses (actually they are current pulses) now travel through the Black wire and into the ignition coils. Each pulse flows back into the ignition coil it came from.

    An unusual aspect of the Fuji ignition coils is that the 'ground' path for two of the three coils actually flows through the primary coil side of the other coils. So you have current pulses from the 'secondary' coils flowing back through the 'primary' side of the coils.

    Since all three coils are connected together, a bad ignition coil can affect ignition on all cylinders. Incorrect spark plugs or bad plug wires can cause tremendous electrical interference that can upset the CDI, causing reduced power or other weird problems.

    This is why Polaris demands resistor type spark plugs, and why some non NGK spark plugs just don't seem to work as well as the genuine NGK plugs specified for the engine.
    Last edited by K447; 07-08-2011 at 10:21 PM.

  2. #2
    Tow-in-Barrels's Avatar
    Join Date
    Jun 2011
    Wilmington NC
    dang....... i found in my newest purchase a small black wire with a broken connector in the box, it seemed to connect the two halves of the box together. it was just a jumper off the CDI ground on the other side and i am hoping against hope that it A. was the problem causing this unknown to me engine from running (hence the cheap purchase price) and B. that having this small black (ground) wire disconnected did not overload either the cdi box or the coils { although i have a spare set of coils, also unknown condition} sl650 1995 ignition system.

    so are both of these ground wires crucial? or is the pigtailed one more of a backup to keep current from arcing from one side of the box to the other?

    either way very interesting fact about the wasted spark, i bet that is what helps give these two stroke tripes that off-beat rhythm. sort of a UK Garage beat if you know what i mean. i bet there are tiny explosions in the pipe when running rich.

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. Fuji 780 in an SL 750?
    By bschinstock in forum Polaris How To & FAQs
    Replies: 7
    Last Post: 08-01-2011, 12:08 PM
  2. Fuji 750 & 780 Engines
    By kwu3757 in forum Polaris Open Discussion
    Replies: 2
    Last Post: 09-10-2009, 06:59 PM
  3. Auto cock and fuel restrictor for 650/750 Fuji engines
    By K447 in forum Polaris Open Discussion
    Replies: 0
    Last Post: 05-15-2009, 09:01 PM
  4. WTB: '91 Yamaha 650 Ignition Coil
    By FLjoyrider in forum Yamaha Classifieds
    Replies: 2
    Last Post: 06-02-2008, 07:57 PM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts