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XJ650 Maxim - remove resistors in plug wires

Discussion in 'XJ Modifications' started by KA1J, Apr 8, 2010.

  1. KA1J

    KA1J Member

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    I was asking some questions about the coils & how they work in this bike in the XJ chat & asked about possible negative effects if I bypass the cap resistors and run straight from the coil secondary to the plug. Then I realized I should ask mod questions here...

    I used to run a MSD-7 on my cars with a distributor and the spark off that sucker would burn the carbon off a pencil. Well, almost... but it made a huge difference in the car's performance. Seems like removing the resistor and getting a full spark at the plug would be a no brainer.

    Anything that seems too easy usually isn't, so what's the downside to doing that on the XJ?

    Thanks!
     
  2. seaguy

    seaguy Member

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    A great deal of engineering went into this bike to get the best performance. They had a reason for the resister caps and the plug type they chose. It might shorten the life of your coils and overwork the already minimal size alternator.
     
  3. KA1J

    KA1J Member

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    Thanks for the reply.

    That may indeed be the answer but the operative word is "might". I'm sure there are people who have used aftermarket coils or replaced plug wires and not used the resistor cap when doing so. I'd like to know what their actual experience has been after doing that.

    From what I remember, the resistor plugs in cars were to reduce the engine impulse noise in the radios. I don't recall seeing resistor caps in cars but some plugs had a resistor built into the plug itself.

    I do know my cars ran markedly better with that MSD system on it and the spark that made was easily 4" long. Engines ran like electric motors. Awesome...
     
  4. chacal

    chacal Moderator Moderator Supporting Vendor Premium Member

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    IGNITION SYSTEM OVERVIEW:

    Before we get into the list of components within your ignition system, it may be useful to explore the basics of the ignition design used on these bikes, as this knowledge may help you to better recognize, troubleshoot, and repair performance problems with your engine that you think may be due to these components.

    The ignition system actually begins at the left end of your CRANKSHAFT, since the rotational position of the crankshaft determines the position of the pistons and of the camshafts. Obviously, since the purpose of the entire ignition system is to deliver a high-voltage spark at the plugs at exactly the proper instant----meaning, as the piston approaches Top Dead Center of the compression stroke----then the ignition system must "know" what the position of the crankshaft is in order to transfer that information (via electrical signals) to the major components: the PICK-UP COILS, then onto the TCI UNIT, to the IGNITION COILS, via the PLUG WIRES and through the PLUG CAPS and finally, onto the SPARK PLUGS.

    But it all begins at the crankshaft, which has a flat metal ROTOR DISC bolted onto the left side snout, and which hides under the left side, round "Oil Pump Cover" (also called a "YICS" cover on YICS-equipped engines). This spinning rotor disc has a small magnet embedded within it's outer tip, and as that outer tip rotates past the fixed magnets within the PICK-UP COILS, the interaction of magnetic fields triggers a small voltage in the pick-up coil wires that lead to the TCI UNIT.

    Note that since the rotor disc is fixed in position and spins along with the crankshaft, this rotor disc "knows" the position of the crankshaft at all times. And since the pick-up coils are bolted in place, and are thus stationary, whenever the spinning rotor passes by a fixed pick-up coil, and thus triggers it to send a voltage signal to the TCI, in this way the TCI unit thus also "knows" where the crankshaft is, rotationally-speaking, and thus where the pistons are in relation to Top Dead Center and when their spark plugs need to be fired.......

    Also note that since there are only two pick-up coils for your four-cylinder engine, that each pick-up coil is actually providing the "firing signal" to the TCI unit for two different cylinders. In these engines, one pick-up coil is responsible for sending the signal to the TCI that eventually leads to the spark plugs firing off for cylinders #1 and #4 at the same time, and the other pick-up coil sends the message to the TCI unit to fire off spark plugs #2 and #3, again, at the same time.


    Although this may seem odd at first, the mechanical arrangement within the engine of the crank throws, and thus the rods and pistons, as well as the camshaft timing, allow this situation to proceed without a problem; in fact, when one of these "paired" cylinders (for example, #1) is approaching Top Dead Center of it compression stroke----and thus is in need of a spark from its spark plug----it's "mated" cylinder (#4) is also approaching Top Dead Center, too.........but on its exhaust stroke.......and so even though cylinder #4 gets a spark at its spark plug, there's nothing in the cylinder to combust, and thus it's a "wasted" (yet harmless) spark that occurs in cylinder #4.

    Obviously, the exact same situation occurs in the mated pair of cylinders #2 and #3.

    In fact, the whole system is known as the "wasted spark" system, since one of the two sparks that always occur at the same time is "wasted" on a cylinder that is on its exhaust stroke............


    Anyway, to continue our journey: when a pick-up coil is energized by the passing magnetic field of the spinning rotor disc, it send an electrical impulse signal to the TCI. Therefore, the TCI unit now also "knows" the position of the crankshaft (and thus of the piston). Using other sensor information.....primarily, the rotational speed (RPM's) of the engine......the computer chip in the TCI is then responsible for calculating exactly when to send a "message" to the proper IGNITION COIL to release it's energy to the proper cylinders. And note that we said "cylinders" (plural), since just like the pick-up coils, one ignition coil also sparks two cylinders at once (part of the same "wasted spark" method discussed above).


    The ignition coils use a rather small (12V) input on their primary side to product a large (20,000V +) amount of electrical energy on their discharge (spark plug wires). When the TCI unit determines that "the time is right" for a particular coil to fire, it grounds that coil, which collapses the small magnetic field inside the primary side of the coil, which thus induces a large electrical field in the secondary (plug wires) side of the coil, which then rushes to ground (the plugs are grounded to the head). This electrical energy rushes down the non-resistive PLUG WIRES, through a resistor in the SPARK PLUG CAP, and finally jumps a small gap in the spark plug electrode on it's way to ground, and thus the spark occurs that fires the air-fuel mixture (in one of the two cylinders being sparked at the same time), things go boom, power gets produced, and you're on the way down the road........


    Now, What Could Go Wrong?:

    Well, remember, all of these components are basically used to transmit knowledge of where the piston is within it's four-stroke cycle, and to be able to deliver a spark at the appropriate time to each cylinder to take advantage of the compressed air-fuel mixture in the cylinder (during the compression stroke). So it sometimes help to understand these component pieces as being mainly responsible for providing this stream of precise information, and the electrical-spark making (at the spark plug gap) as merely being the natural (according to the laws of physics and electricity) end-result of all this information handling and processing.


    Now, unless your crankshaft was installed backwards (an impossibility, by the way!) or the camshaft was installed improperly (or if the camshaft drive chain has skipped a tooth----not a common occurrence, at all), then we can safely ignore all of these issues. And since the spinning rotor disc used to trigger the pick-up coils is bolted firmly in place at the end of the crankshaft, it can be ignored for troubleshooting issues.


    But the pick-up coils themselves can go bad over time, and even though it's rare, it can happen. No pick-up coil signal out----along their wires to the TCI unit-----means no signal into the TCI unit, and the whole system breaks down. Which means the small wires from the pick-up coils to the TCI had better not be pinched, shorted to ground, or broken internally, or the message just won't get through.


    Of course, if the TCI unit itself is defective, then even if it is getting a signal from the pick-up coils, it is unable or unwilling to process the signal, or the signal gets processed incorrectly, and either the signal doesn't get sent to the ignition coils at the proper time, or doesn't get sent at all.


    But even if the pick-up coils and the TCI are performing flawlessly---which they usually are----if the ignition coils are not getting enough voltage input to them, or, if they are not able to multiply the small incoming voltage into a much higher output voltage (that is, after all, their main purpose in life), then we have a problem.

    Worn-out ignition coils, or coils that perform poorly or get cracks in their outer cases (and thus short out when moisture enters their internal shells), are a common cause of ignition system problems.


    And even if the coils are performing properly, if the spark plug wires have an internal break, or an external break, and thus prevent the coil electrical output from reaching the plugs, then nothing good is going to come from all this activity.


    Of course, the spark plug Resistor Caps also play a role. They have a tiny resistor embedded within them, and the purpose of the resistor is to provide, of course, electrical resistance to the flow of electrical energy. There are a couple of reasons why some resistance is necessary-----it helps eliminate electrically-generated Radio Frequency Interference "noise" (RFI), it provides a "cushion" against instantaneous electrical energy pulses (which is really hard on small, fragile electrical components, such as the capacitors and transistors in the TCI unit), and it "slows down" the passage of the electricity through the spark plug, thus providing a sort of "electrical Viagra" to spark at the plugs, allowing the electrical discharge (and thus the spark) to last a bit longer (instead of being instantaneous), thus promoting more-better and fuller combustion of the air-fuel mixture.

    In fact, outside of the RFI suppression issues, it is the control of this "spark burn time" that is really the most critical issue, especially on bikes that are jetted a bit lean to begin with........as increased electrical resistance in the secondary circuit will increase the spark firing or "burn" time, and that longer burning spark assists in the more complete combustion of harder-to-fire lean fuel mixtures.


    But those resistors in the spark plug caps........or in the spark plugs themselves, for those engines that use resistor plugs........do wear out over time. And when they do, their resistance increases, which means that they provide more resistance to the flow of electricity than is needed. So that 20,000 volts of electrical energy, instead of being discharged at the spark plug gap in a rather short (10 milliseconds) amount of time, gets "spread out" over a much longer period of time, and gets reduced in voltage, too. So when plug caps or plugs "go bad", they rarely fail to the point where no spark occurs at all, it's just that the electrical output is being "spread out" over such a long period of time that the energy being created in the spark plug gap is so low that it's not enough to fire the fuel mixture completely (or at all)........and that's what leads to hard starts and poor performance (and reduced gas mileage, too).

    By the way, as you may have figured out by now, a spark plug that is contaminated ("fouled") by carbon or oil deposits, or one which has too large of a gap, fail to operate properly mainly because such situation can greatly increase the electrical resistance characteristics of such a plug.........and now you know why that's not a good thing.


    A very good review of the issue of electrical resistance in ignition systems can be found at:

    http://www.ultralightnews.com/enginetro ... dplugs.htm

    Although the above article references the ignition systems in ultra-light aircraft, the same concepts apply to all ignition systems.



    Okay, so that's your nickel tour, and although it's not as detailed as it could be, hopefully it's enough to get you started. A good voltmeter (also called an ohmmeter) is an invaluable friend when trying to track down ignition system problems, as you must make sure that the "information" between components is actually able to travel from Point A to Point B properly, and that the individual components are, electrically-speaking, able to process and transmit the electrical information properly.


    A good companion write-up to this subject and some trouble-shooting guidelines can be found at:

    http://xjbikes.com/Forums/viewtopic/t=21932.html
     
  5. Polock

    Polock Well-Known Member

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    i use accel solid copper wires and boots, non resistor plugs with stock coils
    never had any tci problems
     
  6. padre

    padre Member

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    It seems to me that I learned in emissions 101 that resistors, be they in the wires or the plugs, improve spark attenuation and a sharper spark allowing wider gaps and leaner mixtures, if you remove the resistors they you may foul plugs and either have to switch to hotter plugs, resistor plugs or closer gaps (bad idea) the lean mixture won't burn as well and the unburnt fuel may foul the plugs as well as rob you of torque and HP and fuel economy. Too wide a gap on the other hand may make the engine run hot (bad idea) ping and blow holes in your tiny pistons.
    If we had a brainmaster, a dyno and a scope with an infa-red then we could probably out think the engineers. Using 8 mm wires and resistor plugs would probably work ok, I believe that Autolite 62s are direct replacements for br7es NGKs. if they don't have NGKs where you live . Both are resistors but if you widen the gaps beyond say .036 you'll probably have to fatten up the main circuit to slow down the flame travel and use more gas to get the same power. Theoretically that is. I use bp7es @.034 which have a projected tip, they are more in the center of the Hemi style chamber and they're a little longer so they raise the compression ratio a tic. All else is stock ignition wise. Lights right up first try almost every time even when temps are in the 40s and the air is below 20% humidity.
     

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