2-stroke vs 4-stroke spark plugs;am I dreaming

Home Forum Ask A Member 2-stroke vs 4-stroke spark plugs;am I dreaming

Viewing 8 posts - 11 through 18 (of 18 total)

  • johnyrude200


    Replies: 782
    Topics: 186
    #20080

    Out of curiosity, would running an old 18hp (stated to run on a J4J) using a J4C cause an overheat condition?

    The reason why I ask, is this is the 2nd 18hp late 60’s OMC I’ve worked on in the last couple of months where it is pumping water like a fire hose, and is showing a progressive overheat condition with a new thermostat and impeller, no water clogs (I pulled the cylinder head and checked all places).

    I just read this post and realized I have a batch of J4C’s NOS. If these run hotter, it pretty much would explain this issue. The motors run OK with no thermostat and stay within an acceptable heat range. With the thermostat, the cylinder head thermostat orifice isn’t even reaching 125° at WOT but the cylinders are progressing beyond 170° so I shut the motor down or throttle down to idle, and everything cools down to normal.

    This does not happen without the thermostat.


    frankr

    US Member - 1 Year (includes $3 online payment fee)
    Replies: 6715
    Topics: 51
    #20084

    No, it would not make the engine run hotter.


    johnyrude200


    Replies: 782
    Topics: 186
    #20088

    well, either it’s not pushing enough water through with a test prop, or I’m just plain stumped again.

    I ran the motor without the thermostat and it’s running at an acceptable temperature, and the plugs are showing normal coloration now.

    Thanks for at least helping me eliminate 1 thing off the check list.


    legendre


    Replies: 389
    Topics: 8
    #20112

    An interesting discussion – I’ve never before heard thermionic emission theory applied to the spark plug polarity debate. But it does make sense, if you think about the characteristics of a vacuum diode.

    In a vacuum diode, electron current can +only+ flow from the cathode to the plate (anode), else it wouldn’t be a diode. There is also a very small reverse current when the AC switches polarity, but that’s due to a lack of full evacuation of the envelope, which allows the few remaining gas molecules to ionize and conduct a minuscule current from anode to cathode.

    If you think about a spark plug in a combustion chamber as an analog of the elements in a vacuum diode – one of which (the center electrode) is much hotter than the other, and so behaves more like the cathode in the vacuum diode, you can conclude that electrons moving from center to outer electrode have greater mobility..

    Now that the issue is reasonably well framed, it might be time to test & measure.. who’s up to it? 😉

    (Any effect, if real, is measurable.. period.)


    johnyrude200


    Replies: 782
    Topics: 186
    #20116

    I’m actually starting to wonder if the motor is over-revving in the test tank with a test prop. I’m going to set up a tach on the motor and see what type of RPM’s it’s turning. Since it pre-dates CDI powerpacks, it doesn’t have a limiter.

    I’m wondering if some of the veteran techs know if test props allow for over-revving a motor, which I would think would cause false positive overheat conditions. The service posters by OMC have minimum recommended RPMs for testing motors out, and I spend the majority of my time testing motors in the test tank with test props. Anything much above 6hp, I generally throw on a test prop.

    I’ve found that, even in my 250 gallon tank (which is only a tad below the minimum recommended size…some day I will build a custom test tank that is large enough to run regular props…when I have a big back yard!), I get a lot of false positives where air pockets form behind or around the water intake at medium to high RPMs. I try to turn the motor to either side to get the water swirling around the tank, but particularly with thru-hub exhaust motors, the water just plain turns into a bubble bath.


    debe


    Replies: 103
    Topics: 0
    #20119

    In relation to spark polarity in 4 strokes in particular. When I was a Ford mechanic the Falcon in Australia, ran 3 double ended ign coils for a 6 cylinder engine. The center electrode wore away quicker on 3 cylinders & the other 3 would still be good. This would eventualy result in the engine not running smoothly. The fix for longer life was to fit platinum plugs in these types of ignition setups. You then got 100,000Kms easily before replacing them.


    rvpapasso


    Replies: 24
    Topics: 2
    #20129

    I started this thread and now I will tell you why. I have been trying to solve hard start issues with a friend’s 1962 Evinrude 40Hp for the last 3 weeks. Engine starts great when cold with choke, idles all day, throttle transitions smoothly and has a great top end. However if shut down and it will not restart until almost dead cold.

    I replaced the magento’s, spark plug wires, spark plugs, grounded the magneto plate to the block and swapped out the ATOM ignition modules with a new set. The float is good and the float needle is not leaking. The temperature gauge (also measured with a thermocouple and DVM on the head) indicate the engine is not overheating. The plugs are NGK B8S and the insulator is tan when I remove them. This really had me stumped.

    On the last trial run the engine ran sweet thru high speed and no wake zones about 45 minutes down river. We shut it down, drifted for about 10 minutes and it would not restart (it is electric start). Fortunately we drifted to a dock and I did some diagnostics. The plugs were slightly wet and I noticed they were side arcing. We did not have a clean set of spark plugs or sand paper (I still am kicking myself for not checking my buddy’s tool kit!) but had a metal file. I took the file and filed the sides of the side electrode clean and around the center electrode. Put the same plugs back in and it fired right up within 1 turn of the flywheel.

    With that we headed back to the marina where we put in at. Once shut down the engine refused to start again. That’s when it dawned on me about the old Champion J6J with the center electrode cut back. Next weekend we pulled the SAME spark plugs and with a Dremel tool cut back the center electrode.

    The engine started 10 out of 10 times, hot or cold, anytime the starter was engaged. No continuous cranking, just a quick blip on the starter switch and it was running. We shut it down for 2 minutes, 30 minutes and even 1 hour with the exact same results.

    My best theory is the plugs were wet fouling when the engine was shut off and winding down still pulling fuel into the cylinders. The starter spins healthy but at low cranking speed the mags maybe were not generating enough energy to blast through the wet electrodes. By having a sharp edge from the side electrode over the middle of the center electrode, the edge doesn’t get as wet and electrons flow better from a sharp edge than a flat surface (aka lightning rods) and that edge is right over the center electrode for the spark to jump. It was like magic!

    I am sure there are going to be many theories as to why this made a significant difference and what the real problem may be. I am just reporting the results and I am sure there was some engineer at OMC or Champion that had good reasons to make the J6J with the center electrode cut back. For now it was the magic bullet for his engine.


    chris-p


    Replies: 2456
    Topics: 153
    #20139

    I believe ya.

    I too cut back my J6Cs and J4Cs to obtain the J6J and J4J style.

    I have a stock pile of J6Js and j4Js, but don’t want to use them!

Viewing 8 posts - 11 through 18 (of 18 total)
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