Finally cured a strange electrical problem

Discussion in 'General Questions' started by Fabian, Feb 8, 2014.

  1. Fabian

    Fabian Well-Known Member

    This post is reproduced from another thread, and it details an electrical problem that was actually easy to solve but harder to diagnose

    My situation is a little different in that the engine runs, but it has a persistent "spark miss", or what feels like a spark miss below 3,300 rpm; getting progressively worse as the engine rpm is lowered.

    I have changed the entire electrical system with new components, including 2 new magnetos and one old magneto from another engine that works. I have changed two brand new Jaguar CDI's and one Jaguar CDI from another engine that worked correctly, as well as installing an original OEM CDI.

    I have changed three sets of Magnecor spark plug leads (all brand new) and 3 new NGK spark plugs.

    I have changed out the original Jaguar coil for 2 brand new Jaguar coils and also the coil that's included in the standard CDI.

    In desperation i have also removed the original engine (which was a rebuilt engine) and replaced it with another fully rebuilt engine, then in further desperation i have installed an engine that previously worked well, but was removed because i lightly scratched the crankshaft trying to replace a crankshaft seal.

    So in total i have replaced the magneto 3 times - the CDI 4 times - the engine 3 times - the spark plug 3 times - the spark plug leads 3 times - the coil 3 times, if you include the coil in the OEM CDI.


    the spark miss replicates itself in every combination i try,

    oh yes,

    and i have been up and down with jetting and removed the diaphragm carburettor and gone back to a standard NT carburettor, followed by purchasing a brand new NT carburettor.
    The end result is that the spark plug miss is ever present.

    I have even tried making up batches of fuel from different fuel companies.

    This is the first time in the history of playing around with these 2-stroke Chinese bicycle engines that i "can't" fix the problem, and yes, the killer switch is disconnected from the ignition system and the (magneto) white wire is isolated.

  2. butterbean

    butterbean Well-Known Member

    I find it extremely difficult to believe, without any actual explanation as to the cause of this problem, that the exact same electrical problem could exist with so many different electrical components and even replacing engines.
  3. keatonx

    keatonx Member

    you never said what was causing the problem lol
  4. Fabian

    Fabian Well-Known Member

    The problem turned out to be the magnet and keyway fit and the interaction between magnet and pullstart mechanism.

    The only thing i didn't change every time i tried new engines was the magnet, because the new/pre-used engines were all partially disassembled or not completely assembled, so i switched out the magnet from one engine to the other.

    What i didn't know is that (as a by product of the newly installed higher compression cylinder head) the pull-start was placing a much higher level of torque on the magnet as it tried to turn the crankshaft via the magnet keyway.

    Even though the magnet was tightened with the aid of a butterfly rattle gun, the torque was great enough to have the keyway and woodruff key taking all of the load when pull starting the engine, especially when i messed up and didn't pull the engine over fast enough to spin it over over top dead centre to start the thing.
    On occasion, the engine spun around and hit top dead centre and stopped in it's tracks on the compression stroke; consequently i ended up pulling the whole bike off the ground by the pull start handle.

    Over time this action flogged out the woodruff key and magnet keyway; allowing the magnet to rotate around the crankshaft in a more advanced manner..
    The typical slop in the system you normally see between magnet and keyway and crankshaft (before tightening up the nut) was 3 times worse.

    The end result was an excess of ignition advance (regardless if i used the original CDI or the Jaguar CDI) and this must have caused problems with the magnet trigger pulse.

    Once i replaced the damaged magnet with a new magnet and woodruff key, the spark miss completely disappeared.
    Surprisingly the magnetism of the new magnet seems less powerful than the old magnet, yet it makes no difference to the visible intensity of the spark.

    The frustrating about the situation was that when installing the damaged magnet/woodruff key, it was not readily apparent that there was so much slop in the system.
    I needed to get a pair of multigrips on the magnet to wiggle it back and forth, because it would take a bit of applied torque to break it free from the standard position to the point where it would move and stop hard on the damaged part of the keyway and woodruff key.

    I can't describe the feeling of relief when finally figuring out a problem that's not readily apparent; and the moment when you "fix it good and proper".

    Having said that, i still don't understand why the spark miss (with the damaged magnet and woodruff key) almost went away when the engine got stinking hot, regardless of all the new electrical components installed on the engine.
    Maybe someone can explain the reason behind that situation?
    Last edited: Feb 8, 2014
  5. V 35

    V 35 Member

    Thank you for that clue ... Magnet loose on crank

    You paid some heavy dues on that one. Good Clue ... Magnet and keyway Good idea to have spare magnet key, and paint mark magnet side that faces out . [ Magnet can go on backwards, less effective ] FWIW ... I wrap a dollar bill around the magnet before replacing armature coil, tighten screws, remove bill, magnet gapped . Us East Coast Guys should be fixin'
    riding season's coming.
  6. jaguar

    jaguar Well-Known Member

    the root cause of this problem was the high compression head which none of these cheap engines should have.
    high compression:
    1) wears down the cylinder liner and rings faster by the extra combustion pressure which partially gets behind/inside the rings to push them harder against the cylinder wall.
    2) wears down the engine bearings
    3) creates too much heat
  7. butre

    butre Well-Known Member

    But it makes more power for a while, and you can completely replace one of these engines for $80. I think it balances out just fine.
  8. jaguar

    jaguar Well-Known Member

    so you think having to occasionally replace the engine is "just fine"?
  9. butre

    butre Well-Known Member

    If it's a daily commuter, I'd hate to be doing engine swaps too often, but for a lot of people these are toys before all else.

    For a daily commuter I'd avoid the two stroke kit all together, they're not particularly reliable to begin with
  10. butterbean

    butterbean Well-Known Member

    Either way, replacing an engine once or twice a year is not practical. I went through 4 engines in two years, and then I went 4 stroke. How many engines have I had to replace in the last year since building my 4 stroke? None. Even if it's a toy, build it right and treat it right and you won't have to throw money at it all the time. No reason to.
  11. Fabian

    Fabian Well-Known Member

  12. Fabian

    Fabian Well-Known Member

    That seems like a disconnected assessment of the interaction between higher compression cylinder head and the magnet. It is not a true statement in the thrust of your argument.

    The combustion pressure is far greater than the compression pressure "and" i typically get 4,000 kilometers out of a cylinder which costs $30 to replace.

    Any engine operation will wear down the engine bearings, but high rpm will prove to be the greatest wear factor in the operation of the engine.

    Prior to my current engine issues with the magnet, the bottom end of my last engine gave me 10,000 kilometers with a CR Machine Manufacturing billet cylinder head. This particular cylinder head is far, far better than the standard head due to it's ability to remove large amounts of heat.
    The other aspect of this situation is improved low and midrange torque which allows usable power at lower rpms. This single effect greatly adds to engine life and a more pleasant riding experience through reduced levels of vibration.

    Not true.
    My bike is fitted with both a cylinder head temperature gauge and an Exhaust Gas Temperature gauge, so i know exactly how hot the engine runs under various conditions; some of them fairly extreme.
    When the carburettor is jetted correctly and the engine is attached with a well designed higher compression head like the CR Machine Manufacturing cylinder head, operational temperatures are noticeably reduced, because it is so efficient at extracting heat.
  13. HeadSmess

    HeadSmess Well-Known Member

    ha ha ha. clutchstart reigns supreme :)

    *giggles to self, in a completely disturbing manner*

    oh my....welcome to the jungle.

    no seriously, its whats playing on my stezza...and having sorted out the suspicious electrical fault in my stereo that i assumed was caused by high temperatures, and bad chinese assembly design (seriously, who puts resistors carrying 420 volts across an earth rail then squeezes high current 6.3 volt wires between said resistors and a chassis that gets to 80 degrees C+ on a hot day?).

    lousy chinese....components! all those resistors burnt out due to a shorted capacitor that gave one tube a 0 volt bias... meaning it was GLOWING :) lucky i was at home...wheres this things fuse?

    so now its cranking :) oh yeah...36 watts of (damaged now. one went redplate when i powered it up, no idea which one was there when it first blew, and then i gently overheated one just to make sure i didnt get lucky and put it back in the same spot-tip. label things before removing!) el34 tubes :) and still clear at full crank :)

    grass is green and the girls are pretty...

    oh. theres no clutchstart with a shift kit.

    oh wont you please...

    shame that...

    take me hoooooome....

    old postie bikes had a high/low range on the box, ontop of the stock 4 speed... wouldnt auto 2-3 speed be nice with a high low range selector for tractor drivers? :rolleyes7: could clutch start posties :p

    ha ha. 2 wheeled tractor.

    oh no. now its cranberries. how did this get on my puter? :eek: