CDI upgrade; why?

Discussion in 'Performance Mods' started by jaguar, Nov 25, 2013.

  1. jaguar

    jaguar Well-Known Member

    The stock CDI is actually designed well for the purpose for which it was intended. Its use with these poorly
    carbureted low compression engines is well matched. Why? It maintains a strong ignition advance all thru its RPM
    range. That is good because these chinese carburetors have around 20% worse fuel atomization due to not having a
    separate idle circuit. The larger than normal fuel droplets take longer to burn which requires more ignition
    advance so that the peak cylinder pressure still happens around 15 degrees ATDC (for best engine power). Also the
    low engine compression (around 95psi) requires more ignition advance. Only with more compression is less advance
    needed because the fuel droplets will be closer together which allows a faster moving flame front for a more rapid
    complete combustion, requiring less ignition advance for the peak cylinder pressure to happen around 15ATDC. Also
    the weak spark allows for a slower burn, matching the needs of this poorly carbureted low compression engine.

    But the engineers screwed up by not properly balancing the crank flywheels and the early ignition of these engines
    contributes to engine vibration at high RPM. With a 10mm or 12mm carbide drill bit you can fix that though.

    So if you are going to stick with the crummy stock carburetor and stock compression you should also stick with the stock CDI (although it is prone to failure). But then you are doomed to perpetual low performance.

    If you want to get decent performance from these engines these are the steps I recommend:
    1) increase engine compression by removing metal from the head mating surface
    2) put on a good carburetor like one of these; Walbro, Mikuni, Dellorto PHBG
    3) install the Jaguar CDI for less ignition advance which matches the previous improvements

    If you stick with piston port intake (without reed valve) then I also recommend extending the intake manifold length. This allows a resonance effect to cram more mixture in at low RPMs for more power.

    I am not a fan of expansion chambers because they have a very narrow powerband whereas these engines need things that allow a broad powerband since they have no gearbox. The only exception to this is my design of torque pipe which you are free to use to make one yourself.

    There are other cylinder heads available but most of them raise the compression too much. I wouldnt buy one unless the seller can give a good educated guess as to what the final engine pressure will be with it. (Engines vary and so its impossible for them to tell what the final pressure will always be). Best target pressure is around 130psi (at sea level). Buy another upper connecting rod bearing also since the stock one is weak and wears out fast, especially with higher engine compression to push against.

    Since the Jaguar CDI is good up to 8500 RPM I also like to see people use a rotary tool to raise their exhaust port and cut some of the piston skirt off (or lower the intake port) which allows the engine to rev higher than the stock 5500 RPM. You can change the ports 1mm at a time till you are happy with the top speed. Going too high (without a torque pipe) causes a regrettable loss of low RPM power.
    battery and SANDSA like this.


    SANDSA Member

    Hi Mike,
    I'm a newbee with a NT carb set-up.I was reading your signature page about extending length of intake manifold.My build is fresh,it fired right up seems to run smooth,I sealed it all up 1st.If I was going to add the Z-pipe+hose would I have to do any rejetting or adjusting the little clip washer?
    *thinking I got lucky first time with the clip placement,up one notch from center,here in mass.
    Kinda sucks it's been snowing on the weekends,I've only rode up & down the block a few times,tune up tinker run.the chain was-a-clattering fenders-a bending sparks -a-flying..naa not that bad.hehe.
    I was real happy on start up ,all I needed to do is,screw the idle pin in all the way,and then I turned it 1 1/2.turn.(thats what I did on my mercury outboard) seems to idle fine..
    Now I get why those double kickstands are so popular.
  3. jaguar

    jaguar Well-Known Member

    Usually no carb adjustment is necessary.
  4. rogergendron

    rogergendron New Member

    hey jag............

    some info on my bike.....and a follow up question for you....

    i run a dax balance motor 4.7cm x 4.0cm bore and stroke for a clean 70cc, i run a puch 70 hi hi head .4mm copper gasket and a decked down cylinder for around 13:1 comp ! i have piston ramp mods transfer and exhaust ramps and the skirt on the intake side is cut 1/4in the full length of the intake side of the piston to lighten it up and hold the port open longer, a titainium wrist pin and an upgraded needle bearing (from info on your site), i have some major port work widening and opening them up, and run a long offset z intake with a 16mm ID bore, running a yamaha y zinger 16mm carb, and high flow filter, transfer port matching and port matching on all the ports, and i also run a tunned strait pipe 7/8ths ID for a verry powerfull but short mid to high rpm powerband,

    people say strait pipes done work on a 2 stroke but from my own experiance i can tell you it does !!! it just has to be the perfect length and ID bore for it to work and have a scavaging effect. i found that 7/8ths is the correct ID for 60-80cc and 16 to 24 in is the tunning length for the china girl rpms.

    for those that still claim strait pipes are no good for 2 strokes i offer you this info.....

    The goal of a tuned straight pipe in this application is to use the reflected negative pressure waves from the open end of the pipe to help draw out the exhaust gases. By selecting the appropriate length of the pipe, the reflected rarefaction wave arrives at the exhaust port just as the transfer port opens thus assisting the flow of fresh mixture into the cylinder, and exhaust gases out of the cylinder. The figure below illustrates this action. In the figure, even though the piston has reached bottom dead center (BDC), fresh mixture continues to enter the cylinder because the rarefaction wave causes P2 to be smaller than P1. A key point to note is that the velocity with which the pressure and rarefaction waves travel down and up the exhaust pipe is for the most part independent of the engine operating frequency (RPM). Due to this fact, the conclusion must be made that for a given pipe length there is an optimal RPM for which the waves will arrive producing the greatest benefit for the breathing of the engine. At this optimal RPM, the engine breathes significantly better and hence produces a noticeable increase in output power. This effect is quantified by calculating the ratio of fresh mixture to exhaust gases within the cylinder as the compression stage begins at EPC. If the rarefaction wave is very large, it is possible that fresh mixture is pulled into the exhaust pipe while both transfer and exhaust ports are open. This phenomenon is known as short circuiting the engine and produces undesired effects such as a decrease in fuel economy and an increase in release of volatile organic compounds. These negative effects can be mitigated by designing the exhaust system such that either no fresh mixture is pulled into the exhaust pipe (i.e. perfectly tuned straight pipe) or further utilizing the exhaust pressure wave to inhibit short circuiting.

    a striat pipe of perfect tunned length will only offer a short albeit powerfull powerband, you will need to get in it and stay there lol so you must tune the pipe length for you average driving rpms so when you reach your normal driving speed you get this real powerfull boost in performance ! def not as good as a big long broad power band torque pipe but its almost too easy to make and tune so any fool can do it !

    any way enough of that ...

    jag my inquary is this .... ieven at 13:1 comp and all those mods and with the motor turnning 9,000 rpms on the regular....... i still run the stock CDI and have never ever had a problem with it or its performace !!! it holds up well even a 9,000 pluss rpms !!!

    what would a JAG cdi offer me for increased performance ? how will it help ? stronger spark ? better timing ? the stock cdi seems timed just fine even for over 9,000rmp driving !

    what would your CDI offer a very high comp race build like mine ? seeing as the stock cdi works just fine, how can you help me justify buying a new high performance CDI from you and how does tyours differ from the HD lightning ????

    if your going to say reduced vibration well then i have to tell you that i run a balanced bottom end and i have a significantly lightend top end trimmed piston light wrist pin, and vibes are almost non existant up to about 7,500 rpms and then they start up real weak and build up to 9,000rpms, but even at 9,000 rpms the vibs are not that bad at all and i would say they are insanly good for a china girl ! almost on par with a real motor !

    Attached Files:

    Last edited: Jan 3, 2014
  5. Fabian

    Fabian Well-Known Member

    The Jaguar CDI might be good for 8,500 rpm but the big end connecting rod bearing is most certainly not.

    Even if you don't feel significant vibration at 9,000 rpm (which i find very hard to believe, and even harder to believe that the engine is making power at those rpms) you will find that the frequency has moved into an area that wants to induce fatigue fractures in the bicycle frame.

    The 2-stroke 66cc Chinese bicycle engine seems to live a long and happy life with optimised jetting (and using a Jaguar CDI) operating between 3,500 and 3,800 rpm and gear changes to maximum power at 4,800 rpm.
    Once you go over 5,000 rpm, the big end connecting rod bearing will have a significantly reduced life span as the needle roller case hardening starts to break up; pitting the large I.D. of the rod (further exacerbating failure time), and sending hardened particles up the transfer port to abrade & score the cylinder, piston and rings.
    Last edited: Jan 4, 2014
  6. HeadSmess

    HeadSmess Well-Known Member

    um...whats the quote?

    "you have your reality, and i have mine"

    g'luck jag, im genuinely interested to see what your response will be.

    i know i had a good laugh just then :jester: (not just over the spelling mistakes, either)

    reminds me of my new signature ;) all true but oh so wrong!
  7. jaguar

    jaguar Well-Known Member

    The stock CDI may seem OK but I know from experience that increasing the spark power and retarding the spark at high RPM will result in even more power. The more the fuel mixture is atomized (with a good carb) and the more it is compressed, the faster it will burn. So in order for the peak pressure to happen at 15 degrees after TDC you need the spark to happen later.

    What is not quite obvious is that an advanced spark puts excessive force on the bearings and creates excessive engine heat. The Jaguar Performance CDI is more reliable and helps your engine live longer. Here is more info on it and how to order it:

    No one ever said that straight pipes don't work on 2 strokes. They just are the last type of pipe I would pick. There are thousands of Suzuki AX100 (2 stroke) on the roads of third world countries and it uses a straight pipe with diffuser and then muffler instead of baffle. That gives it the broadest powerband although weaker than a regular expansion chamber within its powerband.
    Last edited: Sep 17, 2016