Raising compression with stock head!

Discussion in '2-Stroke Engines' started by MAFirst4, Jan 13, 2014.

  1. MAFirst4

    MAFirst4 Member

    Hey guys Iv done a few mods so far to my 66cc, 29in cruiser motor bike, intake and expansion chamber exhaust,41 tooth sprocket and it goes pretty good around 30mph. I'm a bigger guy round 220 pounds and still want some more power without putting a bigger sprocket and losing top end speed.

    I have heard are compression is around 6 to 1. Is that true? I want to go to around 9 or 10 to 1 ratio for more power. I have a lathe and am going to shave Down the head. I have researched but have not found any definite answers on how much I should shave down?? Also what head gasket to use or no head gasket?
    Any info from anyone use had shaved there own head and successfully raised compression will be appreciated! And any tips!!! Thank you...


  2. geebt48cc

    geebt48cc Member


    I am 266lbs, at 6' 7"..............I've milled stock 66cc, and even used without head gasket after milling..........Yes, it did help! I tell ya, the best mod as far as head is concerned, is PUCH head. As far as carb, too many to choose, just go bigger than stock. Ok, the very BEST of all has been TORQUE PIPE!!!~ (JAG is the man to talk about those!~)

    I live in Abingdon,Va. Hills,Hills, and a few more Hills. I can hold 20mph to 34mph and travel miles............Sittng at 880 miles on my SKY 66cc ........Stay on service, use Systhetic , blend castor oil with mix.............once again, there's quite a few good ideas.

    Be safe

    MAFirst4 likes this.
  3. MAFirst4

    MAFirst4 Member

    Thanks man. Sounds like milling the head does really help. Can I ask how much you milled it down? How many thousandths? And what is your compression now?

    Also can anyone tell me how much the compression goes up just by removing the head gasket?

  4. jaguar

    jaguar Well-Known Member

    I don't remember exactly but somewhere around 10psi increase from leaving off the head gasket.
    On my site I recommend making your own gasket from fiber material which seals way better than the stock gasket.
    Start by removing 1mm from the head and with a fiber gasket measure your cranking pressure. Don't go over 135 psi.
    MAFirst4 likes this.
  5. MAFirst4

    MAFirst4 Member

    Thanks that helps. I might just try that first while I save up some cash for a new high compression head. Ill try this in mean time. If there are two gaskets I'll remove one. If there is one and the surfaces look smooth enough ill run no gasket. If not I'll try yamahbond liquid gasket. Guess I'll find out!

    With a high compression do u need to run the speed carb a hair richer?
  6. jaguar

    jaguar Well-Known Member

    increased compression requires little or no jetting change but does require retarding the ignition timing (because the mixture burns faster).
    And no, the stock CDI isn't adjustable.
    MAFirst4 likes this.
  7. MAFirst4

    MAFirst4 Member

    How does she run without changing the Cdi? Just curious. And where do I get an adjustable one?

    Btw took out head gasket runs real good. Tho I think I'll put some yamahbond in to make sure it doesn't leak. Guess when I go to new high comp head I'll have to switch Cdi
  8. velzie

    velzie Member

    Jaguar sells a CDI that is modified from a Kawasaki KDX 2 stroke. It retards ignition at high RPM. Sick Bike Parts sells their Lightning CDI which they claim slightly advances ignition at high RPM. You will have to ask them for more info and I understand they have excellent customer service.

    You should check out Jaguar's signature link since he has done a lot of modification to his motoredbikes. From his information, when increasing compression, you will likely want to change the top-end needle bearing to something of higher quality and also lighten the wrist-pin to help with vibration at higher RPM. These solutions should help failure of the top end bearing that has been know to cause problems when raising compression and alleviate vibration problems at high RPM. I apologize if you are already aware of this information.

    I am not aware of the consequences of higher compression with stock CDI other than greater vibration and likely a greater chance of top-end needle bearing failure.

    Good luck and have fun with your mods!
    Last edited: Jan 17, 2014
  9. MAFirst4

    MAFirst4 Member

    Hey jaguar! Couple questions for ya.

    1. Do you know of anyone who sells manifolds the fit the mikuni vm18 carb to a 66cc engine? That would be sweet I have a mikuni coming

    2. Are there any more jaguar Cdi's available yet?

  10. jaguar

    jaguar Well-Known Member

    hey man
    look on my site at "Jaguar products" to see if I have the manifold you need.
    Any day now JNMotors should be restocked with Jaguar CDIs. Just go ahead and place the pre-order.
  11. MAFirst4

    MAFirst4 Member

    Went to JNmotors and it said they had 9 in stock, went ahead and ordered one! That must be your new one right? The 2.0.
  12. jaguar

    jaguar Well-Known Member

    Did you find my page of adaptors?
    you are going to love that Mikuni!
    yes they are my latest version 2.0
  13. MAFirst4

    MAFirst4 Member

    Yes I did! I believe I will need the m21 or m25. I will have to measure first. Ya I can't wait, I can feel my engine wanting to put out much more and hopefully it will be able to now with the Cdi 2.0 and tune able mikuni... When I do want to order the intake where do I do that from??
  14. MAFirst4

    MAFirst4 Member

    Never mind I see the link now to order em...