Does the aftermarket high compression heads put to much stress on China engine?

Discussion in '2-Stroke Engines' started by SlugMan, May 4, 2011.

  1. SlugMan

    SlugMan Member

    I'm thinking about getting a high compression billet head for my Gumbee 66cc engine, but I'm conserned that it will cause the engine to prematurely fail... These engine are cheeply made... Thus, my thinking tells me not to upgrade to a higher compression, because it would put to much stress on the lower parts of the engine (connecting rod bearings, etc.)
    I'm trying to build a bike that has a good reliability, and some performance upgrades. (I know, I should of bought a better engine :)) But, I'm trying to work with what I have at the momment (First bike build).
    What is your experience with this subject?
    I have a new SNH 15.15 carb and Billet intake on order, plus a CDI / KX80 Coil to upgrade the timing.
    Thanks for your help :)

  2. motorpsycho

    motorpsycho Active Member

    a Gumbee engine?
    does that fit onto a Pokey bike?
  3. SlugMan

    SlugMan Member

    It's the Grubee SkyHawk Silver 66/80cc. Sorry about the Gumbee thing :) Haha
  4. Ixlr8

    Ixlr8 Member

    hi, its not necessarily the mechanical compression that is the main concern. Its the actual pounds that it creates. Most of my china engines run about 70-90 pounds. But I have one that has 210lbs. This was accomplished with a higher dome piston. milled cylinder and a billet radial head to get rid of the heat.

    Higher compression does equal more heat. I know some may argue this point, but should know. Its rpm that kills. Unfortunately its also is what makes horsepower. By this I mean real horsepower. For the most part more compression will equal more lowend, midrange and top end, also increases throttle response and gas mileage

    This is of course if there isn't detonation going on . detonation will kill it fast!..BTW I use a mixture of premium and small aircraft fuel.
  5. Skyliner70cc

    Skyliner70cc Active Member

    IxLr8, would love to hear about your 210 psi compression engine....please start another thread on that engine.

    Slug, both of my current bikes have slant head engines with no issues. I wouldn't own anotehr HT engine unless it had a high compresssion head.
  6. I would imagine it would put more pressure on the crank/bottem end. On my first engine the bearings in the crank failed. That's bound to be easier if there is more pressure on the bottem end I would imagine.
  7. jaguar

    jaguar Well-Known Member

    Of course it puts more stress on the bearings but life without it is unbearable. I don't advocate "high" compression, just higher compression. You can bring it up to 135psi as I have with no problem. I have 1200 miles on mine. I've bought a new rod and bearings just in case but at the rate things are going I doubt I'll have to use them anytime soon.
    It's essential that you buy a cheap compression tester.
    Consider this also, its at high revs that most engine stress occurs and by ordering the CDI you'll be assuring there is much less stress. That is because it retards the ignition at high revs. The stock CDI just keeps on advancing the ignition at high revs, assuring maximum stress on the bearings. Maybe that's why my ported engine, which I rev to the moon, has 135 psi and no bearing problems (because I have the same CDI on mine).
    Last edited: May 5, 2011
    FurryOnTheInside likes this.
  8. Can you send me the link to the site that sells those CDIs?I had it awile back but lost it....
  9. Ixlr8

    Ixlr8 Member

    skyliner70cc sorry I missed and didnt read your post till now. I will be more then happy to start a new thread about this motor.

    Although higher compression will indeed cut the life of an motor shorter, but then doing anything to raise the output or rpm will do the very same thing. I personally go with the theory and use this in my own motors. and most of the high end techs now think that the extra compression can actually cushion the piston and crank. Pretty much everything I own I make turbocharged and supercharged, and to me this would go to prove that this thinking may actually be true. Word is still out on it.

    Its too complicated to get into an explanation at this time to explain why..but if you would like a google search will give you a few hits.

    jaguar?? I noticed you put in a plug here for your custom cdi. dont really consider it also put in few scare tactics there..but to help you out here I do run your cdi and it does for the most part. what you say it will do, mostly just a smoother running engine and it does make it capable of using a little more compression.. but that was kind of a misleading plug.More like you bought it then you made it. and of course you know and suggest the url. its your site...have to add that indeed you do know your stuff, I have kept most of your posts for reference. And I agree , the china motors timing is way too advanced. But some of the other things as much as I do respect your knowledge I do differ in some of what you say, but then again this is the spice of life, look forward to seeing more of your posts!
  10. Ixlr8

    Ixlr8 Member

    I will post the high compression radial head tomorrow, I have too many ****ed off people today. I do think a effective compression of 135lbs is more in the lawn motor category then a true performance one. with husky we ran what would now sound crazy compression ratios. but high compression equals better mpg and raises the response and adds to the power curve thorough out its entire range..look at the high tech morini motors..they all have comps in the area of 14 to 1..the more compression the more power in response..BUT! not if you over rev the engine and if it starts to detonate that will destroy it as fast as an escaping wrist pin clip But one thing only does not make the a carfull mixture of all the modified part together in harmony so to speak ...with all this done you should have a decent running china
    Last edited: May 6, 2011
  11. jaguar

    jaguar Well-Known Member

    JNMotors is selling the CDI along with an ignition coil.
    Ixlr8, I would invite your views on some differences you have with my ideas. Send me a message and we can talk about it.
  12. SlugMan

    SlugMan Member

    Thanks for the replies,
    I did install the Rock Solid Engines high compression head, and I have Jag's CDI, but have not installed it yet.
    Just with the new carb (have not tuned it yet) and the head, the bike runs like a little powerhouse! I'm running super rich at the momment (1/4-3/4).
    I'm just waiting on my wide cranks to arrive, so I can install my expansion chamber. THEN, I will tune the carb. I've got jet's on order from Sick Bike Parts, but the top end seems ok, but maybe could use a little more gas #68 main jet. Plug chop time :)
  13. jaguar

    jaguar Well-Known Member

    If the piston top is all black then the jetting is too rich. Also
    if it runs good uphill but sputters downhill then its too rich.
    If it runs great at top speed but is weak climbing then its too lean.
    For stock NT carb:
    Set the main jet first for correct running at wide open throttle. Instead of buying various sizes you can remove the jet, heat it with candle flame, melt solder into the hole, and drill a new hole using a micro drill bit (20 piece set from for $10).
    Then set the needle height for correct idling and accelerating. If you can't get it low enough (leaner) you can file more grooves on the needle higher than the current highest one and reposition the clip there. If that's not enough then you can heat the needle with candle flame and melt solder onto it to fatten it up to make it leaner. Just the lowest 11mm of the needle should be "soldered". A bottom width of 1.65mm is what was necessary with mine.
    Last edited: May 22, 2011