Cheap CNC Head CC

Discussion in 'Performance Mods' started by skywalker73, Mar 23, 2015.

  1. skywalker73

    skywalker73 New Member

    I bought one of these cheap cnc heads for my 66cc. The original slant head cc'd at 7cc's. This new cnc head is supposed to be 6cc's. Im wondering where to check the volume, at the first chamber where the pug sits, or the top of the outer chamber(see pics) The first chamber measured 4.5cc's and the entire chamber to the top measures 9cc's. my piston/stroke is stock. The bike runs good, but it seemed to be building a lot of heat in the head. After a 34mph run, the plug showed some signs of detonation. I am using a dellorto clone carb with a 65 jet, and a small expansion pipe. head1.jpg head2.jpg
     

  2. Slogger

    Slogger Member

    The jet and plug probably need to go up one number each.
    Try a 66 and a #6 heat range plug, maybe?
    666
    Your motor bike is evil.
    haha

    Your bike is cranking out a little more power, making more heat and using a bit more gas goes with the territory.
    Nice looking head, how much does one like that cost?
     
  3. MotorBicycleRacing

    MotorBicycleRacing Well-Known Member

    Wow, A BP5HS plug should never be used in these motors unless you are in the Arctic.

    Try a B7HS at the minimum.
    I run B8HS
     
  4. skywalker73

    skywalker73 New Member

    Thanks for the input Guys. Sloggers post cracked me up! I was running that plug on a pretty cold day, 35deg(I am in Maryland). I will change the plug to a B8HS and maybe fatten it up a jet. I was running a 65, but I have a 70. Any input on the correct way to determine the Cyl. cc's? I'm just curious. Slogger, that head was $30 on ebay. It was advertised as a 6cc head. http://www.ebay.com/itm/271648522825?_trksid=p2059210.m2749.l2649&ssPageName=STRK:MEBIDX:IT
     
  5. Slogger

    Slogger Member

    Thanks, man.
     
  6. jaguar

    jaguar Well-Known Member

    What really matters is the cranking psi. I don't recommend over 135.
    Detonation happens when compression is too high, jetting is too lean, timing is too advanced (which is true witht the stock CDI), or any combination of those.
    Let us know what the psi is.
     
  7. MotorBicycleRacing

    MotorBicycleRacing Well-Known Member

    I would try the 65 with a 7 or 8 plug.

    How high in the cylinder was your piston?
    what is your squish band clearance?

    http://www.ngksparkplugs.com/pdf/DYK_HeatRanges.pdf
     
  8. skywalker73

    skywalker73 New Member

    Thanks for the input. I will do a compression test and post the results. I am used to fooling with 4 stroke engines where cyl. psi is not that critical because of cam overlap and stuff like that. It seems it is a lot more critical in 2 strokes so I will definitely test it. I have a stock bottom end, and have not checked how high the piston is. I'm not familiar with the squish band. I'm assuming that is the outside part of the combustion chamber? I posted a pic at the top of this post of my head. I can see that if the piston protruded, I would need to measure the difference to find that clearance. Would that be the squish band clearance?
    I have a friend that used to mess with gocarts. He would use a head pyrometer to tune his mixture. Does anyone use a temp gauge on the head?
     
  9. KCvale

    KCvale Motorized Bicycle Vendor

    I run BPR7HIX Iridium plugs in all my 2-strokers here in the desert, 8's if it's a performance engine.

    A 5 keeps an insane amount of heat in the engine, you only need enough heat to keep the plug from fouling.

    Both BRP plugs are longer than stock which gives you a tad more compression and they will NOT work in some high compression heads like the head gasbike sells, the plug hits the piston at TDC.
     
  10. Purple Haze

    Purple Haze Active Member

    I was wondering if that chamber would benefit from some smoothing out the sharp lip. Sharp edges will sometimes cause pre-ignition if it's running hot. Also, wouldn't a flat-top piston work better with that type of chamber? (better flame travel, mixture swirl, quench) My experiences are with wedge type chambers, I know the hemi type reacts differently. On wedge heads, a zero deck height is the hot ticket, but the hemi... not so much?
     
  11. skywalker73

    skywalker73 New Member

    I ran #8 plug this evening. It ran good. I only ran it for about 15 minutes, but gave it everything I had. The plug looked good without signs of detonation, and the head didn't feel quite as hot. I tried to run a compression check when it was still hot, but the wife and kid caused it to cool down completely. lol I did manage to get a cold reading of 105-110 psi. I squirted a little oil down the cyl and tried again. Still around 105 psi. I'm wondering how much the pressure will increase when its hot? I will try again when I have more time.
     
  12. skywalker73

    skywalker73 New Member

    I got some time to work on my bike today. I did a hot compression test and it was only around 105. I put the stock slant head back on, and the compression was 110. The stock head actually ran a whole lot better. I then took the head gasket out, and the compression bumped up to 130psi. The stock head without the gasket ran really good. It also seemed to run cooler. That cheap CNC head is pretty much junk in my opinion, or I don't have the proper combination.
     
  13. jaguar

    jaguar Well-Known Member

    squish is the space between the piston and the cylinder head when the piston is at the top.
    .3 to .6mm is ideal. More than that lessens engine power because the fuel mixture in that space burns later. It should all be in the main combustion area to burn quickly.
    to test it:
    buy solder from Radio Shack. take off the head. position the piston so that when you push the bike it will rise. set it just a bit down.
    put a length of solder on top of the piston from the left side to the right side. put the head back on and then push the bike. if you feel no resistance from the engine having to squish the solder then your squish clearance is ridiculously too much. try not to turn the engine over much past the TDC point. that gives less chance for the solder to fall into a port. take the head off and use a micrometer to measure the squashed thickness of the solder. that thickness is the squish clearance.
     
  14. jaguar

    jaguar Well-Known Member

    when you buy a high compression head with squish it is essential to make sure the squish band is correct. Otherwise the engine will have less than optimal power. and in your case the power is even less than with the stock head because now your compression is too low.
    measure the squish band. if it won't squish the solder then coil two lengths of solder together to make it twice as thick and then test with that. subtract .45mm from the measured squish clearance and that is the amount you have to take off of the mating surface of either the head or the cylinder.
    it is just plain wrong that the sellers of these heads don't make this all clear. they know but they just won't tell. they know that sales will lessen when people understand that often it isn't as easy as just putting the new head on. For some engines it is as easy as that. not so with others.
     
  15. jaguar

    jaguar Well-Known Member

    I see now that some sellers of high compression heads do say that if you have a GT-5 engine that you need to lather down the cylinder till it matches where the piston edge is at TDC. Is your engine a GT5?
     
  16. skywalker73

    skywalker73 New Member

    Jaguar, Thanks for the tips. I don't know if its a GT5. I really have no clue what it is. It was sold as a 66cc. It does have 8mm studs and a 40mm exhaust. I eyeballed the piston at TDC, and it looked like piston edge was even with the top of the cyl.
     
  17. jaguar

    jaguar Well-Known Member

    test it for squish clearance and if it is too much then plane down the head surface using sandpaper and thick glass (or take it to a machine shop)
     
  18. skywalker73

    skywalker73 New Member

    I will give that a try Jaguar. What are your feelings about not using a head gasket?
     
  19. Purple Haze

    Purple Haze Active Member

    Fabian has a video posted on youtube where he explains the various heads available that he has used. A very informative video.
     
  20. jaguar

    jaguar Well-Known Member

    for use without a head gasket the two surfaces have to be planed to perfection first. Then put some copper coat on the mating surfaces.
    I always use a gasket, with my preference to fiber versus metal.
     
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