Cheap CNC Head CC

skywalker73

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Jun 11, 2014
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Maryland
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.jpghead2.jpg
 
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?
 
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
 
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
 
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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

Thanks, man.
 
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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.
 
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

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?
 
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.
 
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?
 
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