Boost porting the 68cc china engine

europorsche914

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Jun 18, 2008
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Lancaster, PA / Newark, DE
Boost porting the 67cc china engine

Saw an interesting thread over on the 2 stroke pocket bike forum and I am plannin to perform this mod this week on my motor. It will be 1 port opposite the exhaust port, about 1/4" wide and 2 mm deep or so to the same height of the top of the transfer ports. I am going to utilize round port edges and angle the top to point at the spark plug. Ill let you guys know the results they claim atleast an extra 1000 rpm gain and more torque across the band. Heres the thread: Added a boost port tonight *pics* - Pocket Bike Forum - Mini Bikes
 
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europorsche914

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Jun 18, 2008
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Lancaster, PA / Newark, DE
Still thinkin this through since i havent torn the motor apart yet to modify it. I just realize that the intake port is currently directly across from the exhaust port so I may do two ports, one on each side of the intake to help scavenge the exhaust charge out of the cylinder. I havent decided what the measurements will be since I will be using two ports instead of one. A boost port acts as if another transfer port was fitted on the engine and also helps force the exhaust out of the cylinder (similar to having a second exhaust port).
 

europorsche914

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Jun 18, 2008
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Re: Boost porting the 67cc china engine

Here is the mod that i did to my motor. I attempted to balance the crank while it was apart and stupid me didnt look up any info on it... So it now vibrates like all **** at 25mph. I didnt half the weight of the reciprocating assy. But not all was lost... Before the crankshaft and boost port mod i could at most do 34mph according to my speedometer. Now the same speedometer read 43mph! Now im not going to tell everyone that it is dead accurate since i do not have a gps to test it but i know im going approx 40 mph since i was keeping up with traffic... The crankshaft can only be holding the engine back and the sound of the motor at that rpm is very smooth. Current mods to my engine include porting intake and exhaust, expansion chamber, homemade glass pack and steel pack muffler, 44 tooth sprocket, NGK b5hs, polish the head and piston top, and 2 boost ports. The two ports were about 1/4" wide and are as long as the ceiling of the transfer ports. They are about 3/64" deep and are centered between the intake and transfer ports. The end of each port is angled towards the intake port and utilizes a steep angle to aim at the sparkplug and help scavenge the exhaust charge out of the cylinder. Im satisfied with this mod and would do it again but will be tearing the motor down again to install a new balanced crankshaft... Please excuse the poor grinding on the one port, they ran out of grinding stones at the store and i will clean it up on the next tear down
 

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jasonh

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Jun 23, 2008
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Re: Boost porting the 67cc china engine

Very cool, glad it helped performance. Too bad about the crank though. I think velardejose balanced his crank. Should see if you can find his thread on it.
 

Home-made J

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Sep 16, 2008
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Santa Cruz, CA
how do those boost ports not effect compression? Aren't you compromising the integrity of the rings' seal? I know you explained it a bit as being an extra exhaust port, but I would be into knowing some more before I do it...and I WILL do it if you're getting 43mph. I allready matched and opened the ports and machined the head, and I'm running a 34T Andy sprocket. If I do these boost ports, balance the crank, and throw a tuned pipe on her, then I will have taken Chinese motors to the wall (unless somebody starts fabricating bolt-on reed cages!). Some dude was making a reed setup, but never followed up on if the thing worked or not.....someday.
 

jasonh

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Jun 23, 2008
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SCSW does make reed cages for these motors. Contact Easy Rider for info.

I think it would only affect the ring seal when the piston is at it's lowest point, as they would get closed when the intake port is closed....I think....
 

europorsche914

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Jun 18, 2008
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Lancaster, PA / Newark, DE
since the boost ports only go as high as the transfers, the compression is unaffected. Dont quote me on the 43mph but i know i keep up with traffic in 40mph speed zones. I may have been the one that you are thinking of, I had a thread about my homemade reed valve but I had a intake leak or possibly too small of a carb bc it wouldnt tune properly, i will hopefully have it mounted within the next 3 weeks. But be warned the boost port with out a reed cage will decrease low end torque (mid and high end torque seems unaffected) but increases rpms. I also noticed that at flat out i do not have much torque perhaps it is out of its power range at this point or it is because of my bad crank. My new crank will be installed within the next two weeks. Those considering this mod be aware that this allows your bike to turn some high rpms and therefore speed that could kill you if something happened...-Justin
 
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i-paint

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May 24, 2008
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Let us know how the new crank performs. I would like to have one of mine balanced. It is a "vibrator" at all rpms, more at lower rpms though. It is a replacement engine from BGF and is the strongest running I have. I've had it on three bicycles same results. Would be worth a balanced crank.
Thanks. i-paint
 

europorsche914

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Jun 18, 2008
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Lancaster, PA / Newark, DE
Heres some great info: it states to account for all rotating mass + 50% of reciprocating mass: http://www.neme-s.org/Model_Engineer_Files/2924-Balancing Single Cylinder Engines.pdf. Here's another thread that states using 58% of the reciprocating weight: 4 Cycle Karting Forums - single cylinder engines can only be balanced at a set RPM
Reciprocating Mass is the weight of:
Piston
rings,
wrist pin,
pin keepers,
rod end that connects to piston.

Rotating Mass:
rod end that connects to crank

Stupid me didnt do any background checking on how to balance a single cyl and I balanced 100% of the reciprocating mass... Bad choice as I now know and wasted a lot of time grindin of hardened steel... Im plannin on using 57.5% of the reciprocating weight (60.375 grams) as the karting guys are doin. From what i take from his thread and pictures, Velardejose uses 52.3% of the reciprocating weight (55 grams) and says his works great but still has light vibration in the pedal area. (it will always vibrate a little since you can only balance a single cylinder to a certain rpm.) I will let everyone know the results and if I need to remove more I will. This way we will know which way to head (closer to 60% or closer to 50%). FWIW i have a one piece crank unlike velardejose's which is a three piece (larger weights attached to a small crank (older style)). Here's velardejose's thread: http://motorbicycling.com/f13/homemade-crank-balancing-4498.html.
I also just ordered a shifter kit I cant weight to see how it hauls with these mods :ride: maybe i will cruise at 70mph!! rotfl -Justin
 

shadee573

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Dec 15, 2008
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CA.
Re: Boost porting the 67cc china engine

Here is the mod that i did to my motor. I attempted to balance the crank while it was apart and stupid me didnt look up any info on it... So it now vibrates like all **** at 25mph. I didnt half the weight of the reciprocating assy. But not all was lost... Before the crankshaft and boost port mod i could at most do 34mph according to my speedometer. Now the same speedometer read 43mph! Now im not going to tell everyone that it is dead accurate since i do not have a gps to test it but i know im going approx 40 mph since i was keeping up with traffic... The crankshaft can only be holding the engine back and the sound of the motor at that rpm is very smooth. Current mods to my engine include porting intake and exhaust, expansion chamber, homemade glass pack and steel pack muffler, 44 tooth sprocket, NGK b5hs, polish the head and piston top, and 2 boost ports. The two ports were about 1/4" wide and are as long as the ceiling of the transfer ports. They are about 3/64" deep and are centered between the intake and transfer ports. The end of each port is angled towards the intake port and utilizes a steep angle to aim at the sparkplug and help scavenge the exhaust charge out of the cylinder. Im satisfied with this mod and would do it again but will be tearing the motor down again to install a new balanced crankshaft... Please excuse the poor grinding on the one port, they ran out of grinding stones at the store and i will clean it up on the next tear down[/QUOTE]


sorry im new here but the intake and ex ports are not open all the way on these motors look at go ped motors online the ports on the cilynder are open all the way thats why my ped runs so smooth. this bike is a womans dream come true lol but i do not like the vibration at all it kills all my power and my a$$ lol the work i did on the motor made it runnnnnnnnnnn sooooooooooo smooooooth i love it now just open the ports you will be happy hope the pic shows updance1daxtitdnut sorry about that i did not know you needed a photobucket account.


my thought was right they closed the ports to keep the hp under 2. silly china!! my bike runs like a dirt bike now im buying another motor and a new bike love this thing and i have a street bike this is waymore funnnnnn !!!!! lol 40 mph plus i got a ticket at 49 mph today good thing they already took my lic. away last year lol i cant seem to get the pic on here so here is the link to photo bucket


http://i554.photobucket.com/albums/jj401/shadee_573/1128081408.jpg HAVE FUNNN make shure your bike can take the vibration at 30 or 40 BE SAFE!!!!!!!!! it hurts to get hit or fall at 40 you can DIE!!!!!!!!!! BE SAFE ONCE MORE BE SAFE whear bright colors .we.
 
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europorsche914

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Jun 18, 2008
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Lancaster, PA / Newark, DE
just got my crankshaft balance and its super smooth... Once the high rpms hit (higher than stock due to the ports) some vibration begins but it is not as intense as the vibration barrier i experience with my original motor. It is amazing the rpm im turning in this motor (in excess of 10000rpm). According to the same speedo metioned before and with a shifter kit, im hitting ~27mph with a 34tooth rear sprocket (i think is equivalent to a 78 tooth rear sprocket for non shifter kit people (used gear calculator spreadsheet)). Now whats limiting me is my carb since it cant flow enough fuel at wot... REED VALVE anyone?
 
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Egor

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Jan 30, 2008
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Hurricane Utah
Euro - What balance percentage did you use? I have been looking for someone to work up one of the reed kits to run on a happy times. I have my eye on what looks like a good one for a pocket bike, I think it would mod for one of our kits. Also if we add a reed we then can use the space in the intake runner to add a boost port. There is some info around the forum about the porting. keep us posted. Have fun, Dave

PS: A way to increase the scavenging ports, a chain saw engine cylinder.

just got my crankshaft balance and its super smooth... Once the high rpms hit (higher than stock due to the ports) some vibration begins but it is not as intense as the vibration barrier i experience with my original motor. It is amazing the rpm im turning in this motor (in excess of 10000rpm). According to the same speedo metioned before and with a shifter kit, im hitting ~27mph with a 34tooth rear sprocket (i think is equivalent to a 78 tooth rear sprocket for non shifter kit people (used gear calculator spreadsheet)). Now whats limiting me is my carb since it cant flow enough fuel at wot... REED VALVE anyone?
 

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Izzy

New Member
Dec 23, 2008
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chicago
alright well im interested in opening up the ports, but i would like more info on actually opening the ports to what diameter and also perhaps a few more pics would be appreciated.
 

runslikeapenguin

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Feb 2, 2009
32
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0
Seattle
is there anyone else out there whos done the fully opened boost port mod? this sounds like it would not really effect anything because the piston already does not come all the way down to the bottom of the transfer ports lip.

i do suppose it would reduce the friction on the rings allowing for faster revs.
 

Egor

New Member
Jan 30, 2008
714
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0
Hurricane Utah
Robin - You need a puller to get all the parts off the ends of the shafts, or all the ends off the shafts on one side, (IE) mag and the sprocket. Then you would lift the left case off the right. You should have gotten a funny looking puller with your kit, I think they all still come with them. Look around the forum there should be some posts about engine overhaul. Have fun, Dave