How fast should my engine go
- Thread starter Sldgehmmr
- Start date
if you have an expansion chamber and the engine has a decent cylinder the 19mm will work very well. on a completely stock china girl wit only an expansion chamber? I have not tried that yet so do not really know. The cylinder I have now is ported but very limited due to the transfer ports being too difficult to modify in my case. the intake and exhaust were ported but the timing was untouched with the ports. so maybe if you have a decent stock PK80 it may take the crb just fine, I think it would be fine to tune with.
The tune on my bike is tailored to city use the MZ pipe is a mild pipe it works at all rpm pretty good makes real good low rpm power and mid range as well. My cylinder has lazy port timing so high rpm is limited but 40 mph at that rpm is good power. very happy with the performance of this china girl so far.
I've got a China girl myself I've put transfer port channels on the piston and grinded a little bit of the piston skirt on the intake side to improve air flow. On my China girl the skirt (at tdc) would overhang the intake port about 1/16 of an inch. I'm pretty sure my hi comp head is a 6mm (I don't exactly remember) I've also pulled all the baffles out of the kit exhaust, port matched it to the jug. A 40mm intake, ngk plug and base gasket delete. That's why I'm considering the dellorto carb and defiantly gonna invest in a performance exhaust
This runs the Dellorto carb on a RSE reed valve with both jug and piston mods to take advantage of the reed valve setup. I'll send photos of the jug machine work and the piston window mod later.
p.s. don't mount a chain tensioner upright like shown on the pedal chain side of the photo above...180 degrees out of position! lol my bad.
Photos may have to be resized, but I'll try first.
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I think the photos will better explain the machine work needed to make the reed valve actually produce power. Big hole in the piston = "piston window" the photo of the inside of the jug = "third port"....using either or will produce power from the addition of a reed valve, but using both modifications produces the most.
The 19 mm Dellorto carb when jetted properly will flow more than enough air and fuel to run this setup and any expansion pipe you choose. I'm running higher than stock compression with the 7cc aluminum head...but nothing radical. Jetted down this same carb runs fine on a stock engine also. This one carb with a selection of jects has been on all three stages of tune from stock to highly modified street and ran great. Jet changes were the only mod to the carb. I've not found it necessary to alter the float level along the way either.
The Mikuni carbs or and other quality cycle "slide" flat plate or tube,carbs of the same basic 18 to 20mm size should serve just as well. I use the 22mm Mikuni on two of my bikes 4 stroke bikes that produce 12 & 15 hp from 125 cc engines respectively and both flow well and run great with the only difference in carb tune is one full size jet up size in the 15hp motor. So what I'm trying to get across is 3 mm difference supports triple the horse power of my reed modified 66 cc motors. & twice the displacement. I'd actually like to try a 20 mm Mikuni on these 125cc motors, but they are really expensive compared to the 22mm for some reason. Factory bikes from some of the Chinese companies use a 21mm Mikuni clone for small 125cc motorcycles they build.
The 19 mm Dellorto carb when jetted properly will flow more than enough air and fuel to run this setup and any expansion pipe you choose. I'm running higher than stock compression with the 7cc aluminum head...but nothing radical. Jetted down this same carb runs fine on a stock engine also. This one carb with a selection of jects has been on all three stages of tune from stock to highly modified street and ran great. Jet changes were the only mod to the carb. I've not found it necessary to alter the float level along the way either.
The Mikuni carbs or and other quality cycle "slide" flat plate or tube,carbs of the same basic 18 to 20mm size should serve just as well. I use the 22mm Mikuni on two of my bikes 4 stroke bikes that produce 12 & 15 hp from 125 cc engines respectively and both flow well and run great with the only difference in carb tune is one full size jet up size in the 15hp motor. So what I'm trying to get across is 3 mm difference supports triple the horse power of my reed modified 66 cc motors. & twice the displacement. I'd actually like to try a 20 mm Mikuni on these 125cc motors, but they are really expensive compared to the 22mm for some reason. Factory bikes from some of the Chinese companies use a 21mm Mikuni clone for small 125cc motorcycles they build.
As the photos reveal the widow in the piston allows the cool and lubricating air/fuel mix to flow directly onto the wrist pin assembly & interior of the piston. The hole is close to 2" in diameter. I smooth out the intake port some especially if the reed clearance to jug wall seems tight the reeds should not contact the cylinder as they open. Also with the RSE reed valve I use, doesn't perform as well when adding a spacer between the reed body and cylinder. I donno' why? I've read that you can space the carb back from the reed body to use an angle adapter to run larger high capacity air filters and still clear the frame, but I've not tried it myself.
The expansion pipe shown, is quite inexpensive & works fairly well, but is available and is close to a bolt on with some frames, though rigging pipe hangers will require some thought and effort. Pretty easy compared to some pipes, but not real easy.
Machine work involved end mill, side mill, and ball mill tools on a real mill! ... & a lathe....not a Dremel moto tool!
Rick C.
The expansion pipe shown, is quite inexpensive & works fairly well, but is available and is close to a bolt on with some frames, though rigging pipe hangers will require some thought and effort. Pretty easy compared to some pipes, but not real easy.
Machine work involved end mill, side mill, and ball mill tools on a real mill! ... & a lathe....not a Dremel moto tool!
Rick C.
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Fog those are great and I need to try one out. I have a digital infrared in the shop but real time data is where it's at & by the time I reach the shop it's useless. Great diagnostic and tuning tool and if I'm not mistaken made by the same company that makes the digital hr. meters and tachs which I have a few and find very useful. Thanks for the link and review!
Rick C.
Javy my setup with both piston window & third transfer port delivers power throughout the range...I feel it mostly in the low and midrange with each exhaust setup I use...but the speed on top is verified by speedometer even though it doesn't feel much different, I'm going faster, if that makes any sense. Of course an expansion chamber designed for top end enhancement will definitely feel faster as well...big kick!
You said your expansion chamber is low/mid range improvement so it should be torquey & have decent throttle response, if you want to get more of this a RSE reed and a large piston window (increased timing for fuel flow) will get you there but could cost you a small amount of speed on top end. This has been observed by myself and a few others. Lots of theories, but I actually don't understand why.
If you make only the third transfer mod to the jug correctly it helps on the high end but at a cost of some low end torque and possibly weaker throttle response. Doing both mods well, both the window and third port will help throughout the power band. Cutting that port correctly really needs to be performed with machine mills. The piston window can be cut easily enough with hand tools.
This is my disclaimer - Other's results may vary from my own. If low end torque is of interest the simple reed and piston window is fairly inexpensive to try.
My first reed build & that was with both modifications took quite a few hours of riding before I really started to feel the full benefit of my efforts. I read a post on reed valve setups and that individual said the reeds are super stiff and need break in time to flex properly...makes sense though I don't know if he was correct as to the reason. My second reed engine was the same... took time to feel the benefit.
Also the bad part of using reeds...they can break, enter the engine and cause catastrophic damage! Big deal that.
Fuel mileage is really bad and running 80 miles a day your going to notice it. A peanut tank isn't going to make it round trip! I ride everyday year round but have eight motorized bikes to choose from so the drawbacks don't mean that much to me and I've put a lot of hours on the two reed engines without any problems , other than poor fuel mileage, but if I only had one motorized bike to get me around. I'd stick with simple till I could build another one to play with.
The heads and cylinders forum has several threads that deal with reed induction and I'd encourage anyone to read up on what's been tried and tested & not just rely on my experience alone and these past few posts concerning this topic.
Rick C.
I slowly add upgrades as things break or wear out or just fail. Upgrading the engine with a 23mm intake and 19mm Dellorto Carb was the best mod I have done for more overall power. The smaller carbs ran good made some power and went 40mph as well but any resistance the engine would experience a drop in power, the larger carb just powers through hills, strong head wind, and accelerates much better than smaller carbs.
Javy the Dellorto carb has worked well for me now going on eight years since I used the first one on a 66 cc CG & I've had a problem with only one of the clones...just wouldn't work, but the next one did. The same carb type, 19mm, has been used on every China girl engine since - stock or modified, as described, and it's been great. I'm not saying it's the best carb out there 'cause I haven't had a reason to try all the others; after switching to the Dellorto. Jet change is easy & once I find the right combo on an engine tune it's golden! As you can tell I jack with my engines a lot, but this carb is the last thing I fine tune on an engine build. I have a lot of confidence in the Dellorto. It may be good, better or best...but to me it's just right.
Rick C.
Rick C.