ok, this is going to sound really dumb, but you never know.but you do have the gas petcock open right?
some carbs have a secondary petcock on them, which is a white plastic "T" shaped knob. if this carb has the secondary petcock, turn it so that the top line of the "T" is in line with the fuel line going into the carb.
is the fuel filter clean? is the fuel line kinked?
the choke is off right?
what are you running the gas/oil ratio at ?
sometimes the simplest things are overlooked when trying to diagnose a problem.
here's what i would do:
try to start it a few times and then pull the spark plug out of the head. if the plug is wet this is a sign that fuel is getting into the cylinder. dry the plug off really good and hook the spark plug to the spark plug wire and lay the plug on top of the head. try to start the motor and watch the end of the spark plug for spark. if there is no spark, then you may have a problem with your magneto (because you hooked the cdi box up to your bike, you know that it works.) the magneto may have given out, which will not give power to the cdi, which will not make the spark plug spark. but if it appears that the magneto is bad...don't do anything yet until you try the other things that i mention further down in this post. don't always assume that the problem is the worst thing it can be.
how is the wiring done on the bike? were the connections made with crimp on connectors, twisted and taped, or soldered and heat shrunk? it is very possible that a wire has come undone, or a bare wire is touching the frame. i suggest that all wire connections are soldered together, and covered with heat shrink. a bare wire touching the frame could ground out the entire ignition and give no spark. a doisconnected wire will give no ignition at all. a bad spark plug will also not give spark. a spark plug will give no warning when it is bad. they either work or they don't and they can go bad at any time...1 week old, or 4 years old , there is no way to tell when a spark plug is going to go bad. you can tell when they need to be replaced by the condition of the electrode, but the plug in question is new, and probably has no signs of a burnt away electrode yet. (but you put his plug in your motor and it ran fine so my guess is that the spark plug is ok)
if you do have spark on HIS motor with all of HIS original parts, then the carb float may be stuck up. if the float is stuck in the up position, it will not allow any fuel to flow into the float bowl. (this happened on my friends 80 c.c. when it was about a week old). you said that you cleaned out the carb, but this problem was there BEFORE you cleaned it out. did you remove the float bowl and clean it out as well? it is also possible that the needle & seat are stuck closed. all it takes is 1 tiny peice of dirt for this to happen and it will not allow fuel into the carb. (same as if the float is stuck up.) it's hard to tell if the float is stuck because when you pull off the float bowl, the float comes out with the bowl. the needle & seat stay in the carb. by removing the float bowl, it allows the float to drop and open the needle & seat. everything will appear fine when looking at it this way. sometimes the float can be jostled a little, which will make it shift slightly and it MIGHT get cocked sideways...enough to stick on the side of the float bowl. the floats in these carbs are nothing more than a plastic ring that is not connected to the needle & seat like most floats are.
i went through this several times on my friends 80 c.c. it was very difficult to tell if the float was sticking.
you will know if you have no spark by the spark plug being wet every time you remove it after trying to start the motor. you will know if it is a no-fuel problem if the plug is dry every time you remove it after trying to start the motor.
here's a long shot....
does he have a headlight or any accessory connected to the white wire coming from the engine? if he does, i would suspect that it is the cause. if he has a headlight hooked to the white wire, it will work for a little while when it's first hooked up. after awhile, the magneto can not supply enough voltage to the cdi to fire the spark plug. this happened to me after having a headlight hooked up to my bike for 3 days. it worked fine for 3 days, and then all of the sudden, my motor would not run at all. i went through the ignition and fuel system and found nothing wrong. i removed the headlight and presto, the motor fired right up and ran perfect. if he does not have anything hooked to the white wire, make sure that the end of the white wire is covered. (the end that you would hook a light or something to) black tape will work to cover it and protect it. all it takes is for a couple of strands of wire to be touching the frame or the motor to short the magneto to ground. also check the wires where they go into the motor. they just run through an open hole in the casting and it's possible that the insulation on a wire has been rubbed through and bare wire could be touching the motor. they should have put a rubber grommet in this hole for to protect the wires more. I know that the wires are covered with a white sleeve of some kind where they go into the motor, but anything is possible.
i am betting that it's something simple, but trouble shooting is the tough part.
"it ran fine and then just sputtered out." that description of what happened sounds like either a fuel or spark problem to me. if it was something worse, you would know because the motor would stop violently (like if the piston seized in the cylinder or something like that). besides that , if something like that happened, you would not be able to turn the motor over at all any more.
man, every time i finalize my post, i think of another thing for you to check.
check the inside of the spark plug boot. there is a little brass connector inside that snaps the boot onto the spark plug. make sure that it's still in there (you will see it...it's bright and shiney brass, and it will have a tiny metal clip on it that looks like a hairpin.) the clip is what holds the boot to the spark plug when you put the wire on. if the brass peice is gone, the spark plug wire will not be making contact with top of the spark plug, giving no spark. the brass peice in my spark plug boot fell out when i pulled the plug wire off pull the spark plug out once. if the brass peice is in there, make sure that it's tight. you can tighten it with a flat head screwdriver (it has slots in it for this) but don't over tighten it because the boot or the brass peice might break. just make it's snug. there is a spring under it which will help it stay tight once it is tight. but in reality, if that brass peice is gone, the spark plug wire would never stay on the spark plug, and you would not have a positive "snap" when you put the plug wire back on.
if it's just loose it will still make contact with the wire and the plug, and will probably not cause the problem you are describing.
