Are they new or used bikes, most of the time it's something stupid wrong with them. Try taking the plug out and see what it looks like, then check the plug for a spark, then put alittle gas down the hole, plug back, see it it fires, if it does, the carb is probably dirty inside the float bowl and the main jet is clogged. Or the reeds on the reed plate behind the carb and manifold might not be sealing.
take apart and reassemble. if something looks broken take pics and show us. if you can describe the problems you are having it also helps.... lots of posts around here you can read to help you figure it out....
cool, thanks for the welcome. i just stuck the spare engine from the first guy i bought and it fired right up but now im dealing with the low end not staying alive but high rpm kicks in and has that little power band. i think i have the wrong fuel oil ratio but i cant find anything about the specs, its one of the zoom 49cc. ive seen mixes anywhere from 24:1 to 50:1 and i have it at 32:1. but im not worried because the bike will undergo the hard core fab and construction. i have many ideas im going to throw out there as for im a custom bike builder so itl be pretty interesting. the whole reason i got one of these.
yeah im on 32 to 1. quick question. i understand with hp carbs theres a high and lowside needle but with the stock carb with no float and just a primer at bottom with the pump diaphragm there is also a high and lowside but i here to adjust both or does the top brass lowside needle need to be adjusted. it looks like this carb here.... http://elescooterparts.com/parts/gas/carb/49ccScarb.html
That's way to much money for that carb. it's a Chinese copy carb. On eBay/motors you can get one $15 or less to your door.
I have a few of them around. That carb is really for a piston port engine, but can be mounted on a Cag reed port eng with the right manifold. I put Walbros on alot of my stuff.
EPR..... i love zenoahs. i have a g62 that i built for a uav that is alcohol ran. the thing is way way way fast and supports a 15' to 20' wingspan and a payload of 500 lbs. id kind of be affraid to throw it on one of these little pocket bikes. but it would not mount up on mine and i need a pull start, clutch and also a gearbox for it.... but on your first pic is the carb that i have on mine. do i need to adjust that top lowside needle??? or just leave it tight. i cannot get this engine to idle. it only stays alive at half and full throttle??