just picked up a non running BMX FA-C70 kids atv. I first just tried cranking the motor over and it did so freely so then i pulled the carb and cleaned it. After carb clean i replaced sparkplug and then tried seeing if it had spark. the spark is very weak but is there. i then tried starting it and after some time got it to fire up and go but as soon as i let off the throttle it would die and could not get it started again. awhile later i got it going and just revved it a few times and then let it idle, after about 3 min i tried applying some throttle and it just choked out and died. Both times getting it started i've put gas into the back of the carb. i can not get it to start on its own without helping it. the airbox is also missing and ive been trying to understand the rat nest of wires and all the safety features and splicing and what not from the PO but its not matching up very well with the wiring diagram i have. any help and guidance would be much appreciated. Thanks in advance.

 

To me it still sounds like a fuel problem since it starts up. The CDI ignition system is at its worst during cranking. All the ignition AC voltages are really low and the spark energy is at its lowest also. The AC voltages are directly proportional to engine speed. The power being transferred is proportional to the square of the engine speed. Cranking speed is 600 RPM, idle is just a tad less than 3 times that. So if you have a marginal (weak) spark it should get stronger and stay stronger once the quad fires up.

It is possible that there is an electrical problem that could produce weak intermittent spark, but adding fuel to the carburetor won't affect that. That all point back to fuel issues unless you have more than one problem.

The problem with "weak spark" is that it is so subjective to the individual observer (and their particular mood at that time). How many pins on your CDI? Once we know that then you could always go through the ignition tests and see if anything unusal stands out.

 

Ok I've got the atv up and running. Cleaning the carb again really helped but finally tracked down the culprit. The rear kill switch was coming loose so I've just unhooked for now. Currently having a loud clicking noise from case and now when applying throttle if not done slowly the motor bogs down. If done slowly the clutch will engage and it will move under light load? Checking fluids in the morning.