I have a Kymco Mongoose 90 that will not idle when the motor is warmed up.

Check fuel filter. It maybe clogged. If still no fuel then check for fuel pump relay and or fuses. If still no fuel then replace fuel pump. And at same time replace fuel filter

You can run sae 10A-40 in it and up to sae 20w-50 as well. It varies in prices. Best place would be somewhere like a Walmart. Usually around $4-$10 depending on what ya get.
The transmission oil for it is sae #90

Bad pulsar unit
Bad CDI unit
Bad gearbox electrical unit
Bad coil
Bad HT lead or caps
Bad spark plug
Bad earth wires
Bad ignition key switch
Contaminated fuel
Dirty carburetor

Cylinoid could be bad which is quick fix. If not check magneto possibly need a new one or sometimes if it has rust on it you can take some sand paper to it...

clicks, wont crank ??? check ground first, then suspect a bad starter as long as the solenoid and battery are good... a 12v test light will help here

broken throwout clutch springs?

Hi Matt only help I can give is look along top & bottom of chain for a tensioner it will be a round rubber wheel. If you cant find a tensioner it will more than likely you need to cut the chain down. If your not sure how to let me know & i'll let you know how. Also DO NOT USE IT while chain is loose as if you do & chain comes off you can crack engine case which then means a new engine

i have the same problem

make sure filter is clean not gunked up..try draining carb if there is a plug if not.Then try to clean the carb the jets could be clogged not hard at all to take apart.Good luck