05 king quad 700, hard to start, wont rev up

[atvoffroad4life]

i have a 700 king quad with very low miles that was recently given to me, the original owner finally got fed up putting parts on that doesnt fix it. there is a new fuel pump and strainer, has 45psi fuel pressure, has new injector, new temp sensor, stock ecm, stock air filter, stock exhaust, (baffles removed from muffler) new spark plug, (very good spark) sometimes it will rev up for a min then it just dies when u give it any gas, when it will run it runs great, it wont idle when cold, will idle warm, starts a little easier when warm, using a heavy duty car battery that is fully charged and hooked up to a charger, low voltage is not a problem, i know this question is out there already, just trying to find some fresh input. i have been a mechanic for over 20 years, own my own shop , have never had this much headache from any kind of vehicle before.