Just took delivery of a 1999 Yamaha YZ 125cc, I knew there were starting/running problems, previous owner informed me that it has,nt long had a new piston, and since that the bike don,t start but it did for about 20 mins after piston was fitted then wouldn´t start again, What I,ve done is; put in new oil/fuel mix 100ml to 5 liters, changed spark plug, dried air filter, and low and behold it started and run brilliantly, but only for about 15 mins, A mate stripped the carb and he seams to think there no fuel getting to the head? but I,m not sure that´s the problem, there´s plenty of spark from the plug when removed and kicked, but just wont start????? confused to hell and back now???? PLEASE HELP IF YOU KNOW WHAT THE PROBLEM IS, as this is my son,s Christmas present THANKS...