hello, i have a 2 stroke engine from thats dax i want to say its a 68cc. i have been riding it and it has been working fine untill the other day when i staretd her up and she would struggle to pull my weight but she would idle(i wouldnt call it very well). when i let the clutch out she will go(sometimes) to around 13-17 mph. but she will struggle to maintain speed and will back fire if i open the throttle up and will shut down if i dont catch it and keep her idling. the guy across the street said there was too much oil in the engine and thats what is causing the problem(he didnt look at anythin just the sound of it). he said to empty the gas out and mix it correct. i did this and she started to run almost at 100% but there is/was still a little studder sometimes. then after riding her a couple more times through out the next couple days she does this again. so this time i take the spark plug out and it is a dark brown color, i put the spark plug in tight and tighten down any loose bolts. change the gas and oil again and peddaled around for a while trying to start her, nothing. the next day i get home from work and wanted to fix her, so i tried to start her once more before i take her apart. and she started right up and worked good for the last couple days. now it wont start and it doesnt sound like it is getting spark all the time, it will spark once in a while. what could be the problem