Just so you know I have not checked anything yet, I only have one cylinder that sees to be firing at idle or low speeds but at full throttle or any other time that I just get on it, it will run smoothly. I have put new plugs in but i have not beenable to test due to a oil plug problem but just lokng for recmmendatins if the plugs dont work i will take a compression test and then start tearing things apart

 

my bet is that you have the rings are broken; but as you run a higher speeds the engine is hotter so the rings expand letting it compress... thats what happend to us atleast...time for a new top end