I can’t start the engine. Do not know what the problem is. Sometimes it does not even fire. I have checked for spark and there certainly is. Carburettor is giving fuel as when I spin the wheel without the plug I can see a mist of air/fuel mixture. Should be no air leakages, I have mounted to cylinder head back on with perfection. Help. Here is a video of me trying it down a steep hill, when it goes fast it will fire sometimes.