I am new the whole mechanic field so i have no idea what is wrong, nor do I know the proper terminology for alot of things. problem: 1980 CM400A has been working fine for the whole time I've had it. now though, it starts just fine, it even goes. but i can't make it go past about 15 mph, and that is after working it. it wants to just cruise at about 3 mph. Now I just replaced the spark plugs, thinkingthat might be it, but no go. My frienddid this little thing where he unscrewed the plugs, put his finger over the hole and told me to hit the kickstart. one of them made a whooosh sound and the other made no sound at all, to which he made a frownie face to. I NEED to get my bike fixed so I have transportation again, and I am all set to just start taking it apart piece by piece hoping i can find an obvious problem, but really I have no idea what I am doing. Also of note, when the back tire is off the ground, it looks like it gets just fine rpm when revved. I hope someone can help me with what little i am able to tell.