what happens when a cam chain is too loose?

[minisman]

Hi
I have a yamaha tt350, i have had this problem which is getting worse, if i hold the throttle constant in the mid range revs there is a really harsh sounding vibration, and its missing a little, but its not there at idle and it goes away when you twist on the throttle .
The plug always shows a really rich colour. I have tried every possible jetting combo, and made it rediculously lean and it still vibrates at those revs. I have also checked the timing marks and it all lines up.
So the fact that it has a funny vibration in the mid range, is rich and misses a bit made me think maybe this vibration is caused by something mechanical, and the missing is a electrical problem being caused by the vibes.
The engine was rebuilt about 6 months before i bought it, so i decided to take the side cover off last night to make sure the balancers marks all lined up and they were fine too. While in there i noticed the timing chain moved quite a bit when you push on it.
So my question is can a loose timing change have such a dramatic effect at only one point in the rev range, and could the looseness of the chaincontribute to putting the timing out so much that it will have complete misses?

Thanks

 

[76GMC1500]

I had a 1986 TT225 that started to lose a CDI box. The bike would miss at lower speeds and seemed like the timing was too advanced in the midrange. If I squeezed the box just right with the bike idling, everything returned to normal, the misses stopped completely. I taped it up real tight and eventually replaced it with a box I found on the auction site.