To be honest its hard to say without inspecting it . There are a number of possible reasons :-

1.It could be the belt slipping

2.There might not be enough petrol getting through

3. Is the air filter blocked at all ?

4.Have you looked at the spark plug ?

5. When you say jerk is there a constant jerk ?

If you require further help don't hesitate to ring us ,my site is in the signature, and then when we find the problem i can post it in here to let other users find a quick solution.

Many thanks from the gmrm team