Steve61,
1. Check the fuel?
2. How old is the fuel?
3. Does the fuel have water in it?
4. Check fuel flow to the carby.
5. Replace spark plug.
6. Try to start.
7. If it does not start check plug for moisture.
8. If wet check carby again.
If you don't have the workshop manual I would suggest that you download it from the parts list and manuals area because it is worth reading.
Also post picture of the units you are working on as it is easier to solve the problem you have because you have not told us what model carbies you are working on as well as machine models.
The information you have supplied is not enough to be able to help with the correct advice.
