Norm are these just hard to get up to speed or is it getting them pass the compression stroke?
I know many Shredder/Chippers can be a pain to get up ignition speed here due heavy flywheel effect of the shredder/chipper head disc. I might need several strong pulls just to get them spinning fast enough to even hit.
The 4 kg is around 8.8 lbs and 2.5 kg is 5.5 lbs. Either is a considerable weight to get spinning by a recoil starter.