I have just fitted a new 2.5 inch exhaust with a 4 inch OD 18 inch long silencer but it's very very loud.
Does anyone have any recomedations on how to make it quieter or a silencer to use.

If it’s a new fibre filled silencer it will with time expand the filling and quieten down however 6 inch would be better. You only have 3/4” padding around the perforated exhaust tube not a lot available to absorb sound

Your need a min of a 6inch or bigger repackable silencer and around 24 inch long with quality exhaust matting fitted

If OD is 4", and pipe is 2.5", there is only 3/4" thickness to absorb sound. So unlikely to do much....
As above, you need a much bigger silencer, or use a baffled exhaust (that will rob a bit of power)

Or a much longer box. My ST170 powered spitfire has about 5' of silencers as I could only use 4"OD without sacrificing ground clearance...

As said above, you need a bigger diameter silencer. Ideally 6" but 5" would be a significant improvement. Diameter has a bigger effect than length. Use accoustafil wadding in it and only use about 2/3 to 3/4 of the amount the online calculator comes up with.

Thanks for info will get a 6" one.