been fighting getting my at3 running well. found compression isn't great. I haven't tore into it yet to look but  looking into insite for best route. do I go .25 over or stick with standard. or tear down and see how things look then decide if its worth having the cylinder bored

Hard to assess based on info given. Any videos? You tube link?

You really need to remove the cylinder and look at the bore before you decide what piston size to go with. You don't know what condition the bore is in yet, could be some gouges from seizures, or just really worn at the top.

Last year I was working on a CT2 top end that did not appear to have seizure issues, but I could feel at the top of the bore how worn that area really was. I think the machine shop had to go 3 oversizes ( ) to get a clean bore. Point is, in order to really assess the condition, it needs to be measured and checked before buying parts.

It's tempting to jump in and start ordering bits before you really know what you need. Better to get a good look first, unless you don't mind having bits you won't use. Just sayin'.

You can have that top end off within 30 minutes, no problem! 

Thanks for the insight! I'll get that off this weekend.

If your complaint is lack of performance then i'd look at the exhaust providing you have reasonable compression to feel on the kickstart.
Also providing you have points gap around 14 thou to get timing in the ball park.

RT is quite right about making sure timing is correct, etc. Worth double checking the timing/points gap certainly before whipping the head off (and that alone may tell you quite a bit- With the head removed, rotate the piston down so you can feel and see the top portion of the bore. If you can feel a wear 'step' at the top you need to continue).

Update: removed the cylinder my case to cylinder gasket had been leaking for what appears to be some time. cylinder and piston look awesome. Gonna get a set of standard rings lightly hone the cylinder new wrist pin bearings and a couple gaskets. See if I can get this beaut can't be ripping again. thanks for the advise

That certainly could be the culprit!!