I'm running a Go B21 with an OS P3. Right now I've been running O'Donnells 20%. I'm looking at switching to Byron's or Sidewinder 30% as that's what my LHS carries.

What differences will I see? I know I'll need to retune, but will I need to change my glow plug?

Thanks.

You may need to re-shim the head button. Not all engines come shimmed for 30% Nitro. Check with a GO-PRO to be sure. Glow plug change not required. You will see increased Power and more constant idle at the sacrifice of some run time

You should probably insert the 0.1mm "break in shim" that comes with the engine.

How do I check the shimming? I got the engine used so I'm not sure what the previous owner was running in it. I thought you only had to retune for nitro changes?

GO engines come shimmed for up to 30% from the factory.
No need for the . 2mm run in shim.
Standard shimming is . 5mm.
Normally 1x . 3 alloy and 1x . 2 copper.
Older motors may have the alloy shim and 2x . 1mm shims instead.
Run a medium plug like the Ok 97T or Werks #6 for best results on 30%.
The GO motors love the higher nitro content.
You will notice a huge difference from the 20% you have been running.
Go with the Byron's fuel - 30 / 9 if you can get it.