Have a quick question.... I have been running 30% nitro in my B3 since I bought it. I just realized today that I will not have enough fuel for a race and if I order it, it wont be here in time for the weekend.

So, my question is...... I have some 20% out in the shed, if I use this how is it going to effect my engine due to me using nothing but 30% for break in and normal running?

Thanks for the help!!!!

Run it for a few tanks and let the engine get use to it and it should be fine.

out in the shed? for how long? nitro can go bad, so be carefull with it
I remember reading some guy used old fuel and had bad results like a burns or an explosion of some sort.

Old nitro is no more prone to explosions than new. But if it was not stored in a tightly sealed container it could have absorbed moisture and thus not burn as well and coud cause corosion if left in the engine.
Having said that, as long as it was stored properly it will keep for a long time. A year at least.
As to the 30% vs 20% issue.
You will have to retune and you may notice that the engine lacks some of it's former power and it may run a bit warmer than with the 30% fuel. It should not be a huge difference though and retuning should should get you close to where you were, performance wise.