In an offset charx smoker, a little spike in temp when adding lit fuel is normal.
To keep this more under control, I use a mini loaf bread pan of water beneath the cooking grate where the firebox joins.
Looks like this:
Beer can is there to give you the general size. And for my personal edification?
The water acts as a bit of a buffer for spikes in temps coming out of the firebox.
Still, a bit of a spike is to be expected and will not usually be a problem. I try to keep my smokes in a 20 deg temp range. That will rarely be a problem in low and slow cooking.
Good luck and good smoking.