In that case, you will simply have to, pour in more acid anyway. It's easier to do it right the first time. When you pour the sulfuric acid on the sawdust, the reaction is almost immediate. The sawdust and acid react in such a fashion as to turn black almost immediately. It resembles an ugly collection of coal tar or oitch.
Bubbles rise up through the solution.
The bubbling is primarily due to air pockets inside the sawdust. Even though the reaction appears to be instantaneous, you should let the mixture sit for a day or two to allow whatever reaction doesn't take place at once to proceed at its own leisure. Once the reaction is complete, you can simply dump in yeast and expect the mixture to ferment. The pH of the mixture is so low, that is the substance is so acidic, that any microorganism such as yeast that you dump in is simply going to explode. Of course, they will be very tiny explosions.