Adjective for engine running at high speed

Solution 1:

Is full-throttle the term you're looking for?

full-throttle: (going, acting, happening, etc.) at full speed or with great intensity -- Webster's New World College Dictionary

Solution 2:

For a single word which is specific to an engine which displays RPMs (cars, for example, usually don't provide direct control of a throttle), you are looking for the word redlining.

The term expresses the notion of taking the engine to its maximum RPM, this region of the tachometer typically displaying its numbers and lines in red rather than white or some other neutral color.

Definition of REDLINE (Merriam-Webster)

: a recommended safety limit : the fastest, farthest, or highest point or degree considered safe; also: the red line which marks this point on a gauge

However, I would not use this to describe a tool like a chainsaw or lawnmower. Here, you have a throttle lever to control, and full-throttle is the most appropriate term. Such tools typically do not have a tachometer to interact with so the user's interaction is not with a gauge but with the control lever itself.

Solution 3:

If you wanted to keep the gerundive form consistent I would go with racing.