I have run as large as a Fuji 86cc twin full throttle on the sawhorse pictured above. The roughly 35-40lbs of thrust does "flex" the horse a bit, but it never threatened to take off. It stayed solidly planted, especially on grass. I made sure when I built the sawhouse that I gave it plenty of length to be stable. Now, if I eventually get into 40% aerobats and run 200cc twins on this thing, I might think about counterweight.

Now here is the sure way of making sure the sawhorse don't fly away - run an engine on either sides! Near end: Minarelli 70 with CH ignition. Far end: Ryobi 31 on glow.