On the one hand, Carnage is stronger, and is far more fearless in his fighting, going for unorthodox techniques that would confuse most other foes, attacking them from multiple angles to catch them out. It is likely that he will get the first strike in, based on his speed and agility.
However, Reed's rubber body means that most of Carnage's offence just isn't going to be effective (using weapons to slice his foes, etc) and that gives him a chance to perhaps warp his limbs to take Carnage out using blunt force (much like what he did with Namor, trapping him and smashing his face in multiple times by trapping his arms as well). Reed's smarts and experience with Symbotes may also play a factor here- while the Carnage creature doesn't have much of a weakness to fire anymore (if we are counting his amps recently with Red Goblin) Reed may be able to come out with some method of exploiting the weaknesses of Carnage, which may in turn make the fight much easier.
All in all, it really depends on the conditions of the fight, but I'll have to go for Reed at the moment. His smarts and experience give him a edge over the other guy.
I feel that with no gear and no prep time, there's little each could do to the other. Reed's not exactly a heavy hitter and Carnage probably can't physically harm Reed. Richards might be creative and resourceful enough to come up with something, but maybe Carnage could incapacitate him by bonding with him? After all, Carnage can bond on a molecular level, right? Unless that would count as BFR. Also, if I'm not mistaken, Carnage has no-sold Reed's sonic cannon before anyway, so don't think there's much Reed can do in a random encounter and no BFR.
Would Carnage bonding with Reed count as BFR? Is there any way Reed could prevent it? Because I don't see any other way this fight could end unless Reed like starts a massive fire or something somehow.