The rpm cycling up and down is called hunting. Our mower did this when the carb got partially clogged with varnish from the gasoline. This happened gradually, over 4 or 5 years of use. I was able to get it cleared up by adding carb clean-out solvent to the gas tank. I don't know how or why the sudden stoppage you describe could cause this problem.

It's possible that you have damaged the flywheel key and timing is off.

Orangedotfever, if that's what happened, is it a necessary repair? Expensive?

It should be checked and the key replaced while you're in there. Remove the flywheel and check the key that keeps the flywheel from spinning on the crankshaft. It's all labor as a new key should be less than $1.

I would say IF your flywheel key is, in fact, damaged, it's a needed repair because the flywheel is going to continue to be in the wrong position, thus throwing off your mower's timing, and it's likely going to fail completely at some point, thus leaving you with a dead mower.

As for the cost of a repair: almost always expensive if the shop does it for you, almost always inexpensive if you do it yourself. It all comes down to whether you actually have a flywheel key problem, and if you're willing and able to fix it yourself. On your ability to fix it yourself, it's like almost any mower repair: not complicated if you have tools and patience and attention to to detail, can read and follow directions and diagrams, aren't afraid of screwing something up, etc. The worst that can happen is usually that you end up having to take the thing to the shop, which is no worse position to be in than if you'd just done that to begin with.

If your mower is still cutting grass OK, I'd certainly try to live with it as-is until the fall, when the repair backlog will be substantially less (a lot of shops now are backed-up over a month on mower repairs).

One more question: is this a repair that I could trust to a good but non-Honda mower repair place, or should I bring it back to the dealer?

Difference is 5 minute drive vs 2 hours, and local probably would cost a lot less.

Any mechanic can do it. Taking the time to properly torque the flywheel is another topic.

Your accident certainly could have partially sheered the key, but in my experience, this doesn't cause an engine to surge at low speed. And you say at high speed it works fine (mowing). It will manifest itself as hard starting, kicking back when pulling, and a miss at high speed.

I don't suspect the key just yet, even though something DID happen.

Any decent mechanic can do the diagnosis and repair. Save your time and money and go with the local shop.

Is it possible you had some sluge in the bottom of the carburetor float bown and when you tipped and jostled the deck around you got that crud in the idle or main jet? Might just need to clean out your carburetor.

I really don't suspect the key is the problem but since you hit something with the blade it's a good thing to check and cheap to fix if you do it yourself. Surging is usually a fuel issue.