I have a Honda it has a standard 4cyl b21a1 1990 Honda prelude (should have specified was in a rush) I recently changed the head gasket(it went bad would puff out white smoke when I cranked on the key and even in the cold of night my radiator hose would be pressurized without driving or even starting it). a fairly simple operation but when I went to start it had lacked power, I would go just as fast if I had the pedal a quarter of the way down as I would with it being full throttle but it was still functional and I was content with it. The big issue is, recently while I was coasting along I noticed my car would drive itself it felt as I had just clicked on the cruise control, I was able to work around it because of my manual but when in gear it will take off especially when u are in the lower gears. and when I sipped it into neutral it would rev up at a constant rate from 1.0k-2.8k being very loud and burning tons of gas in sure. this just happened as I was driving down the road from my slow but still running vehicle im. When I go to start it it instantly shoots up to 3k rpms then jumps between the 1-2.8rpms again and it is constant. Anyone got a good guess? it also smells very rich in gas but my ignition timing was spot on. I changed the head gasket it went bad would puff out white smoke when I cranked on the key and even in the cold of night my radiator hose would be pressurized without driving or even starting it.
I had a similar problem with my 1998 Honda CRV. It turned out to be the idle air control valve (IACV). I cleaned it a couple of times but the symptom returned and my check engine light came on. I replaced it with a new one and it now runs fine. It should be located mid-engine, fire wall side, attached to the intake manifold, held in place with two bolts.
A surging idle indicates a vacuum leak on the intake.
You could have a leaking vacuum line (did you forget to plug something in?) or your intake gasket may not be sealing.
Like Vince pointed out an hour ago, a bad Idle Air Control Valve (IACV) could also cause this. Since it was not doing it before your engine work, my guess is the IACV is fine.
With the engine running, spray some short bursts of starter fluid around the various components. If the engine speed increases, the leak is coming from the area you just sprayed. If it does not increase, let that area dry before moving to a different area. You don't want a lot of starter fluid on your engine. If it ignites, you would be in big trouble.