I have a Honda GC 160 on a troybuilt tiller, it is about 3 years old. Yesterday I was tilling with it and it started to run eratic, the engine would rev up and down continuously while under a load. I changed the oil and air filter before I started tilling with it, this tiller has probly 4 hours at the most on it. I have no idea what is causing this and would really appreceate any help with this engine. It does not rev up and down when idleing, only while the throttle is up or while under a load while tilling.

Thanks
Trey

Sounds like your high speed is out of adjustment.An engine will surge up and down like you say when its over speeding, your high end rpms should be around 3600. It maybe as simple as adjusting your throttle to get it down where it needs to be.The other possibility is your having an issue with your governor, the gc/gcv engines at one time had an issue with the cam gear breaking. But what your describing sounds like a simple adjustment.

How do I adjust this, I know nothing about a engine.

Carburetors on the Honda GC160 do not have a high speed adjustment, there may be some dirt or water in your carburetor causing this issue, or even a bad spark plug.

The governor could definitely be the issue, but they generally do not come out of adjustment on their own. If you have had the carburetor off or have been messing with the linkages then maybe the governor needs to be adjusted.

If you want to adjust the governor, search on "Static Governor Adjustment" you will find the procedure posted on this site numerous times.