Gravely Chainsaw on 4 wheel tractor

I mounted my Gravely 26" bar chainsaw on the front of my 12 hp 8123 Gravely tractor to cut up a fallen live oak tree, that fell about 6 months ago, it was 15" across at the base. I ran it the last time mounted to my 2 wheel tractor and it was very hard to control so I put it on the 4 wheel tractor; much better, I ran the engine at about 1250 RPM. The chain is 9/16" with 60 driving links which is no longer made. The man I bought it from said he ran it on his 2 wheel tractor for about an hour and it was too much to handle.