1970 johnson 20hp.

My outboard I just bought. Tested it in bucket of water and it ran goof. Then took to lake to try out. Runs in reverse just fine put in forward and at low throttle runs ok. When you throttle it up it acts like it's hitting something. It keeps jerking. We kept messing with it and got it to run across lake with no problem. Shut it down and started in high throttle and it started jerking again. Any ideas what could be causing this

Sounds like it is jumping out of gear.-----So do the following.------Remove the wee cover on the side of the exhaust housing.----Now remove screws in brass piece one at a time.----Make sure notch is lined up with hole and put screw back in.----Check adjustment at top behind the shift lever.---Test run.----If it jumps just once it is time to take lower unit apart for inspection.-----May need new forward gear and clutch dog .---This may get expensive.----Your location ?

Holden mo

Holden mo

Since these don't have a detent ball or any way to hold it into gear within the lower unit or gear case, then any slack in the shift rod will cause the inability to hold the shift dog tightly into the forward gear. So if you replace the shift dog and recondition the forward gear you must also correct the slack in the shift arm and rod and associated detent. I have reconditioned dozens and dozens of these gears but that can only be done successfully with careful attention and experience. Some may argue but I have done some of my own outboards over the years and still working nicely after 45 years of use.
Occasionally you will see a prop hub going bad but that will not give you that mechanical jerking like the symptoms that you describe.