I used a JR 662 radio for about a year and I am familiar with the problem that you are talking about. It took me a while to figure it out the first time. The secret is to make sure that you are not using too much sub-trim or travel adj to compensate for poor setup at the servos.

Try the following:

Set the throttle sub-trim to 0 and the travel adjustments at 100%, and make sure the digital trim is at "0". Note- you will have to push the throttle stick forward to set the "high" travel adj and pull it back to idle to set the "low" travel adj.

Now position the stick at center. Make sure that the throttle is approx 1/2 open and the servo arm is perpendicular to the connector rod. This is very important with the 662! You may have to pull the servo arm off and reposition it, and adjust the clevises to get things right.

Now, with the travels still set at 100%, find the holes in the throttle and servo arms that get you nearest to full open and full closed when you move the stick back and forth.

Finally, once all the above has been done, adjust the travels in the radio as needed. To do this push the stick all the way forward and adjust the "high" travel so that the throttle opens fully. Then pull the stick all the way back and adjust the "low" travel so that the throttle is at normal idle position.

After doing the above you should find that the trim and kill button work properly. The last step is to fire off the motor and test. You may have to adjust the "low" travel a bit to get the kill button to stop the motor. And use the digital trim to get the idle where you want it. But do not use the sub-trim to try to compensate!

I upgraded to an 8103 two months ago and I'm still getting used to the manual throttle trim. I actually like th JR digital throttle trim on the 662, but I think that it gets a bad name because many people have problems setting it up correctly.