Quoting from the Universal Ops manual:

" If the gear slips in the forward drive, back out the lockscrew No. 76 (see fig 3) until the end of it is just clear of the notch in the adjusting collar No. 28, now turn collar to the right until the lockscrew No. 76 is in line with one of the notches in the adjusting collar No. 28.
        Then tighten up the lockscrew No. 76, being sure that the end of screw enters the notch in the adjusting collar No. 28.  Repeat this procedure until the reverse gear holds In the forward drive.  An adjustment of one or two notches is usually sufficient.  DO NOT USE EXCESSIVE FORCE TO TIGHTEN NO. 76 LOCKSCREW."

Gosh, that sounds simple, doesn't it!

Last summer I spent a lot of time hemming and hawing about going into my transmission, which had been slipping at high speeds. I am one of those people who belives that there are just certain parts of an engine that were never intended for laymen to see, and that the inside of a transmission was one of them. I had visions of oil pouring out, and images of springs flying out.
But the problem was getting worse, so in I had to go.

Several people had told me that such an adjustment on an Atomic 4 was a pretty easy matter, and it turned out to be just that.

It's a matter of removing the square cover plate over the transmission (careful of the gasket...) and shining a light in -- no springs or oil. I did it thru the access hole in my cockpit. Then,  you need to loosen a locking screw to free an adjusting collar, which can then be moved a notch or two to the right and locked by retightening the screw. (if when opening the tranny you do not see any sign of the locking screw, engage the gear and rotate the motor a half turn by hand and it should come into view) Then,
reattach the plate and try the system to see if it is tight enough. Tighten it too much and you will not be able to put it into gear, so it's best to proceed incrementally. My manual tells me that usually one or two notches is all it takes to eliminate slippage.

IF you are slipping in reverse, it's a different procedure, which looks fairly straightforward, but which I have not tried.

The whole process took me about 20 minutes and solved the problem perfectly.  I have a simplified drawing from my manual showing these adjustments which I would be pleased to fax to anyone wishing to see it.