This is the fabled and dreaded mis-adjustment of the reverse gate (AKA reverse cup and reverse boot).
As you know, there are no gears involved in the outboard jet - reverse is achieved by a gate or cup going over the jet outlet redirecting the jet output backwards. The default position of the cup is hanging down below the jet outlet. Reverse requires that the cup be pulled up and over the jet outlet. What is happening is that your's is mis-adjusted so that when you're going forward, the cup is not haning clear out of the way, but is slightly raised so that the jet stream hits it. If it hits at just the right (wrong?) way, the force of the jet output will actually pull/push the cup up over the outlet and thus causing reverse.
Although all outboard jet lower units are very similar, there are differences in the brackets that are involved in attaching the forward/reverse cable to the unit. I have no experience with the units that use a shift rod instead of a cable. I can't give you specific instruction on how to adjust yours - but I found mine to be pretty self explanatory. As a start, watch it closely as someone (or yourself if you can reach) shifts between forward, reverse and neutral. The bracket that attaches to the cable should show you the spot that will "lock" (a soft lock) itself into position at full down (forward) position. This is the first and most important adjustment.
From there, you adjust neutral. Neutral is just a medium position of the cup which diverts some of the jet flow forward, some backward to be approximately neutral. In my experience it's hard to get a very solid neutral and I usually have a little forward push in neutral.
But again, as you've discovered, the most important adjustment is to make sure forward is good and solid.
I would guess that if you look at it, you can figure out how to adjust it.
To get a user's guide, you need to go to a local dealer of the product, or contact Outboard Jet company directly. I contacted them by phone through their website
http://www.outboardjets.com and found them very helpful. Their website has some printed material, TA and FAQs, but I don't recall a full user's guide - you'll probably have to call them.
Mercury Jet slips into reverse
Linear Mode
