There are 28 (number not important) different tuning techniques. When you learn a level of mechanics, you learn one new technique. Every tunable building has seven techniques which can each improve its level by one. This is all the same as the current system so far, except possibly for the number of techniques which are available.
Techniques are divided into seven levels, with four techniques at each level. When you learn mechanics 1, you learn a random level-1 technique. When you learn mechanics 2, you learn a level-2 technique, and so on. Exactly one technique from each level will be able to tune a buildings. (These techniques may be applied in any order.)
Each technique is given a name. You are told which technique you have learned whenever you acquire a new one. When you attempt to tune a machine, you are given a menu of all techniques available to you. ("Tuning.../optimize gear ratios", for example.)
This system makes tuning deterministic--under the current system, a person with mechanics 2 could fail to tune a machine five times and still succeed on a sixth attempt. This is frustrating, since nobody ever quite knows where they stand. This system also makes higher levels of mechanics more useful--in order to tune a building to level 7, at least one person with level 7 mechanics must work on it.
Best of all, however, this makes the mechanics skill more social. People will start keeping track of what techniques have been used on their buildings, and seeking out people with as-yet untested ones. Level 7 mechanics will be in demand.
An added change, to encourage travelling mechanics: Rather than a 20 minute timer on tuning a building, allow a maximum of 3-5 tuning attempts per day. This way, a visiting mechanic can tune several buildings quickly, without being required to sit around for an hour or two.