Excessive fuel burn after 100hr inspection

First, kudos for the owner-assisted part. Based on multiple experiences, I am firmly convinced that FAA certified mechanics are out to kill me; you cannot pay someone enough to care as much about your machine as you do. Also, kudos for noticing a problem and deciding to take action rather than blowing it off until something worse happens.

Next, the paste is not a sealant; it provides the proper thermal contact to keep the spark plugs hot enough not to foul and cool enough not to cause pre-ignition. A plug would have to be very loose to affect mixture all that much; evidence of escaping gas would mark the base of the plugs in that case.

The problem is likely with anything that was touched, possibly compounded by their "unfamiliarity with Rotax engines". (It's worthwhile to travel, if needed, to find a Rotax-certified tech.) Were the float bowls removed? If so, was it done "in-place", without removing the carburetors? In place, with limited access, it's easy to not drop the bowl far enough to clear the guide pins. Pulling it then out sideways can bend the pins and, in operation, bind the float before it can fully close the little valve. This overfills the bowl, messing up the mixture and blowing fuel out the overflow. <happened to me>

Less likely, but also both <happened to me> is an air leak in the fuel tank selector and/or gascolator. The standard gascolator with the bowl "secured" by a wire bale and screwy screw wheel on the bottom, is fiendishly difficult to get a good seal with. Are you the type who leaves the fuel selector set to "on/both" and seldom if ever cycle it? If it was cycled after sitting for a long time or you just had bad luck, the shaft seal o-ring or seal may have failed. In both cases, a breach on the suction side of the fuel supply may not show as a leaking fuel, but suck air while the engine is running. One would think this would tend to lean out the engine. It doesn't, until the air leak gets so bad that the volume of fuel supplied to the engine gets so low it misses or cuts out. But well before that, air bubbles cause fuel to "splurt" into the bowls which causes fuel to splash out the overflow vents. If you have carb heat from the Rotax intake log, this will be hard to detect directly because most of that overflow goes right back into the engine. Once I found and fixed that leak, my fuel consumption went down from 5.5 to 4 gph, just like you wrote.

It's pretty clear your mech is not at all familiar with Rotax.  These spark plugs don't need "media-blasting".  Based on that alone, I'm going to guess they also did NOT use the right thermal paste.

If you suspect a spark-plug problem, you might think about getting a new set of plugs (the NGKs are quite inexpensive) along with some of that thermal paste, and just replace them yourself.  

What Tyler said...  Plus I would check / re-gap those plugs.  I replaced mine recently, and while 7 of the 8 were correctly gapped, the 8th one was out of spec.  Easy job to do, and legal for any owner/operator, even on fully certificated airplanes (assuming US rules).

It is a Rotax requirement that there is a return line from the carb supply line to the fuel tank. Difficult to believe that such a mod wouldn't be logged-, but maybe that is what the shop retrofitted? Would render any existing fuel flow metering device useless.

Aircraft is an RV-12 so there's only 1 tank and thus no fuel selector, other than the cutoff valve which never gets touched outside of checklists making sure it's open. Carbs weren't pulled/opened as it was at its first 100-hour service, ever.

I ordered a set of the Official And Expensive Rotax Spark Plugs from Lockwood the other night as I couldn't get a straight answer after hours of searching this and other forums about the potential tradeoffs between them and the NGKs. Included in that order is a big tube of Silicone Heat Transfer Compound, and a torque wrench coming from elsewhere.

As was mentioned before, per 14 CFR Part 43, replacing the spark plugs falls under allowed Preventative Maintenance, so I'll be doing it myself. If this doesn't get resolved, there's a Rotax iRC somewhat nearby I'll be running this to (and honestly, probably taking to for its 200-hour).