These days, it's much more likely a pilot will go on about how great the Rotax 912 is. Yes, some grumble about the purchase price, the replacement parts cost, or the cost of an overhaul, but I'd expect to hear such groaning about almost any aviation product. Contrasting a few negative opinions is an entire world of pilots who are intensely loyal backers of the 9-series engines.
Around the planet, I have identified more than 
To most eyes, the RMZ 500 is nearly impossible to distinguish from the original that was taken off the market in 2010. A Russian producer manufactures it to power the snowmobiles they sell. I get that lots of readers may turn away when hearing the engine is a Russian product but many may be able to see beyond the political-class demonization of all-things Russian to understand that regular citizens of that country could have the capability to assemble a worthy engine. U.S. support makes all the difference and that is 
Speaking of exhaust systems, 
FAA did bury the old two-place powered ultralights. The agency's idea was that "properly" regulated Light-Sport Aircraft would replace the role filled for many years by Part 103 ultralight trainers. In case you arrived since 2004, those were simple two seaters that closely resembled related single place ultralight vehicles. They were intended for training only. You were not supposed to fly your buddy or your spouse on pleasure flights. All flights were to be for instruction only.
LSA supposedly fixed the problem — and in a few examples they did (Quicksilver, M-Squared, CGS Hawk, some trikes and powered parachutes). In a Special LSA ultralight-like aircraft, you could fly a passenger recreationally and you did not need to always perform instruction. Pilot licensure also was tightened. Unfortunately, the results were devastating to ultralight aviation — many instructors left the training business.
However, out of sight from most aviators, hang gliders were still permitted to do two-place instructional flying without needing a Special Airworthiness Certificates such as all LSA have. It has continued right on working well to this day as it had done for the decades before the big changeover.
What even less people know is that powered paragliders (different than powered parachutes) were also permitted to keep their training exemption. FAA recognized that almost no one flew two-place hang gliders or paragliders or powered paragliders so they didn't mandate a switch to costlier LSA.
The Aviator Paramotor rig you see with two seats, and the RMZ 500 powering it, remains a legal, exempted vehicle. For those intrigued about flying a powered paraglider as a very affordable and manueverable aircraft, a wheeled carriage with a powerful engine could make an interest purchase. Training is available and Aviator Paramotor can barely keep up. Check this link for more about 
In regular use, Aviator Paramotor uses the RMZ 500-powered rig to train new students — lots of students, some 300 per year. Demand is so strong they actually have to turn some people away. "We could probably exceed 1,200 students a year if capacity were available," said Eric Farewell.
https://youtu.be/oDgjtddwibo
For years — no, make that decades — numerous pilots of light aircraft have told me the Rotax 503 was their favorite two-stroke engine. These days, it’s much more likely a pilot will go on about how great the Rotax 912 is. Yes, some grumble about the purchase price, the replacement parts cost, or the cost of an overhaul, but I’d expect to hear such groaning about almost any aviation product. Contrasting a few negative opinions is an entire world of pilots who are intensely loyal backers of the 9-series engines. Around the planet, I have identified more than 66,000 light aircraft and 70-80% of them use a Rotax 9-series engine as their powerplant. Every other brand occupies the remaining 20-30% space, including some other fine and reliable engines. No matter how you spreadsheet the numbers, Rotax is far and away the dominant brand …although no longer in two-strokes.
Everyone has returned home from the first major airshow in too long (other than the wonderful sector-specific event called 
Just today, I learned that 
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—The 
—The much-beloved Rotax 503 will not be returning to the market. The Austrian company discontinued the popular two-stroke engine years ago, meaning many who still want one must search among used engines. The good news: many have only a few hundred (or less) hours on them; a good used engine is both a bargain and a worthy powerplant for your light aircraft kit. Now, though, a very similar engine is coming from Russia, of all places. It's called the 
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