I bought dual carb 503 with a C box with a 3 blade Warp Drive that was set up ole school on a Quicksilver GT400. No oil injection. When I got the 503 it had 320 hrs and I found out it was carboned up pretty good when I got someone to decarb it for me. Never ever take someones word for the condition of a Rotax is my advice. Set your own base line for your safety as supposedly the engine was decarbed as part of the sale price. LOL
I had heard of the Yamaha product called Ring Free from the Kolb group from an automotive engineer. He swore by it and indicated it had been on the market some 20 years. After I had my 503 decarbed I faithfully used the Ring Free ever time I bought gas. ( now they call it Engine Medic ) When I reached 650 hrs I had the 503 down for maintenance again and the mechanic who did the tear down was so impressed with the Yamaha product he went out and bought a case of it as my e 503 was virtually carbon free after 350 hrs. of operation on pre-mix set up. The stuff works and no problems that I have found with it's use in any engine you use it on.
Bob Brasher
3NJ1
Pemberton NJ