MarineEngine.com does not offer troubleshooting assistance or repair advice by email or by telephone.
You are invited to join our public Boat Repair Forum to seek assistance from other members.
You may also visit the Boat Motor Manuals section of our site to obtain a service manual.
Piston size is the same ( you said 521 cc )------They use different carburetors and air intakes.----------Easily checked on this site----boatengineparts---------Or on ---------shop.evinrude.com