With the information you provided I would say yes to changing the jets and or the needle height depending on if it's lean at idle, mid throttle, or open throttle. You are correct with your ratio of oil and fuel, although you can add or remove oil volume to lean or richen your mixture (more oil + leaner mixture of fuel, less oil = richer mixture of fuel). I do not recommend messing with that to much because you can also run into lubercation problems if there is not enough oil, but you can use it to diagnose but don't do it to repair the problem. Having a leak in the intake manifold can also lean your mixture this can be checked by spraying starter fluid to the outside of your manifold, If the rpm's rise you have a leak and you should replace the intake manifold gasket or the intake manifold because it may be cracked or faulty. There are alot of diffrent things that could be to blame. You can find the parts that you request by clicking the following link and searching the following diagrams. http://www.crowleymarine.com/mercury_parts/359.cfm. If your type BRP in for a promo code during the check out process you can receive 10% off of your order. 

 

Thank you for your business, 

Scott