I would also check out the hoses making sure that they are connected to the proper places and that they do not have a hole in them or a crack anywhere. Replace if any doubts exist. Is tank located properly elevation wise, center of tank at level of carb inlet. Don't be put off by comments on the MDS, they are powerful and reliable engines when adjusted. Look at your owners manual and be sure your needle valves are at recommended settings. You could experiment with opening each on their own and see if that makes a difference. The low speed needle may be set incorrectly. I have 4 OS engines and they have their problems just like any other engine. OS had serious problems with the cylinder linings peeling. believe me I know. I also have Tower and MDS engines and they are great performers too. Find an experienced flyer to give you some suggestions, you will find the problem.