The MDS line were a good running engines of reasonable quality. The same can not be said for the .40. There are several long threads where I and others attempted to get our .40s to run reasonably without success. Maybe you will lucky and yours will run. Try zero nitro fuel with an idle bar plug and if that doesn't do it cut your losses and move on to another engine. I spent an entire summer fooling with a pair of these and it was wasted time. A confusing factor here is that many will jump in and post about the wonderful results they are having with some other displacement MDS product.