I have a 78 bravo that I just got running. Sealed tank, rebuilt carb, the whole works. It starts right up, runs and idles great but has no power. I get power for about 10 seconds and then it gets slower and slower to the point where it is just at idle. I have a feeling that the cylinder may have a crack. Is there a way that anyone knows to test for this. I put my hand down there and I do feel air but I cannot tell if it is the cylinder or the exhaust.