I have a Gen IV (I think) 7.4 L Mercruiser that keeps detonating cylinders. I've gone through 2 motors so far, and hesitant to replace the motor again until I figure out what's causing the pre-ignition. I'm thinking it's a fuel issue, but can't seem to isolate the problem. On the first motor I had the carburator rebuilt, and replaced the mechanical fuel pump with a used one, and it still happened. The second motor has a brand new carburator and the same fuel pump as the first motor. I had the cylinders scoped, and sure enough another cylinder is cracked. I plan to pull the entire fuel line to check the syphon tube, etc. Anything else I should be looking at?