Only use QL77CC in a 3 liter per Mercury racing. I have always put a bottle of fuel injection cleaner in about every 5th tank of gas and never had a fuel issue. A good question is are you using 93 octane fuel all the time? Are you using Mercury Premium plus oil only? Have you checked your throttle to make sure the linkage is opening the butterflies all the way at full throttle? Have you checked your alternator output to make sure it is keeping the voltage up? Also check the plugins on the computers and cdm's they can get patially unplugged when the cowl is put on.Just a few questions that have not been asked but need checked into.
Richard

FYI a 250XB is not on the limiter at 6150 and will make more power above that number.
Good luck