i have a 77 johnson 35, i just had a tune-up new coil put on carb rebuilt. the motor revs up good in netural but as soon as i put it a load on it, it only goes to 2500 rpms no matter what i do.
Check spark at 2500rpms. You may have a bad ground or coil wire not seated properly. Also you didn't mention that you had new champions put in also but I hope you did.
