I have a 98 mercury 225 efi 3.0L that I just recently had a new power head installed. I’ve been in the process of the 10 hour break in period and has been running great. Note: (Oiler unhooked and double oiling during break in.) I just got into the period in the break-in where I could run it at full throttle. The first time I ran it at full throttle I had no issues at all at around 5800 rpm running a 24 tempest 3 blade prop. The next time I took it out I got up on plane and it ran great again for about 20 seconds and started to jump down to a lower rpm around 4500 and would jump back up a little then back down like it was trying to kick in and out. Note this was not in rhythm it is more sporadic. When I let off the throttle and keep it around 5000 it doesn’t cut out cranks and idles flawlessly. My initial thought was fuel. Between me and the mechanic that did my rebuild every single fuel line from tank to injector pump has been replaced. New bulb, filter, rebuilt mechanical fuel supply pump, and injectors cleaned. Since the problem started with my engine I: checked spark plugs (NGK champion replacement) for correct gap and cleaned, no burn on plugs, and checked compression around 98 on all cylinders. Rechecked all fuel lines and connections. I found a crack in one of the small hoses that runs from intake to cylinder heads. I replaced it thinking I had found the problem. Took the boat out and the same thing happened again. This time I played with it some more while running the boat and I started to think it was maybe ignition problems instead of fuel judging by the way it was cutting out ( more of a kick down than bog.) I read that it could be rev limiter so I tried a 26 tempest that I had. Took it out again. It brought my rpms down but started to cut out around 4800 rpms with the 26 tempest. I’m at a loss. The only other thing I know of is I found a water leak around the flush out below spark plugs, maybe it’s getting a spark plug wet when my water pressure goes up at full throttle? No visible water around them after I loaded the boat. That’s my next step fixing the leak but I doubt it will fix my issue. I haven’t checked fuel pressure at the injector pump but I just haven’t felt like it’s a fuel issue at this point but could definitely be wrong. Any advice would be appreciated.
