Sudden RPM drop...

[justrfb]

Hello All.
Long story short, we bought a 2007 Princecraft Vantage 20L last year with a 2007 Merc 4 stroke 75 on the back with 97 hours on the clock. After some initial trouble, resolved by the dealer/marina installing a new fuel pump assembly, it ran like a top all last year! Very basic (but nice) two toon boat that would knock on the door of 20 MPH at about 6000 RPM all day long last year. We tucked it away for the winter. I fogged the engine, changed the oil and lower gear lube, treated the gas in the tank and covered it... Pulled it out last week and launched without getting gas... Changed the fuel filter this year (check that thread if you want a good laugh at my expense...), started up nice, ran it a little bit.. everything good... Light load, little gas, 6,100 RPM, 20.5 MPH... Rde a few more minutes, nailed the throttle again and 5,400 RPM would be all she could get... Parked it figuring the gas is an issue. Took it out again Saturday afternoon, 13 gallons of fresh gas, Techtron treatment, ran it a while and that is all she is still getting, 5400 RPM. Everything else is good... Starts, runs good, no missing or sputtering, idles fine... It is a little rough around 2500 RPM.. Some vibration... Any thoughts as to what happened? Prop is fine and all else is well. I am thinking plugs?... What do you think. Thanks for all of your help.

Sincerely,
Rich

[Bamaman]

When in doubt, start with the spark plugs.

If they're good and you're down a little, it might be fuel issues. Check out the fuel line.

[jrolin1]

Yep - check your plugs. I had a problem on a 2 stroke waverunner exactly like that. It would run fine at idle and on up on the rpms but would not run wide open properly. In my case it turned out that the spark plug boot had gotten a little loose so when I got up in speed and the vibrations increased it would miss occasionally. It drove me nuts until I felt that one plug.

[cwag911]

Does it sound different at WOT? If not, it might be the tach.

[OK Toon]

I would say the plugs since you fogged at the end of last year. That stuff coats everything real well and when you fired it up this year, all the fogging oil burned off like it should. Change the plugs and you should be fine. That's exactly what I used to do with my 2-stroke Seadoo. I'd burn off the fogging oil from the previous winterization, then I'd put in new plugs.

[justrfb]

Hello All and thank you.
I will be doing the plugs soon... Haven't had a chance... I am really praying that they actually are the issue... Someone asked about sound and maybe the gauge is not right. The engine sounds OK, just not reaching full RPM and not full speed. Only goes about 16 1/2 MPH (GPS). I know it could be tricky but can I change the plugs right in my slip or do I have to pull the boat? Any laws or regulations I don't know about? Thanks again.

Sincerely,
Rich

[cwag911]

If you can get to them and don't think you'll drop your tools in the water, then go for it.

[GXPWeasel]

I'd say if you can tilt the enging all the way up, so that you can reach the plugs, then you should be good to go for changing them in the water. Most plug sockets have the rubber inside, so you shouldn't drop the plug. Just don't drop the tools.

[justrfb]

Hello All.
Thanks for the comments and suggestions.. A little update... I went with Annemarie (my wife) to the boat last evening to do the plugs. Ended up not doing them because I didn't bring the right extension for the ratchet but that aside. I went to pull the cover off the motor and the front latch was broke and hanging free. Also, the cover to the top schrader valve was off. I know I changed the fuel filters but I thought I touched the bottom schrader valve cover. I was working with the boat on the trailer in my yard. The boat is in a slip now... I am usually pretty thorough when I work on my cars, house, boat... I can't help but think someone was screwing around with my boat (1'st time slipping at a local marina) . Plus, the NGK plugs... $16.50 a piece!! I am getting really discouraged with the boat. I wish I could just figure out what the issue is with the drop in RPM's, fix the broken cover latch and start enjoying the boat. Thanks for hearing me out...

Sincerely,
Rich

[GXPWeasel]

Sorry to hear you didn't get it fixed yet Rich.

Sometimes, it seems that's the way things go with boats, or with working on boats. You go and try to fix one thing, and all you end up doing is finding another thing wrong/broken, or causing another thing to go wrong.

Hopefully you can get her fixed up quick, and get momma out there on a boat ride to relieve some stress soon.

Oh, and don't forget the

[justrfb]

Thank you All and GPX...
I am happy to say our boat is fixed and running (once again) like a top. Long story short... They said I couldn't work on it at the dock, I asked them to check it, they had my key for a week and a half... Nothing. I told them I was bringing it to another marina to be looked at... They fixed it the next day! My marina checked it out and said it was a bad plug... Plug changed, boat fixed... Now onto the broken front motor latch and the inoperative fuel sending unit. Thanks for your suggestions, help and support...

Sincerely,
Rich