new 50hp 2 stroke performance problem

[devrep]

Hello. A month ago I purchased a new 2006 Cape Craft 15CC flats boat with new 2006 Yamaha 50hp 2 stroke motor. The boat is fiberglass and rated at 60hp max. Hull weight is 850 lbs. Holds 20 gallons of fuel. The first ride I was careful to break in per the manual and kept the rpms low for the 1st couple hours. I did run it up on plane a few times but then throttled back to run. At the end of the days fishing I idled out of a long channel into the lake and when I went to get on plane the engine would not go over 3000 rpm. I shut the engine down and checked for weeds and restarted and it went fine. About 40 minutes later we stopped (I had my son with me) to fish a spot and when finished it did the same thing. I took the boat back to my dealer, who had it for a few days and said they couldn't find any problem.

The next weekend I went out by myself and the motor ran like a top all day, up on plane quick and cruised at about 4900 rpm.

The weekend after that I took one of my friends and his son out. Every time I tried to get on plane the motor would not go over 3100 rpm and the boat would not get on plane. Once while crossing the lake at this annoying bow high speed the engine slowly picked up revs and came up on plane and ran normally but that was the last time it would do it. 3100 rpm max all day.

I took the boat back again last Saturday to be checked out. Today they said they put it on the dyno and it ran great. This is very frustrating. Any ideas?

[Ken]

devrep,
I would first take the hood off the motor and then check the carb linkage while another person gave WOT at the remote. Could just be a carb linkage problem.
Then I would get the motor out and run it up to its max 3100 rpm and have someone check the primer bulb. If it has collapsed, then there is a fuel restriction from there on back to the fuel tank. Maybe a kinked vent line.
It the bulb has not collapsed, then try pumping it several times to see if the rpm comes on up.
If so, then that is usually an indication that the fuel pump is faulty.
Good luck [img]smile.gif[/img] ,
Ken K

[boats.net]

It sounds like it may be over proped if it wont plain out with 2 people on board. You mentioned it would cruise at 4900 rpm's with 1 person onboard, If 4900 rpm's is all you can get at wide open then you might try a smaller pitch prop, go down 1 prop size.

[devrep]

I think you may be correct on the prop. It's an 11 1/4 x 14 pitch. Think I need a 13. I'll try it and post back. Thanks to both of you for taking the time to respond.