RPM issue

[searayjoe250]

Good morning,
I have just repowered my 1987 sundancer 250 with a reman'd mercurisuer 350 260 HP. I have 50 hour on it and not able to get my rpm's above 4300. I am running a 15 1/4 x 15 alum. prop which seems to be the original size prop. The load is usually two adults with 1/2 tank of fuel. any suggestions on getting the rpm's up.... smaller prop? if so what do you recommend.
Thx

 

[Lazy Daze]

How's the rest of the boat? Clean bottom? Condition of prop? Make a run with just you and limited equipment on the boat, first. Just to find out if it's capable/not capable. You'd have to drop to a 12" or 13" prop, otherwise and that doesn't seem correct.

How long have you had the boat? Where you able to get to about 4600 before? Are you trimming all the way up? Tabs out of the way?

These other things need to be addressed before we say there is an engine issue.

 

[Hampton]

Also, these tachs are not the best in the world. You may or may not be getting a completely accurate reading.

 

[searayjoe250]

Thanks guys for the reply,
Thought about the tach, had the mechanic take a seatrial yesterday with the shop tach attached, came up with the 4300 with mechanic and full full load ( I wasn't with him) the prop is new this season, had a 17" before which only brought about 4000, figured it was a tired engine as it was the original, the engine let go at the end of last season so down proped to the 15, thought I would get more.
The bottom probably has some slime but as we're hauling next week I'll wait til then. The mechanic said he'll do a compression test on the engine when he winterizes yet but it runs fine with no skipping etc....so don't expect any burnt valves. Merc says I should be getting 4600 -5000 rpm.

 

[Hampton]

The bottom probably has some slime

Shack! It doesn't take much at all to affect performance. This is my vote. Clean her up and try again before investing too much more time or money.

 

[Southpaw II]

I recall 4400-4600 rpm for that engine...but 5.7s were made in a variety of configurations....fuel injected or carb'ed. Only the very latest models, i believe, like the 350hp models were spec'ed to do that many (5000) rpm.

 

[searayjoe250]

Thanks we'll clean her up, my main concern is to make sure its not the engine as the warranty ends in April of next season, will have about three weeks once she's relaunched.

 

[SeaRide]

Your valve covers may still have the original Merc spec. stickers. including info such as the WOT range. If it is the original engine, a 350 with a 4V carb (260 MerCruiser), your WOT range will be 4,200-4600 RPM.