1999 Mercruiser 5.0 (non-fuel inj.) WOT RPM Issue

[CatainKevin]

Hi Everyone,

First post here on an issue that has been nagging at me. My 1999 SeaRay runs great, with less than 300 original hours. However performance on plane is a little less than what I would expect. I have heard the WOT RPM should be 4600 to 4800, but I have never seen it higher than 4000. I have tried a 21, 19 and 17 prop, which makes little difference to top end speed or RPM.

Also, once I reach about 3400 RPM, I am around 34 MPH, which seems to be about 60% throttle. If I take it to 100% WOT I can reach 4000 RPM and 40 MPH.

Is this what I should expect? Why would I not be reaching 4600- 4800 RPMs and the high 40s some others are reaching with this set up?

-Clean bottom, good gas, light load, good prop (both 17, 19 and 21)

Thoughts are welcome, - Captain Kevin

 

[Bondo]

Ayuh,.... Welcome Aboard,..... I'd do a diagnostic tune up,......

Check the compression, possibly replace the spark plugs if they look like they need it, a new cap, 'n rotor, along with changin' the fuel filter, 'n more importantly, inspectin' the contents of the ole fuel filter for water, 'n crud,...
Lastly, check the timin',....

If yer pullin' 4000 rpms with the 21" prop, ya oughta be considerably higher with the 17" prop,....

If it's a 2bbl. carb, ya might have to be happy at 4400/ 4600 rpms,...

 

[Scott Danforth]

Check compression, also check your exhaust flappers, I have come across many boats with the exhaust flappers laying in the bottom of the y-pipe choking the motor. have you checked your tach with a shop tach?

my 2002 searay 190 would hit 50mph at 4850 rpm with just my but in the seat spinning a 19p stainless prop behind the carbed 5.0 if the wind was right.

 

[Leardriver]

Are you sure that your tach is reading correctly? I would verify speed with GPS. There are plenty of free speed indicating smart phone apps, in case you don't want to take an actual GPS on the boat. You should be turning at least 6-700 more RPM with a 17 pitch versus a 21.
If you look at a horsepower curve for that engine, it makes 40 more horse at 4500 versus 4000, so you need to get it spinning to get your speed out of it.

 

[redmen62]

Pop off the flame arrester on the carb and with the engine off push the throttle to the wide open position and look down the carb to see if the secondaries are all the way open. Had the same problem with both the carbs on my twin 5.0 setup... weekend on the work bench and 2 rebuild carbs later the engines were getting what they needed and I picked up almost 1000RPM and almost 15mph