2.5L lose power after plane and rattle sound

[faurot3]

I just had my the motor professionally tuned up so I'm pretty sure it's not timing ignition or any of those tune up things. It started acting up a little last year and I just dropped it in the water yesterday. When I throttle up she takes right off and at about 3000 or so rpm it starts a sort of rattle sound not really a tick and it gets louder And faster as my RPMs increase. It planes right off and I'm usually doing about 26 sog After a minute or two I start to loose speed and she settles down to about 21 sog and holds there with the rattle sound that I can't tell where it's coming from. Any ideas?

 

[jerryjerry05]

Re: 2.5L lose power after plane and rattle sound

You say a 2.5 leitre? I figure it's an OMC but could use a bit more info?
Year,model and any info you can help with?
Rattling?? Could be the lifters?Timing could be off?Could be spark knock?
Could be the exhaust flappers? Could be the gimbal bearing?Could be u-joints?
Could be the coil?Run it and reach in and touch it.Should be hot but not so hot you can't touch it.
Could be starving for fuel? Clogged vent line,clogged pickup tube,soft hoses from Ethanol?
You in Africa? They run Ethanol there??
Fill out the profile.Location? You might be close to someone who can help?

 

[faurot3]

Re: 2.5L lose power after plane and rattle sound

I'm pretty sure this is posted under the OMC forum so yes it's an OMC its a 1986 2.5L. Timing spark compression is all good new wires plugs cap rotor coil fuel filter. starts right away. It doesn't get hot. I'm not in Africa I don't run ethanol. Carborator was just rebuilt.

 

[Gangly]

Re: 2.5L lose power after plane and rattle sound

Rattling could be many things so lets start with the basics:

Do a compression test to check for leak down. You could have piston slap but not likely.
Get a feeler gauge and check your valve lash. The result from valves and rocker arms out of adjustment is a lot of racket with limited RPM's

Check your lifters. I dont know whether they are flat tappet, roller, hydraulic roller, or what, but make sure you dont have a worn or collapsed lifter. The result is a lot of racket with limited RPM's.

Check your fuel setup, you might be running waaaay too lean. The result is limited RPM and a lot of detonation which can sound like floating valves.

 

[Silvertip]

Re: 2.5L lose power after plane and rattle sound

What does the oil pressure gauge read when the rattling is going on? Low oil pressure says rod and/or main bearings. Have you checked oil level? You say temperature is ok but have you verified the temp gauge is accurate? An engine that loses power after running fine for a short period typically has an overheat condition due to cooling or ignition timing issues, or a problem in the outdrive that is causing things to "bind up".