1992 OMC Cobra 5.8 Won't plane out at WOT.

[CapVic]

Hey y'all, I have a 1992 225 Sundowner 5.8 L engine that has had the out drive changed(again). ( it was supposedly newly rebuilt) I also had the carburetor rebuilt and a new alternator placed on it. And when the boat is on the muffs the engine hits high RPM's and appears to be running as it should. But when it's placed in the water she won't plane out, even when the throttle is pushed completely forward it only hits about 2000 RPM's. We've tried swapping out to different sized props, but that has no effect. I am hoping someone on here can help.point me in the right direction. Thanks in advance!

 

[Bondo]

Ayuh,..... Welcome Aboard,...... How old is the fuel,..??

Has the contents of the fuel filter been examined for anything but clean fresh gasoline,..??

 

[CapVic]

Hi Bondo, yes. I added 30 gallons of premium gas to test her out. And no I did not check the fuel filter yet. I'll take a look at it and update you.

 

[Bondo]

I added

Ayuh,.... I'd be much more concerned about what was left in the tank,.......

It's ashame to load a freshly rebuilt carb with the same crud that caused the need for a rebuild in the 1st place,.....

 

[Scott Danforth]

running on muffs, you could be running on 3 of the 5 cylinders and hit 5000 RPM on bad gas. under load, you need all 8 cylinders firing on good gas.

 

[CapVic]

scott i am a novice when it comes to engine work. But I'm wondering if there is a way for me to identify if all the cylinders are firing or if I need her back to the mechanic (again) lol.

 

[CapVic]

@ bondo, I just talked to my father and he did indeed drain all the gas out of it before I added fresh gas.

 

[Bondo]

@ bondo, I just talked to my father and he did indeed drain all the gas out of it before I added fresh gas.

Ayuh,.... Even so, 1st place I'd look is at the contents of the fuel filter, for anything but clean fresh gasoline,.....

 

[Scott Danforth]

scott i am a novice when it comes to engine work. But I'm wondering if there is a way for me to identify if all the cylinders are firing or if I need her back to the mechanic (again) lol.

At idle, while in the water, with a good shop tach, pull one wire at a time. RPMs should drop about 30-50 RPM as you unplug a cylinder.

Caution, use plug wire pliers

 

[Jammer864]

I had something similar on my 94 Four Winns with a 5.8. Make sure there is nothing obstructing the exhaust where it passes thru the transom. I had a flapper valve come off and almost completely block the exhaust pipe at the bottom of the Y, where it bolts to the transom bracket. I could not see it from above but had to remove the drive and exhaust boot and look straight into the exhaust port at the transom. The boat revved fine on muffs, but couldn't get more than 2000RPM out on the water.

 

[Lou C]

Another subtle cause is a clogged or rusted anti siphon valve on the gas tank. If this is bad it will restrict fuel flow under load and cause similar symptoms.

 

[CapVic]

jammer, today I was able to take off the top of the exhaust, and was able to see she the flappers should have been. Neither of the flappers were there. Completely gone. So I think they may be down farther than where I can see. Taking off the out drive and looking around is a little more in depth than my experience can tackle. Lol
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