1980 OMC 140HP Max RPM

[George05]

I have a 1980 OMC 3.0 Chevy that is rated at 140HP. Can anyone tell me what the recommended max rpm was on this engine? Engine runs pretty smooth until you reach the top end then kind of just stalls out at about 3,800 rpm and 29 MPH. It has a 13 3/4" x 15P prop on it. Seems like it should turn more RPM then that.

 

[Haut Medoc]

Re: 1980 OMC 140HP Max RPM

I'm not an OMC guy, but I would think 4200 would be more like it.....
Someone who is more familiar with your set-up should have more input....
Welcome to iboats, btw......​

 

[whywhyzed]

Re: 1980 OMC 140HP Max RPM

4200-4600 recommended WOT.

What size boat?

I wouldn't go down in pitch though, unless it's pushing a huge boat, I would troubleshoot for lack of power after verifying that your tachometer is correct.

Compression test would be a good start. Then tune up and fuel system evaluation.

Check the boat for waterlogged hull.

 

[George05]

Re: 1980 OMC 140HP Max RPM

It is just a 17' boat. I have done a compression test & cylinders all checked out ok. It is old enough that it has a points type distributor but I have put in new points, condensor, rotor, cap & coil. I have also gone competely through the fuel system. Even plumbed a fuel pressure gauge into the line so I could see if I was holding the proper fuel pressure. Everything checks out ok.

Is it possible that the valve springs are going bad and floating the valves?

 

[QC]

Re: 1980 OMC 140HP Max RPM

There's more to it though. Could be boat weight (see waterlog note above). Could be improper use of trim, weight balance, could be bad tach reading too, but it does sound slow. How are you getting your speed numbers? Speed calc says you're getting slip number of 8% which tells me you're may not even be going 29 . . .

 

[whywhyzed]

Re: 1980 OMC 140HP Max RPM

Is it possible that the valve springs are going bad and floating the valves?

Never really seen that....

time to weight the boat maybe ... do you trailer it?

but even 40 gallons of water soaked into the foam under the floor is only 400 pounds (Like having Two big adults along for the ride)

29 is slow....

39 would be more like it

Are you sure it's a 3.0 and hasn't been swapped to a 2.5 along the way?

Look at the front crank pulley - pulley only = 2.5
vibration damper and pulley = 3.0

 

[George05]

Re: 1980 OMC 140HP Max RPM

Will check the pulley to make sure it is not a 2.5.

Boat is on trailer so I can check the weight pretty easily. I live in a rural agricultural area so it is only 1/2 mile to an elevator with scales I can run across. It doesnt' take any water on when in the lake, usually only a few gallons come out when we pull the plug. Also the boat is kept inside garage except when on the water. Pretty sure this is not the problem but I will check anything at this point.

Someone else, not on this forum suggested maybe the points were bouncing at the upper rpm. May need to consider trying to find an HEI distributor for it.

At any rate thanks for the sugestions, keep them coming because I am at a loss at this point.

 

[George05]

Re: 1980 OMC 140HP Max RPM

Forgot to ask, anyone have any idea about what a 17' v-hull boat should weigh?

 

[john crowe]

Re: 1980 OMC 140HP Max RPM

my 3.0 max is 32 mph so far.

per mfg my boat weight is 1925lbs.....kinda heavy boat i guess for a 18 footer

 

[whywhyzed]

Re: 1980 OMC 140HP Max RPM

My 3.0 powered 21.5' Bayliner capri Cuddy went 34 mph with fumes in the tank and me on a diet for a week.
It did 32 any day... had to weigh 2800 or so....

A few gallons coming out when pulling the plug?

Nothing should come out.

boat speed calculator to play with :

http://www.ultimate.com/omc-boats/speed.html

 

[George05]

Re: 1980 OMC 140HP Max RPM

A few gallons come out because my two sons have been getting in and out of the boat either from tubing or skiing all afternoon. If we just fish there is no water in the boat.

 

[whywhyzed]

Re: 1980 OMC 140HP Max RPM

I'm liking the idea about the distributor as a possible culprit....better than valve springs and easier to fiddle with...I don't think I'd worry about HEI but I might try another distributor.