120 Omc Wot Rpm??

[retmech]

The boat: 1969 Bell Buoy 17 foot fibreglas
The drive: OMC Stringer(?) electric shift, with Mercruiser 120 HP Chevy 4 cyl, 2.5L(153 CID?), old enough that it has breaker point ignition, has MO712EAC stamped by the distributor.
The problem: The best I can get out of it at WOT is 2500 to 2600 RPM. Speed is unknown (no speedometer, and GPS had bad batteries). The boat seems to be almost on plane, but slow. It takes a LOOONG time to accelerate. The prop has 14X18 cast in the rear flange.
The points, rotor, and cap look good, haven't checked the plugs. engine starts easy, runs good with a hose hooked up for water (no load). The engine runs smooth under load, just can't get above 2600 RPM.
If it was an R-3350 with an Aero-Products or Ham-Standard prop, I would know what to do...
RETired MECHanic
PO1, USN, RET.

 

[whywhyzed]

Re: 120 Omc Wot Rpm??

get a 6 gallon gas can and try plumbing that to the fuel pump for a test run. could be a fuel delivery issue from the inboard tank. Other possibility is timing advance weights in the distributor all rusted and stuck. Should run near 4500 rpm at WOT under light load. 120HP is not a powerhouse in an 17 footer - 16P prop might be better than 18, but that's not your immediate problem.
(btw, the OMC pin on props are available still for cheap despite what people will tell you).
Could there be 50 gallons of water trapped under the floor? wet wood?
Definitely put new spark plugs in it too.

 

[retmech]

Re: 120 Omc Wot Rpm??

(btw, the OMC pin on props are available still for cheap despite what people will tell you).
Could there be 50 gallons of water trapped under the floor? wet wood?
Definitely put new spark plugs in it too.

Nope, no water under the floor, spark plugs are good.
WHERE are the OMC pin on props available for cheap? The local marina has ONE (14X11) that they loaned to a guy for a test, which means that it is now a used prop, if the guy doesn't buy it, I can have it for "only" $300!
There was one on eBay, but it had tried to eat a rock and the blades were ground down pretty good.
iboat doesn't have any, any pitch.
Where?

 

[whywhyzed]

Re: 120 Omc Wot Rpm??

propco - $130 + $15 shipping

 

[gangasingh1]

Re: 120 Omc Wot Rpm??

I'm having the same issue ... Please let me know what you figure out .. Only difference is I've changed Dist, cap, rotor, spark plugs, wires, oil, etc ...
Definately fresh gas ... I've been playing with the time and screws on carb .. I have it set to the best running so far .. but still RPM's are not getting past 3300 rpm

 

[gangasingh1]

Re: 120 Omc Wot Rpm??

Without any load (hooked to a hose on the trailer), the boat revs up no problem ...

 

[salmonpro]

Re: 120 Omc Wot Rpm??

it must be in the water as I too am experiencing this same issue please post what you find out I have to check my timing advance yet but I have done everything else ... Thanks for the great post guys.

 

[futz]

Re: 120 Omc Wot Rpm??

you go down the checklist located in the sticky at the top of the forum?

One of my favorites on the list is a compression check, on a warm motor.