OMC 120 HP is a slug

[The Car Doctor]

My dad bought this boat about 8 years ago. It is a 17' Sea Swirl with the 4 cyl. 120 hp OMC I/O. My dad recently gave me this boat (he upgraded) and I'm wanting to take my family out in it and do some tubing, skiing ext. (we have previously mostly just fished with it) The problem is it take forever to get out of the water and the WOT is about 3800 RPM. Needs a prop with less pitch, right? Well here is the kicker, it has a 14X11 prop on it!

I guess I'm looking for ideas. The engine really runs well. Doesn't missfire, smoke, or backfire. It just feels like driving a car stuck in 3rd gear. It has a manual trim adjustment on the front motor mount which had been set at the higher end of the adjustment (front of engine is high). I'm not sure how much affect lowering this would have on anything?

Oh, and does anyone know if this is a 2.5L or the 3.0L?

thanks!

 

[futz]

Re: OMC 120 HP is a slug

what year? 2.5 L.

When was the last tune up done? Got water in the hull maybe.

With a 14 x 11 should be turning at above 4200 rpm for sure.

 

[The Car Doctor]

Re: OMC 120 HP is a slug

It's a 1981. A few years ago we did a complete tune-up(plugs, points, filters, timing) and even replaced the carb trying to chase down a different problem.

I pull out the drain plug after each time I use it. Is there somewhere water can get trapped and not drain?

 

[Daviet]

Re: OMC 120 HP is a slug

You said the the trim has the front of the motor up. Check the cavation plate on the outdrive and see if it is parallel to the keel. Maybe the bow is trimmed down to far. Just thinkin.

 

[Howard Sterndrive]

Re: OMC 120 HP is a slug

Is there somewhere water can get trapped and not drain?

ouch, yeah.

All the floatation foam under the floor. It can hold a few hundred pounds of water.

I'd clean the flame arrester and look at the choke heat tube and make sure the choke is opening all the way.
I'd look at the anti-siphon valve at the fuel tank.
Then I'd look at the advance weights/springs in the distributor.

I'd compression test the engine and double check things like firing order of spark plug wires and timing and dwell.

 

[Gary H NC]

Re: OMC 120 HP is a slug

Sounds like a tired old motor and the trim is staying in the "in" position..
All that equals a slow boat...
A compression test would be my first test too...

 

[cr2k]

Re: OMC 120 HP is a slug

The 120 was not a 3.0 that is the 140.

 

[janpetemar]

Re: OMC 120 HP is a slug

It's a 1981. A few years ago we did a complete tune-up(plugs, points, filters, timing) and even replaced the carb trying to chase down a different problem.

I pull out the drain plug after each time I use it. Is there somewhere water can get trapped and not drain?

The GMC 4 cyl is a 120 hp, 153 CID and 2,5 liter. AC Plug MR43T with a gap of .035. Point gap is .019 with dwell of 31-34 degrees. Timing 4 degreesBTDC witha WOT RPM of 4200-4600. Same engine I have in mine.

The more you set the angle of the drive further outward, the thrust pushes down on the stern and brings the bow upwards, making it come up to plane quicker.

Make sure that when the boat is on the trailer that the bow is raised higher than the stern. With my landing gear fully extended I had to build a six inxh riser to put under the wheel to raise it high enough. Hope this helps.......

 

[KRH1326]

Re: OMC 120 HP is a slug

When I idle out of the harbor, I need the trim all the way in (front of engine up) because my boat like to wallow around a little otherwise.

When I clear the harbor, I trim out alot( engine front down), which , by trial and error, have found to help put the bow up and on plane.

Then trim back in to find comfy running attitude.

Hope that helps...

 

[Bottom Fish]

Re: OMC 120 HP is a slug

I have a 1982 19' Sea Swirl with the 4 cyl. 120 hp OMC I/O. I have a 13 3/4X15 prop it turns 4400 rpm. No power trim. Comes out of the water great with 2 people a little slow with 4. I would check the Tach and try more prop.

 

[searay3]

Re: OMC 120 HP is a slug

a couple of years ago is a long time for a tune up. I do it every season.