2.5L Mercruiser Lack of Power

[MattyD7]

I just got this boat so have no prior experience with it to compare to, just what I've been told. Also, I know a lot about working on and fixing OMC outboards, but this is my first I/O so I don't know where to check for everything that could be causing this. Hoping you guys can help me out.

The boat is a 1990 Thompson Cutlass 170, 17' bowrider, with a 2.5L Mercruiser hooked to a Alpha outdrive. Only 178 hours on the motor, everything looks really good.

Took it out yesterday to try it out. Had 6 people in the boat, plus some gear. Rated for 9 people, and I don't think we were all that excessively heavily loaded. Starts up real easy. Moved along nicely at <10mph to get out into the harbor. Once I got into the harbor I dropped the throttle to go full speed and it got up to about 3100 RPM and wouldn't get on plane. From about 50% to 100% throttle, it made no change in RPM.

I had one of the passengers move up front so there were 3 in the back and 3 in the bow. That helped and it slowly was able to get on plane. Once it got on plane, the RPMs moved up pretty quickly to probably about 4200 RPM and we were cruising about 25mph. This seems remarkably slow to me for this engine and boat combo. My other boat is similar size with a 90HP and with the same load (6 people and same gear) I got 38mph and have no problems getting on plane quickly. I realize this motor weighs a bit more, but come on, that is terrible. I had a 50hp outboard on a bowrider and this was about the performance I would get with 6 people, somewhere around 25mph top speed.

Throughout the day, it was similar. I would have to beg and wait to get it up on plane, then I could get it up to about 25mph. I tried launching with the trim in all different angles to see if anything helped.

Coming back into the harbor we were fighting direct against a 20mph wind and nothing I could do could get us on plane. I was stuck at 3100 RPM and about 15mph. After about 10min of fighting it, the wind died down suddenly and it surged forward as it got on plane.

The prop is a 14.5x19. The previous owner used this same prop for years, so I don't think it is the prop. He said that it would typically max out around 45mph with his family in it, so something really seems wrong.

My first thought is old gas. He filled it up at the end of last summer, so the gas is pretty close to a year old. I burned about half during the day and refilled and it didn't seem to make much difference, but I know that when there's that much bad gas it isn't diluting very quickly. Do you guys think this is the most likely culprit? I will try to drain it all tonight and refill and see if that helps, but since I am new to I/O engines, I am wondering what other possibilities it could be to give me this problem that I'm not thinking of?

It really feels like a lack of power, like I said, it reminds me of when I had a 50hp with a heavy load of people onboard. Once it gets on plane, it accelerates more, but the top speed and RPM doesn't get to where I'd expect.

I checked the throttle cables and linkages and they all seem to be working perfectly. Any tiny change at the controls makes the throttle cable move, and the linkages on the carb. Previous owner said he replaced plugs and points 2 years ago.

Tonight after I drain the gas, I'd like to check as many things as I can, so give me some ideas on what to look at. Thanks!!!

 

[oldjeep]

Re: 2.5L Mercruiser Lack of Power

17ft boat with the smallest I/O you can get (and 23 years of wear on it), combined with 6 people (900 lbs?) and their gear? Doesn't surprise me a lot that it has trouble planning. Doesn't sound like a fuel issue since you are not describing any missing or stalling.

Mechanically the only thing I would check is to make sure that the spark arrestor is clean and then take it out for a test with just 1 or 2 in the boat.

 

[Howard Sterndrive]

Re: 2.5L Mercruiser Lack of Power

Could just need a tune up, but regardless of what the placard says, you can't safely put 9 people in a 17 foot i/o under the calmest of conditions. (unless it's an inflatable) That's scary that it says that.
start with a compression test and check for water on the plugs.

1989 was the last year of the 2.5 just for fyi if you go to buy parts, might be an 89 driveline in a 90 hull.

 

[MattyD7]

Re: 2.5L Mercruiser Lack of Power

Thanks for the idea. I will take off the spark arrestor and clean it up tonight.

I know that it is a small motor for the boat and load, but like I said, my other boat with just a 90hp (even older) motor acts 100x better, something just doesn't feel right here. Especially since the previous owner seemed to hit much higher speeds.

I am planning to go out tomorrow either alone or with one other person because I'd like to see how it acts with a much lighter load. I think that will tell me a lot. If its still struggling to plane with 1-2 people, I've got to imagine something else is up. But if it jumps to life, then yeah maybe it was too heavy and too windy the other day.

 

[MattyD7]

Re: 2.5L Mercruiser Lack of Power

Could just need a tune up, but regardless of what the placard says, you can't safely put 9 people in a 17 foot i/o under the calmest of conditions. (unless it's an inflatable) That's scary that it says that.
start with a compression test and check for water on the plugs.

1989 was the last year of the 2.5 just for fyi if you go to buy parts, might be an 89 driveline in a 90 hull.

Thanks for the reply. Yeah I don't think I'd ever do more than 6 anyways. I know the max people is just calculated by length and width of the boat, not necessarily by motor, etc.

I'll do a compression test and look at the plugs too. I was thinking about doing that already, and since you mention it, I've been convinced to go ahead and do that.

 

[MattyD7]

Re: 2.5L Mercruiser Lack of Power

As an update, I checked the flame arrestor. It was pretty clean, but I washed it until it was spotless, just in case that helped. Bought new spark plugs and brought them to the lake with me.

Tried the boat on the lake with just me and another person. It was quicker, but still seems a little under what I expect. Still didn't plane all that fast or easy, but it did. Trimmed down, it would hit about 4100 RPM and 30mph. Trimmed up to the optimal angle, I would get 4500 RPM and 35mph. Since that's the WOT RPM I should expect, I guess this is as good as it'll get on this boat and I just need to temper my expectations like oldjeep said.

Did a compression check and all 4 cylinders came in right at about 100psi. That's the edge of acceptable I believe, so maybe that's why the power seems a little lower than I expected, losing it from lower than ideal compression.

I still plan to burn the fuel tank down to empty and start fresh to see if that helps any, but I'm not expecting much at this point. Thanks for the ideas from everyone.

 

[Bondo]

Re: 2.5L Mercruiser Lack of Power

As an update, I checked the flame arrestor. It was pretty clean, but I washed it until it was spotless, just in case that helped. Bought new spark plugs and brought them to the lake with me.

Tried the boat on the lake with just me and another person. It was quicker, but still seems a little under what I expect. Still didn't plane all that fast or easy, but it did. Trimmed down, it would hit about 4100 RPM and 30mph. Trimmed up to the optimal angle, I would get 4500 RPM and 35mph. Since that's the WOT RPM I should expect, I guess this is as good as it'll get on this boat and I just need to temper my expectations like oldjeep said.

Did a compression check and all 4 cylinders came in right at about 100psi. That's the edge of acceptable I believe, so maybe that's why the power seems a little lower than I expected, losing it from lower than ideal compression.

I still plan to burn the fuel tank down to empty and start fresh to see if that helps any, but I'm not expecting much at this point. Thanks for the ideas from everyone.

Ayuh,.... Welcome Aboard,... At 100 psi, that's a rebuildable core motor, 'n not really a Runner,...

Yer probably gettin' all there is to get outa it, at this point,...