Merc 150 rpm loss

[MattPark]

I've got a '95 Mercury 150 2 stroke on my boat. It's got just over 650 hours (I've had it since 350 hours or so). It's a carbureted 2.0. Until now, I haven't had a single issue with it.

I noticed a few trips back that it was only reaching 54-5500rpm at wot. Previously it would reach 5800rpm easily. 3 adults (same adults even), full tank of fuel. It's down a few MPH at wot as well, from 43-44 to 39-40. This is with the same undamaged 17" aluminum prop I usually run. It's a on 19' Formula Vee.

Fuel consumption (and economy) at any other RPM hasn't changed, but I never paid attention to WOT (I didn't like looking at that big number) so I'm not sure if it is using less fuel at WOT for whatever reason. Cylinder compression hasn't changed. Fuel filter is changed regularly (but I haven't touched the filter that is between the carbs and the fuel pump.) I will be rebuilding the fuel pump soon (I don't think it's ever been done).

My only thoughts are incorrect air to fuel ratio, or incorrect timing. Bad filters or pump would make it lean, but in my two stroke experience, lean is mean (it makes more power) but that puts the engine closer to detonation and damage.

Next time out I plan on hitting the enrichener at WOT to see what kind of RPM change that makes. It's my understanding that the enrichener dumps extra fuel into each cylinder from the top carb bowl. My guess is if it's too lean, RPM will rise.

I don't have a manual for the thing (never had to work on it), so I'm not familiar with adjusting the timing, but I will be doing that along with removing the idle stabilizer as well.

Any thoughts?

[Just-in-Time]

if you need a manual, email me i have them all

ok you could try pumping the primer bulb, if it picks up u have bad fuel pumps or a fuel line restriction

yes the primer richens the carbs

replace that filter yearly

timing does not change on an outboard unless you played around with it

[MattPark]

I've got a factory service manual, but thanks for the offer.

I'll rebuild the pump before I take it out next, along with the little inline filter, and if nothing changes I'll try priming the bulb. The bulb is only 6 months old or so.

I haven't played with anything on the engine.

[Just-in-Time]

i like to replace the pumps
cheap

[Seahare]

Mercury had to change their inlet needles to a type that's impervious to the fuels containing alcohol. The reason why is that the older type needle/seats swelled from exposure to alcohol and as a result starved the carbs. I'm not sure if you're model has the upgraded needle/seats but you can check the serial number against Mercs service bulletins.

Also, pull all the plugs and run a compression/spark check to see if they're all firing and the cyls. are even.

[MattPark]

Like I said in my original post, compression is perfect and hasn't changed since I've owned it. It's within 2psi on each cylinder. I'll call that acceptible:D. I just don't want a lean condition changing that scenario. A 4 stroke Suzuki 175 sure sounds nice, though.

If spark was an issue I would assume that it would at least have some effect at cruise (4200-4600rpm), where fuel efficiency hasn't changed one bit. It's always got 3.5-3.8 mpg depending on load.

I'll check the carb update. It doesn't seem like any one cylinder is running out of fuel, like a dry bowl would have, but just an across the board drop in power. I suppose with 6 cylinders, one here and there dropping out could be hard to pinpoint.

Before I run it next it will have the fuel pump rebuilt, and both filters changed. With any luck that will be tomorrow.

[MattPark]

I thought for sure I checked to see if it was getting full throttle. It wasn't. Maybe 60-70%. I feel dumb now:redface:

I adjusted the cable to the point where it starts raising the idle up, and it still won't reach 100% throttle at the carbs. It's a good improvement, but not good enough. I opened up the control box and didn't see any adjustment in there.

I'm thinking now that the cable is stretching or just worn out. There aren't any numbers on it that I can see. Anyone replaced one of these?