1985 Mercury 90 - Very sluggish

[patrickintx]

I just bought my first boat this week, so please excuse the ignorance.

After looking through the boards all night, I have not found anything close enough to my problem that makes me jump and say "oh that's it!"

Sooo, the question I have is this, when I give throttle in neutral, I can get the RPM's to about 4000. When I have the prop engaged, the RPM's never go above 2500, and it seems to be going a whopping 10 -15 MPH. The boat is a 1985 sun tracker 24' pontoon boat. I'd imagine that it's relatively light and should be going much faster on a 90 horsepower engine. Am I correct in this assumption, and would checking the prop pitch be a step in the right direction?

 

[WillyBWright]

Re: 1985 Mercury 90 - Very sluggish

The first step is to ask the previous owner what's up. It's far more likely that you have a powerhead problem than a propeller problem. I'd guess the boat would be fitted with a 15p prop.

 

[Silvertip]

Re: 1985 Mercury 90 - Very sluggish

Stop reving the engine so high in neutral. If proves nothing and you could experience engine runaway and engine damage. An engine running on half its cylinders will rev up freely. You need to do all the basic checks such as spark check on all cylinders, compression check, fuel system check (blockages, kinked lines, air leaks, filter check/change) and carbs rebuilt. Do the easy and inexpensive things first. If you plan to work on this engine yourself you are advised to obtain a service manual specific to that engine.

 

[Don S]

Re: 1985 Mercury 90 - Very sluggish

And don't start pulling the carbs and rebuilding them before you do the rest of the basic test.
1. Check for spark on ALL cylinders
2. Pull the plugs and examine them, There appearance can tell you a lot about how it's running.
3. Do a compression test.

Once you have that info and all is good, then move on to your fuel system and other checks.

As mentioned, without the manual, you are just blindly tinkering with things, not knowing why for anything.

 

[patrickintx]

Re: 1985 Mercury 90 - Very sluggish

Thanks for the advice! I have ordered a service manual and will start from there. I will be doing part of this myself, but something like re-building carbs will be done at the shop.

As far as the previous owner, he said "it was just always like that, I think it comes limited from the factory that way." I'll take that with a grain of salt.

-Patrick

 

[jimmbo]

Re: 1985 Mercury 90 - Very sluggish

A 90hp inline six when it is running on all cylinders can make a light boat literally leap on to plane. It actually pulls harder below 4000 rpm than the inline 6 115s and 140s. As said before, check to see if it running on all cylinders. My 115 lost a cylinder due to a bad switchbox and it took forever to plane the boat and was severely short on the top rpm. What prop is on there now? rpm range for that motor as stated by merc is 4500 - 5000, although some people, for whatever reason, like to prop it to run higher. It is possible that the main jet of one carb is plugged, that effectivly kills two cylinders.

 

[castaway68]

Re: 1985 Mercury 90 - Very sluggish

first thing i would do is a compression check,,after that i would stick it in the water,,dont even have to take it off the trailer,,fire it up give it some gas till u notice the bogging,,then hit the choke,,if it picks up u have a fuel problem if it dies its in the ignition system somewhere,,this is a crude test but it can point you in the right direction MOST of the time,,its not full proff but its a good starting point.

 

[emckelvy]

Re: 1985 Mercury 90 - Very sluggish

It's also a symptom of a bad stator so when you get to that point, a resistance check of the hi & low speed stator windings should be done to rule out any problems in that area.

Here's some links with good general ign info and also some specific guidance on troubleshooting a Merc ADI (Alternator Driven Ignition):

http://www.outboardparts.com/mercury/troubleshooting/printguide.htm

http://outboardparts.com/mercury/troubleshooting/troubleshooting10.htm

http://www.marineparts.com/partspages/Outboard/trouble2.htm

http://www.maxrules.com/fixsparktips.html

HTH & G'luck...........ed

 

[patrickintx]

Re: 1985 Mercury 90 - Very sluggish

Hi all, just wanted to thank everyone for the great advice. Here's what I've done:

1) compression check, all good
2) spark test, 1,4,5,6 good. 2,3 no spark
2a) replaced both switch boxes (expensive little suckers)
3) changed impeller
4) new fuel filter
5) replaced fried rectifier

On a 24' pontoon boat I can hit about 27mph (with a light load) and WOT is at 4300 RPM's

Thank you everyone for the great info!! My first boat is surely a great experience.

 

[jimmbo]

Re: 1985 Mercury 90 - Very sluggish

If the motor is running right you have too much prop. Rpm range for the 90hp inline six @WOT 4500 - 5000. prop it to turn 5000