1981 Mercury 80 hp outboard has no power

[RNDavid]

Hey everyone. I recently put a 1981 Mercury outboard, 80 hp, on my 18 foot pontoon, to get a little more towing power for pull behinds. With the ears on it, the outboard achieves high RPM's,(no work load) but in the water will not get over 2000 RPM's. It will bog down or shut down if the throttle lever is opened too quick or too far. In neutral, or out of gear, the RPM' s may reach 800. It sounds like it idols well. I attempted to remove one of the plug wires, while in idol, and was given a pretty good shock, If that may be any indication of its electrical health. I figure that even pushing a small pontoon, I should see a higher RPM range. Any and all help will be appreciated in diagnosing the problem, and getting up to speed. Thanks.

 

[racerone]

Did you just clamp it on and hope for the best ?-----Was any preparation work done ?-----Compression test done ?---New impeller ?----Fuel pump checked ?-----Carburetors cleaned ?

 

[Chris1956]

So if you rev it slowly and smoothly, you will see 2000RPM, and if you rev it fast, she will die? You may have two issues. Clean the carbs and link and synch that baby. Post back on improvements or more issues.

 

[RNDavid]

Thank you for the responses. The outboard was on an older boat that had seen its last days. The outboard started, and ran for the guy I bought it from. I placed it on the pontoon, got it running, but discovered more problems. Today I checked the firing of the spark plugs to find that the number four cylinder was getting no spark. I found a an almost disintegrated wire going to the positive side of the number four coil. I ran a new wire and all plugs sparked. I tested it on the lake, and managed to get almost 2900 RPM's. She ran well at 2000 RPM's, but when I pushed the throttle all the way forward, the outboard seemed to sputter and jump a little, but still did not reach her full potential. What's next? Does this carburetor have high speed and low speed jets? What is the best way to blow out or clean the jets without taking the crabs all apart? I don't want to risk messing things up more than they are right now. Is there anything else that I might check?

 

[jimmbo]

Welcome to iboats
Your engine develops its best potential between 5000 - 5500 rpm. So 2900 rpm is still a tad below it. First thing to do is a compression test. Engines can run out of the water quite well with a dead cylinder. Also a new impeller should also be very high on the list of things to do
The carbs have fixed high speed jets and adjustable low speed needles. The correct way to clean the carbs is to remove and disassemble them. That said, I have has success in the past by dumping a couple of aerosol cans of combustion chamber cleaner into a half full 5 gallon fuel tank, with extra oil mixed in. Running at various speeds dissolved the varnish and the carbs seemed to work well again. Your results may vary and could result in a damaged engine if fore mentioned procedure were to plug a fuel passage in one of the carbs and a lean condition developed.
Once you get the carbs clean, check to see if the timing is advancing properly and synced to the carb. After that I would spray combustion chamber cleaner into the carbs to do a decarb. Then you can start your mission to select the correct propeller for your boat

 

[RNDavid]

Thank you, Jimmbo. That was very well put and you gave me all the information I need for my next step. I am not looking forward to carb work, but I guess it must be done. I have a stainless prop on the boat right now. I will have to calculate the pitch.

 

[Chris1956]

A prop with a too great pitch will cause your symptoms. What boat did it come off of?

 

[RNDavid]

It was an old fiberglass walk through, tri-hull, Chris. All the wood in the boat was rotted and the transom was about to fall off. I was thinking that about the prop, but I should get a higher RPM with a higher pitch prop(speed prop) and a lower RPM with a low pitch prop( load prop). Or am I misunderstanding the concept?

 

[racerone]

You mis-understand the concept.-------When you go to a bigger / heavier boat you have to go down in prop pitch !------No different than a truck with a heavy load needing to shift down to go uphill.

 

[RNDavid]

My goal is to get my RPMs up to 5000. The prop may have to wait for now. I do appreciate the knowledge, however. Thank you.

 

[RNDavid]

What pitch of prop would you guys recommend for an 80 hp Mercury, on the back of an 18 foot pontoon??

 

[jimmbo]

Here's what mercury's prop calculator came up with

Black Max?	Mercalloy	3	13	19	RH	77346A45
SpitFire?	Mercalloy	4	12.5	19	RH	8M8026600
Vengeance?	Stainless Steel	3	13	18	RH	16988A46

I used a 1990 75hp Classic, which is a prop rated version of your engine, same displacement and gear ratio, on an 18ft pontoon. I myself think it is a bit optimistic on how high of a pitch to use...

You do have spark to all 4 cylinders? And, you never have told use what pitch you current prop is, or even what model of prop it is

 

[RNDavid]

I replaced a corroded wire coming into the positive coil pack on cylinder four. All four cylinders are now firing. I started simple and easy to clean the carbs with fuel additives and spray jet cleaner. She cold starts first turn of the key, now. I think that my RPMs are a little fast in idol, just with sound of the gears engaging. I don't think that my tachometer is working properly. With the carbs working better, I can slow it down a little. I have a stainless Prop that came with the outboard. I will have to take it off to calculate pitch. Meanwhile, I want to make another lake run this week, open her up, and see what I have got to work with next. Thank you Jimmbo

 

[RNDavid]

Okay, I got my prop off, and if the calculations are correct, from what I have found on line, my pitch calculated out to 16.5, margin of error, I will say 17 inches. According to the Mercury guide, that should be roughly correct. I could not find my engine on the Mercury calculator, so I used what i thought was close. I am still not sure. Needing input please.

 

[jimmbo]

With the mercury calculator choose 2000 for year, then 75 classic. When I posted earlier, I said 1990. Damn I hate when I'm wwrrroooo uhm, mistaken

 

[RNDavid]

Yes sir, Jimmbo, I did that for both the 75 and 90 hp. To increase push and rpm, the lower of the two seemed to be the choice. Every two inches of pitch can effect RPM by up to 400. That heavy steel prop that I have now has a 17 in pitch.

 

[CharlieB]

Before changing the prop you need to make sure the motor is running properly, or you will be chasing your tail needlessly.

You say you now have spark on all cyls. That's a good start. Now you need to ensure that you have ADEQUATE spark. The voltage needs to be enough to arc across at least a 7/16 inch gap with a hot blue arc, any less and the rpm is limited by the ignition intensity.

The ADI Ignition system has TWO sets of ignition windings, a low speed set and a high speed set. A failure in the high speed set limits spark and the motor will not rpm. The stator voltage output needs to be tested and verified using a DVA meter or a DVA Adaptor with any Volt meter. Complete instructions are posted elsewhere in the forums as are direction and parts list to build your own DVA Adaptor.

Dirty carbs, weak fuel pump can also limit rpm and power.

ALL systems must be verified or the motor will not perform as expected. A weak or poor fuel system can cause damage running lean so it is critical that all systems be verified prior to attempting full throttle on the water.

 

[RNDavid]

Okay, my latest lake test... the outboard idols much better with the crabs chemically cleaned. On the low end, the outboard runs well. On the high end, full throttle, I am still not getting the RPM's that the outboard is capable of. It is like the outboard wants to take off, but is being helped back, sluggish , for some reason. If it is the high end ignition, would replacing the stator, fix the problem? And how hard would it be to replace?