Mercury 1983 model 115hp RUNNING ROUGH

[bexi]

Hey guys,
I bought a boat a few months ago with an old 115hp and one cylinder only had 50 psi so i took that motor off and bought a 1983 model mercury 115hp and bolted it on.
I bought it through a mate of a mate who checked it all and said everything was sweet with good compression.

The 1983 model engine i have bought is in beautiful condition and looks to be extremely well looked after. I had it laying on it's side on the ground for a couple of weeks before i had a chance to fit it to the hull.

Now that the engine is on, it's running rough.
I'm really confused about what this prblem could be.
I am a motorbike mechanic by trade so i know the basics... suck bang blow!
I have checked spark which is good on all 6 cylinders but i did replace all spark plugs just in case which made no difference.
I am suspicious with cylinder number 2 as after running the motor for 10 minutes i pulled the plug out and it was as black as anything. dry sooty black. but very black.
cylinder number 1,3,4,5,6 were all nice and cleana nd a little bit wet.
I pulled the top carby off which i presume feeds cylnder 1 and 2 and removed all jets and made sure there were no blockages (keeping in mind that cylinder 1 is running well). There were no blackages in the carby.
It looks to me as though these engines have a common manifold that feeds all cylinders but not sure.
The only thing i can think of that might be a problem would be the intake reeds on cylinder no. 2???
Does this sound like a possibilty.
The other interesting thing is that when i remove no.2 spark plug the engine runs no different is not better.

My next step was to put a fuel pump kit in just to eliminate that as a problem too.
I'm not entirely sure that no.2 cylinder is the issue here... maybe it's timing or something... how do you adjust timing on these electronic ignition engines? Could i have bumped it in transport or storage?

The only other thing i noticed funny is that when i push the key in (which operates the choke) whike the engine is running, the engine stalls instantly

Any help would be greatly appreciated as i'm stressing out after forking out all this extra money and still not having a boat to use and now winter is on it's way here in australia!

thanks guys
bexi

 

[Droll]

Re: Mercury 1983 model 115hp RUNNING ROUGH

Clams Canino wrote a nice howto here .

The only thing i can think of that might be a problem would be the intake reeds on cylinder no. 2???
Does this sound like a possibilty.

Not likely, but ...

Have you checked ALL spark plugs for good spark ? ( use a timing light ).

• Take a comp test ( just to be sure )
• Verify good spark on ALL plugs ( your engine got multiple ways to "lose" a cylinder ( coils, wires, switch boxes ))
• Inspect the piston tops ( tru plug holes ), look for signs of water ( clean spots as in steam cleaning )
• Do a link/sync job ( see link above )

If nothing helps...you might try to check out this service bulletin ( 74-6 )( PDF file ), a missing blead restrictor will give the engine bad idle .



Arne Kjetil

 

[bexi]

Re: Mercury 1983 model 115hp RUNNING ROUGH

I had a good look at the link/sync post.
Awesome info for when the motor is running properly but my motor is not running right on all cylinders so is it worth going through all that?
here's some pics too of the engine. the picture with my finger in it is pointing to an "un-connected" wire?
looks like it's been snipped

 

[Chris1956]

Re: Mercury 1983 model 115hp RUNNING ROUGH

Bexi, The top carb feeds cylinders 1 and 2. The intake manifold and crankcases are seperated fom each other, although there is a small hose that pushes excess oil from cylinder 6 to cylinder 1 to lubricate the main bearing.

I would suspect that the float in carb 1 is set too high, or defective and slightly flooding cylinder 2. Put a timing light on # 2 spark plug wire to assure it is firing, as well. It is unlikely the reed valves for #2 are bad, and this only really affects idle. High speed is usually unaffected.

 

[jimg984]

Re: Mercury 1983 model 115hp RUNNING ROUGH

mate.,, i hope didn't go the pub too m,uch,,,, and put the wrong engine on da boat J/K

 

[jimmbo]

Re: Mercury 1983 model 115hp RUNNING ROUGH

As for the snipped wire. It most likely is/was connected to the idle stalibizer. If so, nothing to worry about.

 

[CharlieB]

Re: Mercury 1983 model 115hp RUNNING ROUGH

Each carb feeds two cyl's, so that's probably NOT the problem.

You really need to check spark quality on ALL cyl's, you may have a bad plug wire or weak coil, possibly even a switchbox that's weak on one coil feed.

Using an adjustable gap spark testor set to 7/16 inch test all cyl's. If #2 is not hot sharp blue crackle try switching #2 Coil with an other cyl and retest.

 

[bexi]

Re: Mercury 1983 model 115hp RUNNING ROUGH

one thing i havn't done is check the reisistances of plug leads so i'll definitely check that this weekend.
When you say that a possibility is a crook coil, on these engines, there's only 2 coils right?
one for the top 3 cylinders and one for the bottom 3?
the reason i ask is that my top one is newer (replaced at some stage) than the bottom one.
As far as the spark plug gap tester... any other way of testing without buying that tool?

thanks so much for all the help so far.

 

[AMD Rules]

Re: Mercury 1983 model 115hp RUNNING ROUGH

Hey Bexi.... six coils (one per plug). There is two powerpacks (switchboxes). Did you check for the presence of the bleed restrictors yet? Even if you don't perform the whole lync and sync right now, you might want to check primary and secondary timing just to be sure.


>>>>>>

 

[bexi]

Re: Mercury 1983 model 115hp RUNNING ROUGH

no i havn't checked for bleed restrictors... from what i gather, i have to pull the motor apart to check them?
the boat is at my mates house so i havn't checked anything yet as i can only get out there on weekends. so this weekend i've got a few more things to check.
does anyone have the specs for primary and secondary resistances for ignition coils?

 

[Chris1956]

Re: Mercury 1983 model 115hp RUNNING ROUGH

The Bleed restrictors are under the transfer covers. I do not think they are your issue as they have a small effect on idle smoothness. If you can adjust the top carb idle mixture and float operation, I think you will fix your problem.

 

[bexi]

Re: Mercury 1983 model 115hp RUNNING ROUGH

hi everyone.

Finally got a chance to drive out to the river and spend some more time trying to fix the motor.

Here's what i've checked today.

* Took timing light out and whilst engine running at about 1500 rpm (i don't have a tacho but about that) put the timing light over each plug lead and all six were firing the same - firing well.

* Checked resistance of all ignition leads. they were all the same - around 0.2 to 0.4 ohms.

* I then got some carby cleaner and while the engine was running at around the 1500rpm, sprayed little squirts at a time in each carby. when i sprayed in the top carby, the motor ran slightly better but still not right on 6 cylinders. when i sprayed a little squirt in the middle carby, the engine stalled immediately. The same thing happened when i sprayed in the bottom carby.

* I then thought maybe the top carby had a problem (even though i've already had it off and cleaned it all out) so i pulled the top carby off my old 77 115hp merc that was originally on the boat and put that carb on this motor. if anything, the motor was coughing more and running worse. I then put the original top carby back on as this test had indicated that i don't have a problem with my top carby

* When i used to work on Harley's, a common problem was manifold gaskets wearing and the engine would suck air in through there. I thought this'd be another easy thing to check so i got my carby cleaner again and sprayed it around where each carby bolts on to the head or whatever you call it. Strange thing here was that the engine stalled again when i sprayed around the mounting area of the middle carb and bottom carb but not when spraying around the top carb. To me, this indicates that that carby cleaner was sucked in and stalled the engine on no.2 and 3 carby?

* The next thing I did was to try to check exactly which cylinders were the problem so i individually took off one spark plug lead at a time.
if i removed the top or 2nd from the top spark plug lead, the motor would continue to run but if i removed any of the bottom 4, the engine would stall. This has to indicate that the problem here is definitely with cylinder 1 and or 2!

* The last thing i did was remove all spark plugs again to check their condition. (bearing in mind these plugs are new. All plugs were clean and wet apart from cylinder no.2 (2nd from the top) which was dry and black!???
i have tried to attach a pic of no.2 plug and another one to show what the rest of them looked like but the forum says they're 30 kb too big!?
can anyone help me here?

Hopefully i can get to the bottom of this soon coz it's really driving me in sane spending so much time and making no progress.

Thanks guys

Bexi

 

[trussmonkey]

Re: Mercury 1983 model 115hp RUNNING ROUGH

hey bexi,
Did you check your compression? (if so, what was it?)
How old is the fuel you are running? (probably fresh, but, gotta ask)
get some sea foam spray into #2 to get that carbon cleaned out
you might want to pull all the carbs and clean them, cleaner is better

 

[AMD Rules]

Re: Mercury 1983 model 115hp RUNNING ROUGH

Bexi.... you can email me the pics...I'll post them for you.

I'd agree that is appears you might want to focus on #1/2 cylinders based on you tests, but it is odd that your spray test on the the gasket area affected #3 onward. Perhaps the fumes were simply being drawn into the carb throats?

Rather than spraying something into the carbs, try covering them up with a putty knife instead to check for response.

When adjusting the low speed jet on the upper carb, does the motor respond?

Has the fuel pump been recently rebuilt, fiitings all tight, and is the routing of the fuel lines to the carb as per original?

 

[CharlieB]

Re: Mercury 1983 model 115hp RUNNING ROUGH

Trim the motor up just a little past even and squeeze the primer bulb, HARD, does the upper carb flood?
Even a slight amount of flooding while running will seriously richen the 2nd cyl

 

[bexi]

Re: Mercury 1983 model 115hp RUNNING ROUGH

Charlie B - i have done this test with the motor running and all the way trimmed down and it made no difference to the running of the motor? do you mean i need the motor positioned between all the way trimmed down and up?


Hi thanks AMD, will email you the pics. can i get your email address?
When adjusting the mixture screws, it does affect the way the motor runs yes.
The fuel pump has just been overhauled with a gasket kit yes.
the fuel lines all look like original routing yes.
how do we explain the fact that the motor won't start if i use the choke and if i activate the choke while the engine is running, the motor instantly stalls?
this is an electronic choke - looks like a little solonoid thing that allows extra fuell to flow through to one of the carbs.

trussmonkey - The compression has apparantly been checked (not by me) and is supposed to be good on all cylinders.
fuel has been drained and fresh fuel in.
this sea foam spray - how do i use/apply it?
As said previously in this post, i have cleaned the top carby which is the carby feeding my problem 2 cylinders

 

[trussmonkey]

Re: Mercury 1983 model 115hp RUNNING ROUGH

Did you clean ALL your carbs? (disassembled, cleaned inside and out, then reassembled and installed with new gaskets)
If so, swap your carbs around, top with middle,etc...
if the problem doesn't change cylinders then it's probably not fuel related

I'd look at the ignition, if you've still got the other 115 HP maybe try swapping coils. how's your spark on #2 ?

you really need to get some actual numbers on your compression,

worry about the sea foam later, just follow the directions on the can,
your local auto parts store should carry it.

 

[bexi]

Re: Mercury 1983 model 115hp RUNNING ROUGH

hey good point about swapping carbs although i did swap a carb off my old motor (which ran well) and it made no difference.
i havn't cleaned all the carbs no. i've only cleaned the top carb (the carb that feeds the problem area)
i think i will remove them all and kit them all to eliminate the possibilty of a problem there and i will do my own comp test too.
that'l be my next step to take i think
does anyone have any resistances for the coils and trigger packs or whatever they're called?

 

[AMD Rules]

Re: Mercury 1983 model 115hp RUNNING ROUGH

Here's the pics Bexi had trouble posting earlier, of the spark plugs.

Bexi,

Given your running issues, I agree that cleaning all three carbs is a good plan. New needles and seats along with new gaskets, and they should be good as new.

Only reliable way to measure the electrical performance is with a peak reading volt meter (DVA). That said, there are some resistance checks you can do. I'll dig out the specs.

You might want to invest in a spark gap tester as well. They are only a couple of dollars and are adjustable to test how strong your spark is.

I use an inductive timing light to check to see if the spark pattern is irregular.

Good luck.

 

[bexi]

Re: Mercury 1983 model 115hp RUNNING ROUGH

AMD, Hey did you have any luck getting those resistance tests yet?
bexi