93 60HP - under powered and not reving up all the way

[twr7cx]

I have a 1993 Mercury 60HP 3 cylinder 2 stroke twin carb outboard which has been a pain for quiet some time now.
Recently with the help of some fellow forum members I managed to identify and resolve the problem with it ( http://forums.iboats.com/showthread.php?t=362469 ), and got a bit over 2 hours of perfect run time before new issue started!

The current issue is that the outboard starts and idles perfectly, but has no kick too it (feels like a 20HP is pushing the boat instead of a 60HP), it also doesn't rev up all the way - previously, from memory, it was reving to around 5,500rpm, the other day the max rpm was a bit over 4,000rpm and then yesterday it was around 3,500rpm.
The tacho is intermitent and kicks in and out when it likes, but this problem has been ongoing and I havn't had time to look at it yet.

In the last six to 12 months, to resolve the previous issue, the outboard has had the entire ignition/charging system replaced - stator, trigger, ignition module, rectifier, 3x coils, 3x spark plugs, 3x ignition leads.
I have also replaced most of the fuel system including new fuel tanks, fuel lines, fuel filter and completely rebuilt both carbs.
Compression was test some time ago, and was good. I've been meaning to check it again.

So far I have run the resistance tests on the stator and trigger to confirm that they are fine - they are, the results were in the range (as I thought it might be the high speed on the stator blow).
I have run with without the cover on the carbys and can confirm that the petrol is flowing well through them.
I have tried different ignition coil set - no change.
I have tried a different ignition module - no change.
I have tried a different rectifier - no change.
Writing this list up has made me realise that I havn't changed the spark plugs in a while, althought they are about 12 months old they would only have 10 hours max of run time on them - I have a spare set at home so will try that tonight.

Last night I removed the prop from the outboard and have put it into a 44 gallon drum so that I can run some tests in the driveway, but ran out of time last night to do so.

Does anyone have any particular tests that they can recomend or any ideas as to what the problem would be?

 

[twr7cx]

Re: 93 60HP - under powered and not reving up all the way

I got a chance to have a bit of a play this afternoon. I have the outboard with the prop removed running in a cut down 44 gallon drum.

Did a compression test with the engine cold. Results (from top cylinder [number 1?] to bottom [number 3?]) are:
116 psi.
115 psi.
121 psi.
So all are reasonably close and I believe still within an acceptable range.

While doing that I checked out the spark plugs, which as I mentioned above only have about 10 hours max of run time on them - most of that is low rev idling while I was testing and fixing other problems over the past 12 months.
What I found interesting while looking at the plugs was that the bottom 2 cylidners (numbers 2 and 3?) had a black oily film over them (as I expected for a 2 stroke motor), the top cylinders (number 1?) plug did not have much at all. Instead it was filled with a silver looking sludge.
It was hard to get a picture of this, but here's a few attempts, the plugs are from left to right, 3 to 1. Plug 3 is a bit cleaner than plug 2 as I accidently wiped it a bit with my finger.






Anyone know what this silver sludge build up could be? What would cause it?
I fitted new plugs and ran the motor in the 44 gallon drum, it reved out fine - which could be due to the new plugs (particuarly the new plug in number 1 cylidner which had the silver sludge build up), or could be due to the prop being removed the engine was not under any load. I guess I'll have to put the prop back on and put it into the water tomorrow morning to test out which it is.

 

[saumon]

Re: 93 60HP - under powered and not reving up all the way

Silver is bad! Look like aluminum transfer to me. Piston is overheating, expand and scuff the cylinder wall.

Maybe a cooling problem (bad impeller), as #1 is always the first to suffer, a lean condition (clogged carb), an over advanced timing or a lugging engine.

The bad news is that you'll probably have to split the crankcase to know exactly what's happening (thoses triple don't have removeable heads)

The good news is that, wich such great compression readings, maybe you catch it early enough to be OK with only a honing and new rings on that cylinder instead of going oversize (rebore + new piston). You'll know it by the depth of the vertical grooves, if any, on that sleeve. If you can feel them with your fingernails, you'll have to rebore.

 

[AlistairHenderson]

Re: 93 60HP - under powered and not reving up all the way

Hi,

I have had the same problem as you when I first received my 1991 Mariner 50. It would never reach fullRPM even though it was ticking over fine. I did a number of things to the engine, some you have already done.

Stripped down all 3 carbs and replaced jets and gaskets. Very important you properly clean all the tiny channels. Easiest way to do this is to use an air compressor.

Replaced all 3 spark plugs.

Stripped down the fuel system and replaced the pump diaphragms, check valves etc... You have yet to do this and i think it was one of the main causes for my low rpm.

Adjusted all the timings and fuel/air mixes on all the carbs. Worth getting hold of a good timing light to get these right.

All these repairs were done quite easily and you can buy service packs for all the jobs which contain all the spares needed.

Once all this was done, put back together, gave it some new clean fuel (after draining ALL the fuel from the old tank and throughout the system. Now it runs at full speed perfectly happily. Now the piston rings are on their way out, but that's just due to its age and heavy use.

As for the silver spots, you will probably need to strip down the water pump unit. Bit harder as you need to take off the drive leg. Probably got the channels blocked up with salt if its used in salt-water. If its in freshwater you might of picked up sand and ruined the impeller. These things happen and need to be checked. Try running it with muffs on and keep an eye on the tell-tail temperature.

Hope this helps.

 

[saumon]

Re: 93 60HP - under powered and not reving up all the way

What AlistairHenderson said is true but, for the silvery flakes, there's no shortcut: it's probably aluminum deposit resulting from the piston melting. Doing all the above without opening it will only delay the problem.

If you choose to do it, i HIGHLY recommend you to get an OEM service manual as you'll need all the measurements like clearances, ring en gap, torque specs and such. That's probably the best investment you can made.

BTW, i have the exact same engine wich is also a 1993 but with 3 carbs (they probably change the design in mid-year). How's the setup with 2 carbs? The top carb feed piston #1 and the bottom one feed #2 and #3? Or the other way around?

 

[achris]

Re: 93 60HP - under powered and not reving up all the way

... How's the setup with 2 carbs? The top carb feed piston #1 and the bottom one feed #2 and #3? Or the other way around?

Top carb feeds cyl#1 and #2. Bottom carb feeds cyl#2 and cyl#3...

Chris......

 

[saumon]

Re: 93 60HP - under powered and not reving up all the way

Thanks for the explanation Chris! So there's 2 reed openings: one between #1 and #2 and the other between #2 and #3?

What do you think about his #1 plug?

 

[achris]

Re: 93 60HP - under powered and not reving up all the way

Thanks for the explanation Chris! So there's 2 reed openings: one between #1 and #2 and the other between #2 and #3?

Not sure if I understand you correctly... each carb feeds to its manifold, that has 2 reed openings. (I'll do a drawing and PM it to you....)

What do you think about his #1 plug?

Not good... Aluminium on the spark plug can only lead to tears... Piston is coming apart... MUST find out why or it will happen again.... Over-propped?

Chris......

 

[twr7cx]

Re: 93 60HP - under powered and not reving up all the way

Not sure if I understand you correctly... each carb feeds to its manifold, that has 2 reed openings. (I'll do a drawing and PM it to you....)



Not good... Aluminium on the spark plug can only lead to tears... Piston is coming apart... MUST find out why or it will happen again.... Over-propped?

Chris......

Any chance of sharing that drawing?

When you say over propped, you mean prop size is too big?

 

[twr7cx]

Re: 93 60HP - under powered and not reving up all the way

As for the silver spots, you will probably need to strip down the water pump unit. Bit harder as you need to take off the drive leg.

Water pump was rebuild around 12 months ago. Has had about 3 hours of run time in the saltwayter, and probably 7 hours in tap water with either ear muffs on or in the 44 gallon drum.
The engine has a nice hard tell tale spitting out.

 

[achris]

Re: 93 60HP - under powered and not reving up all the way

Any chance of sharing that drawing?

Yes, when I've drawn it...

When you say over propped, you mean prop size is too big?

Yes (too much pitch).... The manufacturer specifies what the WOT (Wide Open Throttle) revs should be. That's the revs that the engine should be making when run at full throttle. If the engine isn't making those revs, it is overpropped, and the engine is overloaded. That overloading affects the engine at all throttle positions, not just WOT. The effect on the engine is to raise cylinder pressures and temperatures, which leads to detonation, and eventually the crown of the piston starts to melt, depositing aluminium on the spark plug. Although there are many other causes of detonation, this is the most common.

Do you know what revs the engine was making at full throttle (should be 5200-5500)? If not, what size prop is on the engine (pitch, in inches. It's stamped into the prop hub) and describe the boat.

Chris.........

 

[twr7cx]

Re: 93 60HP - under powered and not reving up all the way

I am certain that you gents are correct and that it is an issue with aluminium from the top cylinder (number 1?). This is clogging the plug which is effecting the outboards performance, not to the point that it's only running on 2 cylinders, but to the point that it has reduced performance in terms of power output and ability to rev.
I have just come back from testing the boat out in the sea after changing the spark plugs with brand new ones and it performed perfectly once again.

Any tips of guides on replacing the top piston? I have both the Clymer and Seloc manuals for this motor, are these sufficient?

Aside from a new piston and rings, what other additional parts do I need? Which gaskets will I require?

Is it acceptable to just replace the one top piston, or should all be done?

I've just looked on the Sierra Marine Catalogue and the piston Std. Cyl. Dia. (Bore) is 2.875.
Piston part numbers - 18-4505 (Std.) 18-4520 (.015 O.S.) 18-4535 (.030 O.S.). Ring part numbers - 18-3918 (Std) 18-3919 (.015 O.S.) 18-3920 (.030 O.S.).
Can anyone confirm that these used to be 3 ring pistons, but Sierra now manufacture them to be only 2 ring pistons?

 

[twr7cx]

Re: 93 60HP - under powered and not reving up all the way

Yes (too much pitch).... The manufacturer specifies what the WOT (Wide Open Throttle) revs should be. That's the revs that the engine should be making when run at full throttle. If the engine isn't making those revs, it is overpropped, and the engine is overloaded. That overloading affects the engine at all throttle positions, not just WOT. The effect on the engine is to raise cylinder pressures and temperatures, which leads to detonation, and eventually the crown of the piston starts to melt, depositing aluminium on the spark plug. Although there are many other causes of detonation, this is the most common.

Do you know what revs the engine was making at full throttle? If not, what size prop is on the engine (pitch, in inches. It's stamped into the prop hub) and describe the boat.

Thanks for that explanation. I will test and see what rpm the engine does at WOT, but from memory I believe it to be 5,500rpm. Any idea what the max rpm from the manufacturer is?

I will get the info on the prop shortly.

 

[twr7cx]

Re: 93 60HP - under powered and not reving up all the way

Do you know what revs the engine was making at full throttle (should be 5200-5500)? If not, what size prop is on the engine (pitch, in inches. It's stamped into the prop hub) and describe the boat.

As said above, I'm fairly certain that the engine was reving at 5,500rpm on the dash's tacho. Now the dash tacho reads 200rpm higher than my testing tacho, and I don't trust it too far, as it can be intermitent and hang. So it's more likely that the engine was only reving at around 5,300rpm at WOT.

The only marking that I can find on the prop reads "48 73136 A40L 13P".

Boat is approximately 14 foot, fibreglass, shallow hull, just a little run about. I'll take some photos.

 

[achris]

Re: 93 60HP - under powered and not reving up all the way

The only marking that I can find on the prop reads "48 73136 A40L 13P".

That's the number I'm looking for, 13P. That prop has a pitch of 13 inches.

Never trust the dash tacho unless you have had it checked. They are an indication only. They can be as much as 1000 rev out. You need to get a hold of a 'shop tacho' and then check you WOT revs....

Spec is 5200-550rpm....

Chris......

 

[twr7cx]

Re: 93 60HP - under powered and not reving up all the way

You need to get a hold of a 'shop tacho' and then check you WOT revs....

Yeah, the tacho that I've checked it with is a AUD$300.00 automotive multimeter with the tacho pickup attachment.

Does 13P sound about right though?

 

[achris]

Re: 93 60HP - under powered and not reving up all the way

13" sounds ok, but the only way to be sure (and you need to be sure or you'll blow a piston... that's right you've blown a piston) is to check the max revs on the water, with the shop tacho. It's no good checking the dash tacho at idle and saying 'It's 200 revs out, therefore is must be 200 revs out over the whole range". Doesn't work like that..... You need to check the WOT revs with a known accurate (shop) tacho.

Chris.....

 

[saumon]

Re: 93 60HP - under powered and not reving up all the way

I am certain that you gents are correct and that it is an issue with aluminium from the top cylinder (number 1?). This is clogging the plug which is effecting the outboards performance, not to the point that it's only running on 2 cylinders, but to the point that it has reduced performance in terms of power output and ability to rev.
I have just come back from testing the boat out in the sea after changing the spark plugs with brand new ones and it performed perfectly once again.

Any tips of guides on replacing the top piston? I have both the Clymer and Seloc manuals for this motor, are these sufficient?

Aside from a new piston and rings, what other additional parts do I need? Which gaskets will I require?

Is it acceptable to just replace the one top piston, or should all be done?

I've just looked on the Sierra Marine Catalogue and the piston Std. Cyl. Dia. (Bore) is 2.875.
Piston part numbers - 18-4505 (Std.) 18-4520 (.015 O.S.) 18-4535 (.030 O.S.). Ring part numbers - 18-3918 (Std) 18-3919 (.015 O.S.) 18-3920 (.030 O.S.).
Can anyone confirm that these used to be 3 ring pistons, but Sierra now manufacture them to be only 2 ring pistons?

I repeat: an OEM service manual is the best investment you can made for a teardown. Not a generic one like Seloc or Clymer.

If it's only detonation (top of piston melting), you MAY be ok with only a std pistons with rings. You'll also need the engine gasket kit, some sealers, a hone and a flywheel puller.

But if that piston expand from overheating (you'll know if it leave vertical grooves on the cylinder wall) you'll first need to rebore and get oversized piston.

And while going that far, it would be foolish to not redo the waterpump (only the impeller), the fuel pump (diaphragm kit) and the carbs (clean and put new gasket kits).

Anyway, to know what have to be done, and before ordering anything, you'll first have to open it...

BTW, i'm not sure it's a good idea to ride down the lake at WOT with a deteriorating engine. You could cause a lot more damages...like if, let's say, a ring catch a port or something funny like that!

 

[twr7cx]

Re: 93 60HP - under powered and not reving up all the way

I repeat: an OEM service manual is the best investment you can made for a teardown. Not a generic one like Seloc or Clymer.

Do you know where I can get a part number listing on the OEM service manuals?

 

[saumon]

Re: 93 60HP - under powered and not reving up all the way

Here's some places where they sell them:

http://www.marinepartsman.com/Mercury-outboard-manuals_6.html

http://www.marinepartsman.com/Mercury-outboard-manuals_6.html

They're not cheap. Sometime you can find them on ebay but i haven't seen yours now.