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Mercury 1985 75hp four cylinder problem

D

daselbee

Advanced Contributor
Hi Merc guys....

I post regularly on the J/E forum, but now I have this Merc that is giving me fits.

It came to me not running. New to owner, never test ran it, nothing.
I checked compression....150 range on all four.
I checked spark. No spark. Debugged it to a bad switchbox, replaced, now good spark.

Ok, so....went thru the carbs (2) and found some usual things, cracked bowl gasket, missing drain plug gasket, floats not quite right but close, nothing real major. I replaced the welch plugs and sealed them with Indian Head shellac.
Set the floats with the float arm level with the bottom of the carb body, by adjusting that 1" long arm that goes over to the needle.

Re-installed the carbs, started it up.

Here is the problem: It will NOT idle any lower than 2500 RPMs. I have a tach hooked up.

I have indexed the flywheel and all sparks are happening at the proper time for all four cyls. 1-3-2-4 firing order.
I have fixed the timing lever (tied it down so it cannot move) at exactly TDC position for the timerbase.
Both throttle butterflies are absolutely closed and in sync. It should idle normally with those settings.
I have actually disconnected the carb butterfly linkage so that the butterflies cannot open.
I have looked up at the sun thru the carb throat, checking for butterfly sealing/closing all around the throat.
All checks out.

The engine is on my test stand, with the lower unit removed, and being fed water via a hose up the water tube.
The fuel line is plumbed directly to the carbs, bypassing the fuel pump. The primer bulb gets firm as it should.
No gas leaks. I put the fuel line directly to the carbs to eliminate a torn fuel pump diaphragm.

After running it, large amounts of unburned fuel and water mix drip out the bottom of the midsection onto the floor.
Tells me it is way, way too rich.

It has that weird enricher valve that turns to enrich for choking. (no choke butterflies) I have been thru that and cannot find anything wrong.
Idle mix screws are set at 1-1/2 turns out.

Directly in the front of the carb throat, off to the right, is the idle air bleed orifice. I am measuring that as about .038" in diameter.
I can plug that orifice with a toothpick. If I plug either carb's idle air orifice, the engine runs on the other two cyls. If I plug both orifices, the engine dies. That tells me the extra fuel is coming from the idle circuits of the carbs. Is this a fair conclusion?

Where can all this extra gas be coming from? Why is it pulling so much gas that it revs up to 2500, does not flood/choke itself out, etc.
It almost seems as if it is getting gas and some air to keep from flooding out.

I have sprayed carb cleaner all around the intake, carb mounting gaskets, etc....no detectable difference looking for a leak somewhere.

Is there some quirk about a Merc ignition system that can cause this racing condition even tho the timing lights show everything is OK?
Does it need the LU on it to provide back pressure to slow it down? Even so, that crap dripping out says rich unburned fuel...

I am tearing my hair out.

Any Merc experts out there that can lend some advice? Much thanks.....
 
Well I'm no expert,they'll be along later when they quit fishing,but the 1 1/2 turns out are the starting point,and yes you need the lower unit on,in water and under load to set the idle.
When you get it in water,warmed up and under load,turn the idle screws in a little at a time, 1/8turn, until she runs rough,then back them out until she runs rough,set it back to where she runs right and then back it out a 1/4 turn.
 
I am not talking about idle fine tuning and adjustments.....this thing will not slow down at all.

Races at 2500....I gotta get that fixed first, then fine tune it.
 
What happens as you rotate the ignition trigger forward and backwards. I believe that the timing advance is what controls the engine RPM until the butterflies open.
 
Yes, when I rotate the advance (timer base) manually, it does control the speed. I can slow it down by retarding the spark significantly, i.e. to about 15* ATDC, but that is way too retarded. Spec says it should idle at 2*BTDC. Right now I have it fixed down at TDC. Engine races at 2500.
If I turn the idle screws slowly in, leaning it out, the speed increases on up to 3k.

As I said in my first post, I have indexed the wheel, and all sparks are where they should be...right on TDC for each cylinder.
 
Sorry but the motor does NOT need to idle at 2 degrees.------------That is where the throtlle opens up.-------If it idles at 15 degrees ATDC it is a bonus.
 
Sorry racerone...u are wrong on this one....
When I get it to 15*ATDC, the timerbase has hit it's max limit of rotation in the clockwise direction. It won't go any further, and it still is running way too fast.
Pickup point is never seventeen degrees away from the timing where it idles best...never, on any boat motor....

Hell, the total advance of most two strokes is maybe 25 degrees total....
So you say that it should idle at 15* ATDC, and then pickup at 2*BTDC and then hit WOT timing at 25*BTDC...a total of 40* travel?

The mechanics of how the linkage works won't even allow that.
 
Thx...been trying to post back, but "server too busy" message....
So, plugs.....the plugs are surface gap style, and it is very weird. I cannot tell that they have been running. They look new.
They are wet with oil and gas, but there is no indication of any burning or sparking combustion, etc.
They look new.

I will critically compare #4 to all the rest today.

I am not sure how I would go about checking the lower crank seal without pulling the powerhead...

I will also turn the idle mix screws in til it leans out to the point just before it stalls. It will be racing, and I will get a tach reading.
But I am doing this to see if it drips crap out the bottom at the new leaner idle mix setting.

Is there anything in the Merc ignition system that I could have wrong, that would make it look OK, but still be firing wrong? I know it is a stupid
premise...I know for sure the sparks are happening properly on the indexed flywheel for all four...

Local guys are saying the stator could be the wrong one, that it could be mounted wrong, etc. Weird stuff like that. The stator mounting holes are not symmetrical. It will only go on one way. The crank is splined, so the flywheel will only go on one way. No key to shear....

I will pull the carbs again to examine for air leaks. Can leaking welch plugs be the whole problem? I have sprayed carb cleaner directly on them, and cannot detect any difference in the way the engine is racing.....
Is there a better fluid to detect a leaking welch plug?

I have a small handheld butane torch. I can trigger it, and sometimes it will not light, but gas is being expelled.
If I hold this source of unlit butane right next to the welch plugs, what would that tell me?????? (besides BOOM).
Would that butane gas cause a difference in the running speed?
Is there a trick to this?
 
I often see people searching for how to " set the idle timing "--------All that is set is maximum advance at full throttle !!----------Idle timing looks after itself.-------Throttle pick-up timing is a carburetor / linkage adjustment.
 
Racerone...thanks for your input, but respectfully, I think you are missing the main point....

The timing right now is hard fixed at exactly TDC. Bolted down.
The throttle link that opens the butterflies is physically disconnected so that the butterflies cannot open.

It should idle at those settings, maybe badly, but it should idle.

It races at 2500 rpm, with no way to slow it down.

I am not looking for "how to set the idle timing".
 
Hey Fazt...

Yes, that is the same setup I have.....that small valve that rotates when the choke solenoid pulls in is called an "valve assembly, enrichment" on the Merc parts lists. Part number is 6198, it is NLA, but I cannot see anything wrong with them, and I have studied how they work, etc. Seems to be OK.
One question....directly above the rotating shaft of that valve, in the top of the carb, is a small hole. It allows air thru that enricher valve system.
Is there supposed to be a BB in that hole? Should it be sealed? Right now both are open. If I plug them off, there is no change in the motor.

I have also pulled the carbs, and blocked off the hole where the fuel feeds to the intake when enrichening. In other words, I was trying to keep any fuel flow from coming from the enrichener system. When I blocked it off, re-installed the carbs, it ran even faster! Almost like a runaway.

I didn't get a chance to do the butane test yesterday. Will do it today.
If those welch plugs are leaking....I wonder if that would cause my racing problem....
I put new ones in, staked them like I always do, and sealed them with Indian Head shellac....shouldn't be leaking....but who knows.

One other thing for those reading....anyone out there have a set of these carbs that could measure the diameter of the idle air bleed hole? I need to know if someone has drilled these out sometime in the past. Best I can figure, mine are about 0.038".
This is the hole off to the right side of the center of the venturi. Small booger, not removable, and it kinda goes into the carb casting at an angle.
Long shot....but could be....These are the only holes that I can block off that actually affect the running speed of the engine.
 
well, I finally got around to testing with propane to see if engine speed changes as propane is supplied around the block, carb gaskets, intake, etc.
I took a propane torch, turned the gas full on, and for a control test, I put it directly in the throat of the carbs. Engine slowed down slightly.
It was running at about 2500, and it slowed down. I did not anticipate the gas would actually slow it down....wow.

But, the torch was at full gas flow, it was being fed to the throat of the carbs fully, so there was no chance in hell that me waving it around the block, intake, etc would have ANY effect at all. The air leak would have been too small to cause any change (slowing).

Failed test. Not a valid test. No need to try this one at home, friends!!!

So, brings me back to why this is idling so high.

A Merc guy said the sealing between the cylinders could be leaking. He said "It will never idle slow with a leak there."
That would be the sealing main bearing assemblies with those reeds.
Any validity to that? That is a big teardown just to find nothing wrong.

Any way of testing this? How can I test for leakage between adjacent cylinders. like 1-2 or 3-4?
I cannot figure out how to introduce any flammable liquid into an individual cylinder, because they share carbs. Only two carbs.
 
I'm a bit confused here, but bear with me. That idle bleed you plug with a toothpick...that sounds like the infamous backdrag circuit that destroyed lots of motors. If there is also a rubber tube going from the float assembly to the left side of the carb, that;s what it is. If so, remove the tube from the float assembly and plug it off with a screw.

Interestingly, one of my triples does the same thing--it won't idle below 2,00 rpms. In its case it was BAD gas (believe it or not). I tossed the gas out after the motor I replaced it with did the same thing. With fresh gas all is well.

Jeff

PS: I'll be testing the first of these motors today with some good gas and get back to you.
 
Correct on the carbs--missed that. One thing to check: are the enrichener circuits closed? The lever pulls them up to "choke" and if it doesn't come all the way back down, that's what can happen.

Jeff
 
Thanks for the responses. Much appreciated.

There are no rubber tubes internal to the carbs. There are two carbs, each one servicing two cylinders. Top carb is one and two, bottom carb is three and four.
Just to repeat, the carb butterflies are totally closed. The linkage is disconnected in such a way that the butterflies cannot open. (testing)

The enricher valves are linked together with a rod. This rod is also removed. All this for testing and verification....
The valves are spring loaded, and I have verified that they are closed. The fiber seal is intact and seems to be sealing, because no change in engine speed is noticed when I spray them with carb cleaner.

The idle bleed is in the throat of the carb, on the right side when looking in, and runs in at an angle. This passage intersects with the same passage that the idle needle valve lives in. Air flowing thru this bleed results in fuel being pulled from the bowl to feed the engine. All fuel appears to be coming from this circuit, because if I plug those bleeds, the engine dies.
Another thing I have noticed....I can turn the idle needles IN, and the engine picks up to 3000 rpm. But the mix is now correct, in that the engine doesn't drip unburned fuel and water out the bottom. The mixture seems right, if that makes any sense.

I have obtained another set of carbs. They are in bad shape and unusable, but I did it to verify the idle bleed size, and the size of the jet that is located on the left front face of the carbs. I cannot get those out, but they are clear, and I do not know the size. The sizes of all these holes are the same as on the working set of carbs. In other words, no one has drilled these orifices out....

I had a "Merc Mechanic" come out. He is suggesting three things.

1. The stator might be installed "rotated" from the correct position. Now, I had the flywheel and stator off AFTER this high speed problem was noted, and I think I remember that the stator mounting bosses are not symmetrical, in that you cannot mount it but one way. Even so, how can the stator being rotated cause this problem? In my opinion, that is a long shot, but I will check it again.

2. The sealing between crankcases might be leaking. He said "If the seals between the crankcases are leaking, you will never get it to idle low."
What do you think?

3. I am running it with no lower unit on it. I have my reasons for doing it that way. The reasons are valid. I do all my engines that way.
I mount the engine on a low stand, remove the LU, attach a water hose to the water pipe, and go to it.
Now....he suggested the absence of back pressure and LU mechanical drag might be causing the engine to race up. I know other engines I have worked on
will run faster with no backpressure....like maybe 1200 or so, but not 3000. So, what are your opinions on the no back pressure theory?
Am I chasing a problem that doesn't even exist?

Again, I have the timing mechanically bolted down so it cannot move, right at TDC. The carbs are closed. It should idle.

Once I get this racing problem fixed, then of course, all linkages and controls will be re-attached and put back in normal running condition.
Then the fine tuning.

That's the way I do almost every engine that comes in. Never had one race like this, but then I never worked on this model Merc before either.
 
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One other thing. There is a fixed size air bleed jet on the left front face of the carbs. I cannot get them out, but have compared the sizes to the junk set of carbs I obtained. Sizes look the same as best I can tell using small drills to measure the hole.

If I plug those orifices while running, there is no change in the motor. So it is getting a lot of air from somewhere, and it is not thru those orifices.

Welch plugs are verified sealed...no leaks.
 
..." the stator mounting bosses are not symmetrical, in that you cannot mount it but one way."


Correct. I've run those carbs wth and without ANY air jet in the top carb (doh!) and it made no difference.

You said turning the idle screw in gave 3,000 rpms--and that is as it should be.

You're running it with the LU off; that alone will make it idle sky high! THe drag of that massive LU is substantial.

I think:

1. The LU needs to go back on

2. The idle mixtures need adjusting, and

3. The idle speed setting needs setting.

Do all that and all will be well.

Jeff
 
Well, thanks to all who responded.
The work that I had done to make this engiine run right was correct.

BUT!!! As suggested above, the engine was racing simply because it had no load. The LU was re-installed, it was submerged in the tank, and it now idles properly.
I will do a complete link and sync and adjust timings, but it is idling right where it should be now.

You guys put this hard earned lesson in the back of your heads...you cannot run an outboard without the LU installed, and still expect it to behave as if it were all together. I state it that way, because I have done countless J/E engines this way with no problems. Mercs...not so much. Yamahas...I did have one that was getting water on the spark plugs....could find NO obvious problems...I now suspect it was sucking back up into the exhaust ports because of the lack of the proper back pressure in the exhaust system...
Anyway, learn from my mistakes....

Thanks again for all your help.

Doug
 
When reading the forums I see the difficulty people have with diagnosing faults. Outboards are quite tricky to diagnose, especially to detect if a cylinder or two has gone off, more so which cylinder, as the engine usually feels the same temperature all over and the spark plugs often give no good clue either.

Perhaps reading how I found a fault may help others to diagnose odd running faults with multi cylinder outboard engines.

I have an old 1980's 4 cylinder 2 stroke Mercury 75hp and the engine had a misfire below 3000 and would go lumpy at low rpm. I knew a cylinder was dropping out but couldn't really tell which one.

My careful diagnosis led me straight to the fault and a strip down revealed the lower crank bearing to be severely worn and allowing the crank to oscillate, therefore allowing air to leak past the lower crank seal. The bearing was so worn I was surprised it still ran so smoothly and quietly.

The leaking seal caused a loss of primary compression (crankcase compression) on the bottom cylinder only. This fault can't be detected by a compression test as the test only measures upper cylinder pressure, not primary compression.

So if you care to read, here is how it's done in my world.

Do all the obvious stuff first, there isn't really much to check. Each carburettor runs two cylinders, so carb faults will usually knock out both relevant cylinders and not just one. I gave the carbs a once over but expected there would be nothing to see, they were fine

The ignition can be easily tested by making up a long rod extension to go between the top of the plug and the plug cap, long 4mm screws may work. You can now run the engine and short out the spark on any cylinder by offering an earth connection to the metal extensions. Be careful here not to get a shock, but now you can see how huge or not the spark is on each cylinder. The ignition timing wasn't in question so didn't need checked.

The cylinder will go off as you earth the spark away, of course, so keep the rpm up just enough to keep it going.
The spark, if good, will jump to the earth lead before you even touch it, and as you draw the earth back the spark will jump a good inch or two, or possibly more.
Make comparisons between the cylinders, the spark should be the same strength.

Also earthing the spark on one cylinder at a time will help you diagnose it, if it makes no difference when you earth the spark, that cylinder wasn't running anyway, if the engine staggers, the cylinder was running.

So if the spark is good we can diagnose the fault further by running the engine on one cylinder at a time with the other three spark plugs out of the cylinders and earthed to ground.

Beware, you must earth the plugs which are out of the cylinders, push them back into the caps and wire them together or somehow electrically ground them all so when you start it up it doesn't kill the ignition box. Also be aware of fuel coming out of the open plug holes,, don't make sparks near them.

On mine the top cylinder was first to be tested. I put in the spark plug and connected the cap and started it up (all other plugs out and grounded) and set the throttle so the engine idled smoothly at about 1000rpm or so, on just the one cylinder.

I switched it off leaving the throttle exactly where it was and removed the spark plug from the top cylinder and put it in the next one down. The engine started and ran exactly the same speed on that cylinder, so all ok so far.

I moved to the third cylinder, it ran about the same but was not just quite as strong as the two above it, so that was noted.

When I tried the bottom cylinder, it was barely able to start or run at all, I had to open the throttle way further to make it run, so here was the culprit for sure.

So I knew the ignition was strong, the carb was good, so it had to be a primary compression loss, caused by either a leaking crank seal, leaking reed petals, or piston skirt damage. It was the crank seal after all.

Bruce Hart
 
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