sparking sound when idling

[tavacska]

Motor is 1977 mercury 1150 with distributor.

Motor starts ok. Idles the same as before, a little rough as usual. It idles a little rough in water, but always runs good on WOT.
But I want to give it a check why it idles rough. When it idles, I can hear the sparking sound. Since it is too noisy from motor, I can just tell it seems to come from the spark plug hole #5 or 6, with a paper towel channel aid to hear. It pops up occasionally, whenever it pops up, the engine acts like lifting head hearing something.

I took out the #5-6 plugs, #6 rust on out skirt. I took a ohm test between distributor pins and the spark plug core, all 1-2 ohm.

To me, it seems to spark outside the hole not at the tip of spark plug. But when I crank to test it, the sparks seems to be just at the right place.

Any experience or recommendations? Time to change the spark plugs and wires? Wires are 40 yrs old, spark plugs are 2 yrs old.

PS: one more thing to mention, the #5 & #6 is covered with white wet skin. On the first start, the exhaust with water has white on the surface like oil on the water. The white is not huge amount, but you still can see it spreads around on the first start. Is it water intruding or just from fuel? The boat runs super good at high speed and doesn't seem to lose any speed. And the WOT RPM is 5000.

 

[racerone]

Run it at night to see if spark is jumping out somewhere.-----Could be a gasket leaking on the internal water jacket cover.

 

[Chris1956]

Those old "Tiger Tail" spark plug wires can arc to ground. That seems like what you are hearing. They were avail for $6 or $7 each, but you had to install the spark plug spring connectors and boots. Pre built they were $20 each. Try one of those old merc web sites. You may get a link by searching on tiger tail, as that was the official name.

It was common for those motors to leak water thru the seal between the water jacket and cylinder liner. It dribbled out next to the spark plug bases. You can clean, dry that are and rub some silicone on the seam to seal it.

If you are really getting compression into the water jacket, that is very bad, as that indicates a cracked cylinder liner.

 

[tavacska]

updates:

Found the fault plug, the spark is pretty long, from the boot to the outskirt. I have a spare one, so the sparking now looks fine. 100% confirmed.

However, problems could come from the water intruding.

When idling on muff, I am tuning the idling fuel screw. The top 2 respond very well. But the bottom one doesn't work. I can close it with nothing noticed on engine. I checked the air import side, the top 4 chamber is around 120F, the bottom two is 80F. The bottom two cylinders don't fire at all. The idling is all supported by top four cylinders.

It seems obvious now there is water intruding into the bottom cylinders. But still not serious yet, because the WOT works well.

Need confirmed by you guys.

 

[tavacska]

Those old "Tiger Tail" spark plug wires can arc to ground. That seems like what you are hearing. They were avail for $6 or $7 each, but you had to install the spark plug spring connectors and boots. Pre built they were $20 each. Try one of those old merc web sites. You may get a link by searching on tiger tail, as that was the official name.

It was common for those motors to leak water thru the seal between the water jacket and cylinder liner. It dribbled out next to the spark plug bases. You can clean, dry that are and rub some silicone on the seam to seal it.

If you are really getting compression into the water jacket, that is very bad, as that indicates a cracked cylinder liner.

There is one set of wire in speedway motor store, named "20 ft red 7mm solid core spark plug wire". Looks like a quality set. Do you think I can use that to replace the old ones? I already have the screw-in terminals. So another set of boot terminal is good to go?

 

[racerone]

If water is leaking into the motor this spells CORROSION.----It will destroy your motor.----I suggest you look into this issue,----Also clean the bottom carburetor.------Test compression too.

 

[tavacska]

I have dealt with the water intruding problem twice. Changed all the gasket and inner plate, exhaust out plates. The motor, I guess, has reached his limit. The water path corrosion is pretty bad. The inner plate has several holes. Although I replaced the inner plate, but I guess there must be somewhere I didn't catch.

The water intruding problem is not that bad last year. Now it's time to replace the power head. The cylinder is still in its original size, very good compression on all cylinders, 130 psi, except #2 is 125 psi. I have a brand new power head as back up. It's time to replace it.

Do I have to replace all the rings? The rings were brand new three years ago, which has at most 100 hours on it. Really hesitate to replace them with a new set.

 

[Chris1956]

Water in the bottom 2 cylinders is often due to bad crankshaft seals. These are easy to replace, once you pull the powerhead.

As for the ignition wires. They need to be solid wire. The original wires were stranded stainless steel. I have heard of replacements being stranded copper. If you can crimp on the screw terminals, then that wire is likely suitable, I bought the replacement tiger tails with the screw terminal already attached.

 

[tavacska]

Water in the bottom 2 cylinders is often due to bad crankshaft seals. These are easy to replace, once you pull the powerhead.

I did change the crankshaft seals. I tried twice. the first time is "1 new + 2 old seals", the second time is "2 new seals". But symptom keeps going. That's why I choose to believe water comes from exhaust part or cylinder head, maybe from small hair cracks.

The only concern about bottom seal is the large o-ring of the base. The o-ring for the bottom shaft holder I bought with the gasket package seems to be too thin. Could that be the problem that water comes in?

Is there any effective way to tell if the water come from shaft down side?

 

[tavacska]

I am now concerned about the water from bottom. If that is the case, there will be still water in cylinders even if I replace the powerhead.

Just read some youtube, that water can intrude to bottom cylinder 3 ways:
1. lower crank shaft seal
2. exhaust port
3. air port

I never checked the air port. How can water come into the cylinder from air port? Seems there is no way the air path way can contact water.

 

[Chris1956]

Do you mean carb intake, when you say air port? Not sure how water could get there.

If the block is not cracked, water can only enter via the crank seals or via a leak or hole in the exhaust baffle, or inner exhaust cover.

You should be able to see holes in the baffle or inner exhaust cover. leaking gaskets are very hard to find by inspection.

The cylinder liners and head are 1 piece. They cannot leak water into the cylinder, unless they are cracked or have a hole in them.

 

[tavacska]

Thanks

Do you mean carb intake, when you say air port? Not sure how water could get there.

Thanks, Chris, it's air by path way, I guess, on the opposite way of exhaust. I checked it out, there is a hole tube from the side chamber to the bottom shaft holder, a very similar position to the one-way looping 1/8 tubing outside the powerhead. But, that could happen only after water intrudes first and then accumulate in bottom.

In my case, I really doubt that it comes from below, because I have been too cautious and serious when I sealed and installed the lower shaft cap before. inner exhaust cover is brand new. Baffle is like new condition freshwater used only.

My suspicious is the broken bolt on the exhaust side. I got it out but the hole enlarged too. It is not a clean enlarged one and located between 3rd & 4th cylinder. The bolt holes of baffle and outer cover have to be enlarged too. That could be the reason of leaking. But anyway, I am going to replace the whole powerhead. Hope it can fix everything.

 

[wn6ngp]

updates:

Found the fault plug, the spark is pretty long, from the boot to the outskirt. I have a spare one, so the sparking now looks fine. 100% confirmed.

However, problems could come from the water intruding.

When idling on muff, I am tuning the idling fuel screw. The top 2 respond very well. But the bottom one doesn't work. I can close it with nothing noticed on engine. I checked the air import side, the top 4 chamber is around 120F, the bottom two is 80F. The bottom two cylinders don't fire at all. The idling is all supported by top four cylinders.

It seems obvious now there is water intruding into the bottom cylinders. But still not serious yet, because the WOT works well.

Need confirmed by you guys.

I have been misled a few times when using muffs and tuning. The back pressure on exhaust when lower unit is in water makes a significant difference on idle on my 90 elpto.