sciclunaneil
New member
Hi everyone,
I searched a lot through this forum and found a lot of help to get to where I am now, however after doing a lot of work and ironing out other issues, I'm stuck with this one.
This motor isn't a heavily used motor, used maybe 2-3 days in the summer season and recently it had been sitting around for a few years without use, it is very well taken care of and always cleaned after use and never left in the sea (it runs in salt water). It's the Mercury 90 HP 3 cylinder 2 stroke, 9506481 - 9589038 (Belgium) model.
Since it hadn't been running for some time, we did a full teardown of it (except the engine block) and refurbished the gearbox, water pump, rebuilt the oil pump, fuel and air hoses, replaced reed valves, gaskets, rebuilt the carbs and put in new plugs. At this point we tried running it and noticed that we had very weak spark and the motor was not running well at all, we isolated this to an issue in the stator and trigger (both readings were out of whack) and we replaced the stator, trigger and switch box, and the spark leads, checked the coils and they are good. Since then, we had very good spark and the motor out of the water runs like a champ, in the water it starts up quickly and idles well.
However this is where the teething issue started, after starting we let the motor get up the temperature and try to put it in gear, and when this happens it stalls. After a few times trying, it finally gets into gear without stalling, what seems to help is revving it up a little bit in idle before and then putting it into gear. At this point we did a full link and sync, adjusted the idle screw, idle RPM is to spec and it improved a bit but still happens from time to time. Carburetor mixture screws are around 1.5-2 turns out each, not much difference between them.
At this point after struggling a bit to get it into gear, we can use the boat and the motor goes up to full RPM like a champ, plenty of power, no issues at all, never stalls except for when trying to dock and going at very low speeds. We start noticing that the problem gets worse and the motor starts running rougher the longer we stay out on the boat (we stayed out for maybe 3-4 hours). Important to note here that most of the time spent running the boat is on plane and not at low speeds, this is only for docking or placing the anchor.
After and inspection, everything looks good, except for the spark plugs. The top and bottom plugs are shiny clean, no oil, as should be, but the middle one is jet black full of carbon soot, but it's dry to the touch, not wet. It was perfectly clean before we went out. The carbon cleans off with a brush and cloth.
From this point, to try to isolate the issue we tried, changing the plug, no difference, swapping the coils to see if the coil is causing the issue, swapping the lines on the switch box, again nothing, the issue stays on the middle spark plug only. One thing to note here is that the middle spark plug hole was re-threaded and a heli-coil was fitted. I am not sure if this could be contributing to the issue, but the spark plug fits properly and we're able to tighten it properly. On top of that we also did a compression check and the middle cylinder is not lower compression, all 3 cylinders are just under 90 psi.
Not sure what else to try at this point, also tried a different brand spark plugs, was using NGK originally and tried Denso, no difference. Anyone has any experience with a similar issue and can point me in the right direction? The only thing that's on my mind at the moment that we haven't checked are the crank-case bleed check valves, however in the manuals I can't seem to find exactly where they are, does the crank-case need to be split to test and/or replace these? I attached some pictures of the motor, as you can see it still looks new.
Appreciate any help. Thanks for taking the time to read through.
I searched a lot through this forum and found a lot of help to get to where I am now, however after doing a lot of work and ironing out other issues, I'm stuck with this one.
This motor isn't a heavily used motor, used maybe 2-3 days in the summer season and recently it had been sitting around for a few years without use, it is very well taken care of and always cleaned after use and never left in the sea (it runs in salt water). It's the Mercury 90 HP 3 cylinder 2 stroke, 9506481 - 9589038 (Belgium) model.
Since it hadn't been running for some time, we did a full teardown of it (except the engine block) and refurbished the gearbox, water pump, rebuilt the oil pump, fuel and air hoses, replaced reed valves, gaskets, rebuilt the carbs and put in new plugs. At this point we tried running it and noticed that we had very weak spark and the motor was not running well at all, we isolated this to an issue in the stator and trigger (both readings were out of whack) and we replaced the stator, trigger and switch box, and the spark leads, checked the coils and they are good. Since then, we had very good spark and the motor out of the water runs like a champ, in the water it starts up quickly and idles well.
However this is where the teething issue started, after starting we let the motor get up the temperature and try to put it in gear, and when this happens it stalls. After a few times trying, it finally gets into gear without stalling, what seems to help is revving it up a little bit in idle before and then putting it into gear. At this point we did a full link and sync, adjusted the idle screw, idle RPM is to spec and it improved a bit but still happens from time to time. Carburetor mixture screws are around 1.5-2 turns out each, not much difference between them.
At this point after struggling a bit to get it into gear, we can use the boat and the motor goes up to full RPM like a champ, plenty of power, no issues at all, never stalls except for when trying to dock and going at very low speeds. We start noticing that the problem gets worse and the motor starts running rougher the longer we stay out on the boat (we stayed out for maybe 3-4 hours). Important to note here that most of the time spent running the boat is on plane and not at low speeds, this is only for docking or placing the anchor.
After and inspection, everything looks good, except for the spark plugs. The top and bottom plugs are shiny clean, no oil, as should be, but the middle one is jet black full of carbon soot, but it's dry to the touch, not wet. It was perfectly clean before we went out. The carbon cleans off with a brush and cloth.
From this point, to try to isolate the issue we tried, changing the plug, no difference, swapping the coils to see if the coil is causing the issue, swapping the lines on the switch box, again nothing, the issue stays on the middle spark plug only. One thing to note here is that the middle spark plug hole was re-threaded and a heli-coil was fitted. I am not sure if this could be contributing to the issue, but the spark plug fits properly and we're able to tighten it properly. On top of that we also did a compression check and the middle cylinder is not lower compression, all 3 cylinders are just under 90 psi.
Not sure what else to try at this point, also tried a different brand spark plugs, was using NGK originally and tried Denso, no difference. Anyone has any experience with a similar issue and can point me in the right direction? The only thing that's on my mind at the moment that we haven't checked are the crank-case bleed check valves, however in the manuals I can't seem to find exactly where they are, does the crank-case need to be split to test and/or replace these? I attached some pictures of the motor, as you can see it still looks new.
Appreciate any help. Thanks for taking the time to read through.
