2003 115 Two Stroke Missing at low RPM's

[btomlins]

My motor has started missing at low RPM's recently. I thought fouled plugs and changed them but it didn't completely eliminate the issue. These models are the ones that only use two cylinders under a certain RPM and then kick the other two in when throttling up. It seems once I get the boat up on plane it runs ok. Where do I start checking components? Coil, Ignition module?

 

[Bt Doctur]

Not telling us the type of motor makes it impossable to give any kind of answer

 

[btomlins]

As you can see from the title of the post it is a 2003 115hp mercury two stoke. sn OT792840.

 

[achris]

The 'dropping' of the cylinders is done by leaning out the lower 2 carbs. I would suspect you have the opposite problem, one of the carbs is running too rich and causing the cylinder to fire intermittently. I've seen that before. I would remove the spark plug leads from the lower 2 cylinders (and ground them out) and see how it runs. If that cures the problem, put one back on at a time. Which ever one causes the miss, clean that carb out. Actually, if one carb needs cleaning, they probably all do....

Chris.........

 

[Bt Doctur]

My motor has started missing at low RPM's recently. I thought fouled plugs and changed them but it didn't completely eliminate the issue. These models are the ones that only use two cylinders under a certain RPM and then kick the other two in when throttling up. It seems once I get the boat up on plane it runs ok. Where do I start checking components? Coil, Ignition module?

Must be going blind, where does it say mercury?

 

[achris]

Must be going blind, where does it say mercury?

It doesn't, but I don't know any other engine that drops 2 of the 4 cylinders at low revs. So by deduction, it's a Merc....

 

[btomlins]

My bad. I figured if I posted this under the Mercury/Mariner sub category it should be self explanatory so excuse my error.

 

[ondarvr]

When someone clicks on the "new topics" or "today's posts" sections it just lists the threads in one group, not by category (although in smaller print it says the category), so once you open it you don't know the the category, it's not listed at that point. sometimes it's obvious what's being discussed, sometimes it's not. So you have to back out of the thread, look at the category and then click on it again.

 

[hunter200524]

On the starboard side of the engine there are two check valves that screw into the engine block on the bottom two cylinders behind the plate that holds the electronics.They run off the accelerator pump system. If they are not functioning properly they will allow fuel into the bottom cylinders causing your misfire problem. If you can pinch off the accelerator pump lines that runs to them and it clears up that would be your problem. They can be bought new, or if you want to try removing them and taking them apart and stretching the spring out that works too. There are also check valves behind the reed plates that could cause erratic idle.

 

[Texasmark]

It doesn't, but I don't know any other engine that drops 2 of the 4 cylinders at low revs. So by deduction, it's a Merc....

+1. I know because my service manual for my 90 triple covered 60 Jet thru 125 4 cylinders with the cylinder deactivation below 2500 rpm. Wink!