2005 Mercury 25/20 jet missing

[RiverBlazer]

So I've got a 2005 Mercury 25/20 jet where to me it is running on one cylinder at idle or low speed then when I give it a little throttle it is like I flip a switch and the second cylinder kicks in and runs perfect all the way to full throttle. It has always ran perfect and started very easy before this issue. I always run the gas out every time. I run non ethanol gas and put a little star tron in the tank as well. It doesn't sit for more than a couple of weeks, fish in winter as well. I called a boat dealer and told the mechanic what was going on and he mentioned checking the pilot jet, I took the carb off and apart and the only jet I saw was the one inside the float bowl drain screw which I thought was the main jet. Anyways there was no dirt inside. To me I'm thinking a coil is going bad or maybe the cdi box. Anybody had this issue or can somebody give me some advise on what you think. Thanks in advance.

 

[gm280]

Well :welcome: aboard RiverBlazer. Nice to have you join us..

Since you stated you took the carb (singular) off, then both cylinders have to be working from that one carb. So that rules out the carb issue. However, you stated that it seems like only one cylinder is working until you get up in the RPMs range from idle. So if you think that is happening, remove one spark plug boot at a time during that idle time to see if the engine change RPMs. If it does, reinstall that plug boot and remove the other one. If both seem to change the RPM range, then they both are firing. However, if you remove either plug boot and it doesn't change the engine's RPMs, then that cylinder is not firing. I would then swap spark coils around to see if the problem follows the coil. If so, you just found your problem. If it doesn't follow the coil but still doesn't fire on the same cylinder, you have whatever feeds the spark coils as your problem. There is a switch box and a trigger assembly. But you will need you engine's serial number to determine which part you would need. JMHO

 

[RiverBlazer]

Thanks for the welcome and great advise! I'll try to get on it this weekend and get it figured out. It's been doing it for the last few trips and I don't want it to finally quit all together.