1987 Mercury 35hp missing when cold

[PhilAtOmni]

I purchased a 1987 Mercury 2 cylinder 35hp long shaft outboard, SN 0B177856, electric start with tilt and trim. Motor had been sitting unused for 3 years. Carb was rebuilt. I installed a new starter, replaced the entire fuel line from tank to fuel pump. Rebuilt the fuel pump, installed new plugs, removed and cleaned all electrical connections then routed and tie strapped all wires. Cleaned and filled tank with ethanol free fuel and measured oil. When put in the water, the motor starts right up, pumps water fine, but only runs on 1 cylinder until warmed up for about 10-15 minutes. Then all of a sudden the other cylinder kicks in and the motor runs perfectly, sometimes for a few hours, sometimes all day. What is the deal? Any help much appreciated. Thank you.

 

[Texasmark]

What kind of ignition system do you have? How about some "well focused" pictures of the engine, left, rear, right as close in as you can get and still show the whole power head and all components and wiring. When you say all of a sudden that smells like ignition thermal malfunction to me. Solution depends on ignition type.

 

[PhilAtOmni]

Ignition is CDI switch box # 114-7452K1, I am new to this and keep getting "Invalid File Data" error when I try to upload ,jpg file. Help.

 

[PhilAtOmni]

Gonna try again...

 

[Texasmark]

Go to the CDI site and browse around for anything associated with your switch box. They have troubleshooting guides among other things.

The trigger mechanism should be one pickup coil under the flywheel that feeds both trigger circuits with 2 magnets embedded in the under side of the flywheel, 180 degrees apart. With that easily manufactured, minimal component, common sense design both plugs get fired twice per rev. Doesn't matter as one cylinder is ready to fire the other just wastes the spark....no big deal. That means you could swap CDI high voltage outputs and see if the problem is in the CDI or elsewhere. It sounds like an internal problem to me but more info will certainly help to locate just where.......In later engines the CDIs were separate, like one per cylinder and you could easily swap the whole module ($95 per) and be done with it. This way you have to attack the right spot in the whole switch box.

I don't know if the spark plug wires can be removed from your CDI so that you can swap cylinders, without damaging them or not. Possibly the CDI troubleshooting guide will aid in answering this question.

Come back when you get some new info and we will continue.

 

[PhilAtOmni]

Tex... Found 2 butt splices in the trigger circuit. Removed them, soldered and covered with heat shrink.
Been out 3 times for 2 hours each and motor has run perfectly. Thanks so much for your help.

 

[Texasmark]

All right. Chalk up another one for the feeble minded guesser. Grin