help needed please

[paulj1014]

I have a 1990 yamaha 40hp 3 cylinder (40SD model number) it is only running on 1 cylinder. I have cleaned all 3 carbs. replaced all plugs, cleaned/swapped reed valves, and have compression @105 psi on all 3 cylinders.I have also swapped plugs wires to check if it could be those but they all work on the good cylinder I know it is only running on one cylinder b/c I have taken spark plug wires off the top and bottom plugs and there is no change in in engine performance. I am truly at a loss for what could be causing this. I have spark, compression and fuel. This is not a trick question this is really happening. if anyone can help I would greatly appreciate it.

[rodbolt17]

first thing,make or buy a spark tester.
verify good spark at 1/2" gap minimum.
that system uses 3 pulser coils and 1 charge coil.
lose a pulser and that cyl quits.
lose a charge coil and all 3 quit.

[throrope]

These outboards fall under the old gas or spark rule with spark the least likely.

I did see a video on youtube about a newly rebuilt powerhead that catastrophically failed because the schleps didn't install O rings between the carbs and powerhead. Since you have compression in all three, your problem is minor.

I suggest returning to the fuel connection and systematically and sequencially checking every device in an organized and documented process that eliminates the part as a source of difficulty before moving to the next.

Take the upstream fuel system out of the equation. I rebuilt my carbs over two weekends to promptly undo my work within twenty minutes due to contamination.

When you say you cleaned the carbs, was this cleaner through the throttle body or a compete disassemby and verification of every air and fuel passage? If the former, you're at square one.

I also suggest getting the service manual through Yamaha Publications online. One use will pay for it.

If your near enough to Wanchese NC, you can easily get back on the water by bringing it to Tom at Joe's Marine Service.