2004 2 cycle 15 hp only runs on one cyl

[r2sail69]

bottom cylinder not running, has 105 lb compression, spark with plug removed and running on #1, swapped spark plugs-same, swapped coils-same, what am I missing?

[99yam40]

If you have good compression and spark your problem is probably fuel to air ratio is too rich due to fuel pump diaphragm leaking into crankcase or too lean due to plugging in carb or bottom seal sucking in air

[r2sail69]

fuel pump is part of the carburetor, how can it leak into crankcase?

[rodbolt17]

what powers the fuel pump??

[r2sail69]

I don't know what powers the fuel pump or how it works however it is attached to the carb as you can see in the parts list from this site, use this url to see it!
2004 15MSHC Yamaha Outboard CARBURETOR Diagram and Parts

[99yam40]

pulse from crankcase moves the fuel pump diaphragm from what I have seen on motors, but I have not looked at a Yam 15.
Would not hurt to look at diaphragm to see it is leaking, just to make sure

[rodbolt17]

and still no one is thinking.
how and where does the poor pump work?
dunno if its your issue but I am trying to make you think like a trained tech.

[r2sail69]

Thanks everyone for you suggestions, it turns out there is a small pin hole in the fuel pump diaphragm, replaced it and all is well.

[rodbolt17]

see where we were leading with this?
single carb engine with good spark and compression.
cant be but so much.
glad ya got it fixed quick and easy.

[1fastcar]

Hmm!

I have the same problem. Bottom cylinder not running. With a single carb how would it only effect the bottom cyliner? Mine looks like weak spark or intermitant spark. Iv'e swapped the plugs and coils but its still the bottom cylinder. The engine runs but bogs when giving it thottle. I rebuilt the carb because I thought it might be from sitting too long and it still does it. When it's running I can pull the bottom plug wire off and it runs the same, pull the top wire and it dies right away. Does the CDI control the spark if it's not the coil or spark plug. If I hold it wide open in the water youcan hear the bottom cylinder trying to kick in but seems like weak spark.

Any thoughts?

[rodbolt17]

after testing spark and compression.
the fuel pump is driven off of crankcase pulses from?
the bottom cylinder crankcase passage.
its doesnt work on magic.