Yamaha 25 running lean

Dirty carb, sticking reed, or air leak will cause that.
Small amouint of water entering the cylinder will do it too.
Only things I can think of that would affect the fuel/air mix ratio.

 

If the compression is okay and no bolts are missing or loose I would check the reeds. If there is no silencer on that engine and it is stored outdoors a rag stuffed in the carb will keep muddobbers and lizards out.

Frank_S

 

Once a fuel line has been primed, there should be minimal bubbles going to the carb.
Large bubbles means somethings not right in the fuel lines.
The fuel system should be airtight, from the bottom of the pickup tube
all the way to the tip of the float valve. Any leak between those 2 locations
can cause engine problems or a fire hazard in your hull.

 

The way a 2 cylinder 2 stroke crankcase is engineered
each cylinder is fed by it's own intake chamber, and the flow
to each chamber is controlled by it's reed valves. If a reed is sticking
it interferes with the amount of fuel/air mix entering the intake side of that cylinder.
An air leak into that chamber will cause the same problem, possibly a reed block gasket split.
If there is a coolant leak due to a failing gasket or a hairline crack in the block
that small amount of water will vaporize in the intake chamber due to engine heat
and change the amount of fuel/air entering the firing chamber.

Take a look at the linked diagram, and remember you have 2 cylinders.
One cylinder is working as expected, the other has a problem.


http://science.howstuffworks.com/transport/engines-equipment/two-stroke6.htm

 

If a reed is broken or cracked, that's an easy to spot problem.
But an air leak is rarely found by just looking at the reed block.
A mechanic who can do that has way more skills than I do.
Only way I know how to find an air leak is to pressure test.
Rotate the flywheel until the piston intake port seals,
introduce compressed air (4 to 8 psi) into the intake side with an adapter.
Then listen for the hiss of escaping air or brush the intake
from the reed block to the engine block with soapy water
and look for bubbles. It's not going to be a big leak.
Your engine is still running, just running lean.

Plenty of how-to's on the web.