2006 Yamaha 150 2 stroke

[TownsendsFJR1300]

Try re-checking the compression in the bad cylinder with a just a little 2 stroke oil in the cylinder and see if it rises.
Should it rise, it points towards the rings. If it does not, your probably looking at potentially a head gasket, corroded head.

How does the spark plug in the bad cylinder look?

Also, what "looks like it's repaired before", please elaborate.

[rodbolt17]

the old oil trick is worthless in a two stroke and terribly messy.
bore scope it or pull the head.
however this is not your intermittent issue.
mechanical failures do not come and go.

[TownsendsFJR1300]

the old oil trick is worthless in a two stroke and terribly messy.
bore scope it or pull the head.
however this is not your intermittent issue.
mechanical failures do not come and go.

I've used this method on small two strokes and it does in-deed work.

Were talking maybe two teaspoons and not messy at all.

Depending on the normal compression I've seen it raise usually around 30 PSI..

**I've had some (two exactly) engines, not run for years (2 stroke weed eater and chain saw)and won't start with fuel, compression, etc. Add a tad of oil to the top end, the engine would crank up and run great after that... I assume/think the little bit of extra sealing, helped pull in fuel...

[99yam40]

oil will not help his motor, and it will not tell him what his problem is with that cylinder.
inspection is needed