[bjs65stang]

I'm looking at a Yamaha 4 stroke 9.9hp that has 90lbs of compression. Is that low?

[JB]

If both cylinders are within 10% of each other that is okay, BJS.

4 strokes usually come up higher than that but differences in guages and technique can cause the numbers to vary by as much as 50psi.

The probabilities that both cylinders have problems that lower compression to exactly the same level are about zero. One exception to this is a blown/leaky head gasket.

[bjs65stang]

The motor runs but it stalls when put in gear and on acceleration

[rodbolt]

without a year model, beats me.
most not all small yamaha 4 strokes have an automatic decompression device rendering a compression test useless. do a leak down test.
your describing a clogged carb.

[bjs65stang]

It's a 2000 model