What the compression to be on a 2 cylinder onan motor?

When checking the compression on an outboard motor... you aren't looking for the final number as much as the percentages of variation. There are so many things that can vary the compression numbers, like throttle position, air temperature, and even the tilt of the motor on a four stroke, that the manufacturers don't put the numbers out. What you are looking for is less than 10% variation between all cylinders. So if 3 cylinders are giving you 120 psi and the forth cylinder is giving you 105 psi, that hole has a problem. In the old 2 stroke days what we would find is people were using air cooled 2 stoke oil, which has a much higher burn temperature, and fouling the piston rings with tar.

If you have 2 cylinders with little compression, you need to rebuild the motor.

Some engines have a low oil cutoff switch. If the oil level drops to a certain point it kills the engine. Your oil level may be close enough that after starting and running for a little bit the level drops enough to kill the motor. After sitting for awhile the level rises enough to allow the engine to run again check the level and add oil if necessary.
Hope this helps.
Bob

The head on the motor you bought is bad; or the head gasket is leaking; or the rings in the 2 center pistons are bad. 30 lbs is far too low of a compression, and 5 lbs is garbage. They should be closer to 80 or 90 psi and not have more that 5% variance between the cylinders.
The first thing you will have to do is pull the head off and check it. Or you can take it back to the salvage yard you got it from, show them the results from the compression test and either get another motor, or have them refund your money.

Does it bog down? if so have engine checked for ability to deliver HP. Cylinder compression, leakdown test. When was it tuned?

A mechanic doesn't know why the motor
has low compression ?

Wouldn't you conclude that person has no
test equipment or doesn't know how to use it

Obviously you have bad valves or a failed head gasket,
if only two cylinders are an issue

All four cylinders should have over 100 #'s of pressure-ideally over 150#'s-and they should be even or close to even.
You could take off the timing belt cover and check the timing, but it sounds like 2 bad cylinders-which could be a valve train problem.

Sounds like low compression. Check the compression or have a small engine shop do a leakdown test on it for you

It is not okay to run with gas in the oil ! I assume you mean the lubricating oil? If you are getting gas there you need to perform a leakdown test on the cylinder to make sure the rings are not shot, and also the intake valve seal. A compression test will not reveal this.