Chainsaw Repair

Put a fuel line on it year or so ago been on the shelf since.  Got it out to service it as my other MS290 I sold keeps coming back w/little probs.  Told the guy I would swap it if he wanted or buy it back which I really don't want to do.  Fired it up 30 sec running gas was spewing out around the gas cap.  Shut off opened cap tightened made sure the flippys were locked 10 sec. gas spewing.  What the heck never had that happen before.  What causes that?  Bad carb diaphgram.  Plan on kitting it.
Shep

You could test the tank vent with your Vacuum/pressure test set up that you use to check engine crankcase pressure/vacuum integrity.

I checked the gas tank vent on the last chainsaw (025) I did with my Mity-Vac 6500 and the tank vent performed as the Workshop manual says it should. The vent passed both pressure and vacuum slowly. Use about 7 psi pressure or 15 inches of vacuum (About 1/2 atmosphere)

There should be nothing making gasoline pressure in your gas tank?  However, gasoline has a vapor pressure of 7 or 8 psi, so the tank should build pressure with the chainsaw just setting and not running if the vent is plugged? Running a chainsaw should not have an effect on tank pressure?

Trying to consider how the MS 290 is made . . .

This is a clamshell design so there is not connection possibilities like a leaking gasket in the crankcase. The 290 has no gaskets about the engine except the exhaust gasket.

There are only two connections to the tank.
One is the fuel line
and the other is a combination vent line/primer bulb return line

There could be a problem with this combination system of the vent line but there would have to be additional air pumped into the tank some way to build pressure while running?

Normally, the purge pump is only pulling fuel through the carburetor, so the fuel itself is merely in a loop.

If the fuel line from the tank to the carburetor has a air leak, Then there could be a source of additional air? But with all that pressure there would certainly be a fuel leak also in the fuel line?