I recently bought a used Panga Marine 18. I have seen the invoice and it has an 18 gallon tank.
I filled the tank and ran about half a day and engine acted like it was out of gas. Ran rough sputtered stopped and would not crank. I had a spare gallon. Put that in squeezed bulb and engine cranked and got me back to landing. Tried to fill it up but only took 8 gallons.
Could be a bad squeeze ball, clogged filter or clogged vent line/closed screw vent
A clogged vent line will cause what happened, and when you opened the fill cap to add the gallon, it may have relieved the vacuum… or do you have a screw on the top of your fill cap? That is a vent as well.
Should be able to tell by sight between an 8 and 18 tank, no?
My 6 gallon tank is about 24" L x 14" W x 12" T (guesstimating)
Or, is it hidden from view?