You may have a problem that I have run into with replacement carburetors for Onan. If you did not buy it for the direct model number, but for the basic engine type, that may be the problem. Many Onans are run with a gravity fuel feed (squat for pressure) and so have huge opening in the needle seat. This means that almost any pump will over power the float system.
I don't what you can do if this is the case, because I had to rework that seat to a smaller port size.
If the pump you got delivers more than a couple of PSI, that can do it. Check the information for that pump. Low pressure pumps are available, but you have look for them.
Good Luck
Matt
