From Continuouswave.com
It is also advantageous if the primer bulb can be position in such a way that during priming it can be oriented vertically, with the direction arrow pointing skywards. This will allow gravity and the weight of the fuel inside the primer bulb to help with the operation of the one-way valves. If the primer bulb is oriented with the flow arrow pointing downward, gravity and the weight of the fuel in the line above the inlet check valve may spoil its operation, and the primer will not work properly.
This subtle but important point—orienting the primer bulb skyward—is often overlooked in many installation, but it really helps the primer bulb to do its job. This trick was shown to me by an experienced Mercury outboard mechanic, after I complained that the new primer bulbs he had installed did not work well. The difference in results is amazing, and just by changing the orientation of the primer to vertical, its operation is much improved. In just a few squeezes it should be possible to fill the bulb with fuel.
Primer Bulb As Diagnostic Tool
The primer bulb is also a diagnostic tool that will tell you a number of things about your fuel system.
Engine Stalling or Running Roughly at Speed
If your engine begins to stall or run roughly several minutes after starting and when running at higher speeds, use the primer bulb to force fuel to the engine. If this restores the engine to smooth running, you have a fuel supply problem. There may be a restriction in the fuel line, or your engine fuel pump could be defective. If this happens even at idle speeds, the primer bulb may not be working properly.
Primer Hard to Prime
If the fuel line is difficult to prime, you likely have an air leak in the system. Check all fuel hose connections. You could also have a defective primer bulb, or not be orientating the primer bulb properly during priming. A likely place to investigate is the fuel disconnect on the engine. Be sure the connector is properly seated and is making an air-tight connection.
Collapsed Primer Bulb
If the primer bulb collapses after the engine has been running or stays collapsed while priming this indicates a fuel line restriction upstream from the primer bulb. A defective check valve in the primer bulb could also be the cause. The fuel line filter or the fuel tank pick up could be blocked. A blocked fuel tank vent could also cause this. A quick way to check the tank venting is to temporarily open the fuel tank filler fitting. For some reason the fuel tank vent line thru-hull fitting is a favorite spot for wasps to build mud nests.
Primer Bulb Never Gets Firm
If you can fill the bulb with fuel but it never gets firm, you probably have a fuel leak downstream of the primer bulb. This could be a stuck float valve on a carburetor, or a fuel leak in the engine fuel pump. Many engines use a diaphragm pump, and if the rubber diaphragm has a pin hole leak, gasoline will be forced past it into the engine crankcase. It should only take a few squeezes to get a primer bulb to become firm.
If you have a fuel disconnect downstream of the primer bulb, disconnect the fuel line from the engine and see if the primer can now be pumped to firmness. If so, your leak is in the engine.
Primer Bulb Gets Soft While Engine Is Running
Most primer bulbs will lose some firmness when the engine is running. This is normal. The bulb should not run dry of fuel or collapse in suction, but it will tend to lose some of the hard firmness it had when initially primed.
Because the primer bulb never gets firm and you have a rough idle my money would be on a stuck float valve in one of your carbs
