no you will not as Bondo said, the muffs have to fit well to allow the water pump to prime...when you do a Cobra impeller there are a couple of little things to keep in mind:
that hose hooked up to the nipple on the housing and its fitting on the housing have to be open, it lets air out and allows it to prime, you can see the nipple I'm referring to in the lower pic of the housing above, see it on the right hand side, if I am not replacing the impeller that season I might just blow this out with a bit of compressed air since we get marine growth due to salt water moorage...
the oring gasket on the housing can slip out of place, I stick it in there with OMC triple guard grease to hold it from popping out
after you replace the impeller, leave the cover off the first time you run it on the water hose, check for leaks around the housing...
That all said this is the easiest to replace impeller there is, you can even do one in the water if you are careful lol!
they are usually good for at least 3 seasons if you don't suck up sand.
if you have a bad problem with it not priming and your muffs are good, then you may have rotted water tube gaskets in the lower unit. Not fun to replace but I've done it before. A bad overheat can melt them.
