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I had to use an impact screwdriver, will be replacing them with new ones

Like Warren mentioned, i've seen similar when the lined "red-coat" tank material breaks down but you say it's not. You might drain the tank and have a peek down at the very back and bottom, where the petcock is. Or even pull the petcock and have a look-see. Someone could of in the past "tried" to of sealed or of removed a former coating and left behind crap at the bottom. Do you still have the pleated paper air filter or an over-oiled K&N? That certainly is a mess! 

I will have a better look in the tank to see if I have missed something, and remove the e tap, it has the standard paper air filter set up, 
thanks