I read in a forum somewhere that I can not find again that someone was getting 25 MPG with there Magna . They thought maybe there valves were too tight. I get that or less with 1600mi. on bike,is this a possability? Air cleaner is clean plugs are good shape but sooty tailpipes are also sooty.
If there has been any mods to the bike which it sounds like there has but anyhow sounds like you are running way rich and that would for sure bring your mileage waaaayyyy down ..... But the plugs would be sooty too so maybe a little more history if you can would help a bunch if you have it....
Don't know what bike you have, but the link you might be V65 MPG on the list. It is still near the top of the Garage forum at the moment. On my V45 I get 45-50 mpg, you should get better.
This isn't much info for you but it is all I am qualified for.
I found my problem when the guy I got the bike from admitted he had used a drill bit to unclog the low speed jets!! I put in new jets and this tankful has gone 113 mi. and still on main tank have to wait for warmer weather to know just how good but it,s better than the 65 mi. I was getting CB