You should be getting twice that much.
I didn't think that plug type would affect mileage, but if that and a throttle sync is all you did.
You could also get the right heat range of plug, and try and swap out the ones that are in there now.
I would try and check the sync again to see if it is still in spec, and check all the hoses are on correctly. If a vacuum hose is off or cracked I think that would cause some loss of mileage.
You must be able to smell gas from the pipe with it running like that.