Re: have REAL power loss now!
To all that are mentioning leaks in the intake, I have a question for you - if he's got a leak in the intake manifold, wouldn't he be running lean instead of awfully rich?
Anyway crabdance, do check for leaks, start the motor and get it up to temp. Then with it just idling, spray some carb cleaner where the intake meets the cylinder, where the carb connects to the intake tube, etc. If the idle changes, you probably have an air leak.
Your plug should not be turning black after one day. If you open up both of your carbs, you can compare the size of the jet. I'm wondering if your new jet is bigger than the old or something. Norman's carb thread should have the info you need to be able to compare them.