I once owned a 200 GT Johnson and it was a 89-90. Something simliar happened to me. It ran incredibly rich and almost quit running. There is a fuel enrichment valve that provides the engine with more fuel when started cold. I was told it was very common for going bad. When it does, it dumps large amounts of fuel directly into the engine bypassing the carbs. Yours might not be as bad as mine was, but mine fouled the plugs and basically the engine almost didn't run. Plugs were dripping wet with gas.
Plus, an over-rich condition will almost always cuase a loss of performance, unless in a modified case where you can supply more air into the cylinders to make that perfect balance. But, in your case, you have more fuel and the same amount of air as always and in return you get lower RPMS and lower power.
Leaner conditions always run higher RPMS and make more power--in an outboard---until you get TOO LEAN---then it's KABOOM! Hope this helps.
Sure wish I could rembmer the exact name of that part. Dealer should know!