The 3.0L Merc has a MerCarb carburetor that was known for this problem so they came up with a spring in the float needle. Had the same issue with my 92 shortly after purchase (bought new). The engine should idle in the 800 -900 rpm range in neutral which should put the idle at 600 rpm in gear. A lot of info online about the MerCarb and this problem. Hope this helps. Looking forward to another year on the water!
Dean (Go Big or Go Home)
The 91 1800SR with the 3.0L engine should have the same 2 brl MerCarb that I have on my 92 model. I have found that Nikki built those carb's for Mercury Marine so specs & re-build kits are available. You will need your engine serial number to get the proper kit, mine was OD803211 which has a 1389-9562A1 MerCarb carburetor. My rebuild was successful and now I am ready for the water. If you enter Mercruiser Carb Rebuild in your search engine on your computer (Google or others) you will find help for your issue. Good luck and let me know the results!