2000 Mercruiser 3.0. Problem started with blown headgasket, shaved head replaced gasket only to blow again. complete teardown and rebuild with all new parts and had block deck checked. Machine shop had to cut .0018 off to level deck. Knew there was a problem when I bought it so no scoffing. Rebuilt Mercarb 2bbl. Might sound funny but my ASE Cert. goes back to 1966. Only 3 adjustments on carb. 1. Linkage, 2. Idle cam screw and 3. Mixture screw. When all 3 are set IAW seloc manual Idle is 1200-1400 RPM, back out cam screw all the way Idle is 1200. Mixture screw setting is 1 1/4 turns from seat, turn it in 1 turn idle drops to 1000 RPM. To get it lower the timing has to be retarded about 8-10 degrees which will only lower it to 900 RPM on my tach not engine tach. OH BTW Linkage was set to absolute zero, fully closed. Any Ideas? I'm considering a new Carb.