Tonka

I have a 95 Barracuda that has no low end. It's a 640 with a single carb and will climb onto plane only once or twice. Even when it does get on plane, it's VERY slow to do it. You can run it wide open all day long and it runs smooth and strong as long as you keep the revs up. If you idle down more than twice, it will no longer climb onto plane. The plugs seem pretty fouled and if you let the ski cool down, you can usually get it to climb on plane again and you can run a tank of gas through it as long as you keep moving. Is there only an idle mixture screw on this carb or is there a way to lean this mixture down. Do I just have to re-jet the stupid thing or am I completely missing something? I am still using the factory oil pump on it. Can these things start getting the ratio off and mixing in too much oil? By the way, it starts and idles perfect. Thanks in advance for any advice.