I think 1st of all, many of the fox motors were designed to run on high nitro. I had a couple of Eagle 60 that required 25%. Probably the 3rd screw is for the barrel adjustment. Start with the HS needle about 2 turns and the low speed about the same for starters. Make sure the spray bar is not plugged up. See if you can blow fuel through the needle valve opening with the needlevalve removed and putting your finger over the fuel inlet coming from the fuel tank. You should see a spray coming for the spray bar when you do this. If not then it is plugged up. Hope this might help.