[pettaw]

It might be to do with the mixture. is the throttle butterfly electrical clicky thing set properly. I'm sorry I don't know the correct technical term for it. Check that it click's just as the throttle is moved from shut to open.

If that is OK, then check the continuity. The middle terminal is common and one side will connect when the throttle is closed and will disconnect when the click occurs. The other side should connect when the throttle is fully open. You won't hear a click on that side, at least you don't on mine.

Only other things that would cause an over rich mixture is a worn air volume meter, where you adjust the mixture. That's expensive to replace. Mine needs doing next year as I'm only getting 2.5% CO at idle with the screw all the way out, but I'm holding off as long as possible.

Hope that helps