Pilot screw adjustment by ear.
This may be a pointless exercise if the pilot circuit plays a much greater role over the rev range than just idle, as it has been reported for various carburettors. However for interest's sake if nothing else.
On all of the Mikuni and Keihin carbs, that I know of, the screw is an air screw. As you screw it out you allow more air in, therefore leaner. On Dell'Ortos and Bings it is a fuel screw, the opposite. Adjusting a fuel screw by ear is very easy. Well it is for me. For many others it seems to be difficult. I do not know why. You adjust the screw to achieve the highest idle. Then lower the idle and move to the next cylinder and repeat. The more cylinders, the more difficult it becomes. With an air screw it is exactly the same process but more subtle. It takes more turning of the screw to achieve the same dramatic effect. It helps if you can hear the intake at the same time as the exhaust. I have seen it mentioned that you can adjust the fuel screws altogether. For example, if they are all at 1 1/2 turns out then take them all to 2 turns out. In other words do not set them individually but use the highest idle technique by moving them in unison. This might be the best way for a novice to start with, but I am going to describe the individual method because that is what I do. The unison technique means to find the best average, but it seems a bit rough to me.
"You adjust the screw to achieve the highest idle." Ok this needs a lot more detail.
1/ Set the idle at the lowest speed where the engine is idling smoothly, not labouring.
2/ Imagine your screw is at 1-1/2 turns out. Wind it in, slowly, until you hear the idle lose speed, a very slight drop. Now lets say that occurred after you had turned the screw 1/4 turn, so now it is at at 1 1/4 turns out.
3/ Okay wind the screw back 1/4 turn and you are at your start point again.
4/ Now repeat the process but this time winding out, again slowly. Lets imagine the idle faltered at 2-1/4 turns out.
5/ So you move the screw to the midpoint of the two points which will be 1-3/4 turns out.
6/ If the idle has risen then lower it and move to the next cylinder.
7/ As you repeat this process the number of turns between too lean and too rich becomes smaller and smaller.
If the compression is low or the carbs have vacuum or seal leaks then the carbs will not be responsive. In fact the more responsive the carbs are the better condition the engine is in and vice versa. For example, the pilot screw will do nothing if you have leaking valves.
The intake makes a gasping, moist sound when it is too rich. You will never distinguish it without practice and being able to hear the intake easily. Too lean does not seem to make much intake sound, just dies. Of course all bikes make a different sound too. Okay I better stop now. Sounds a bit like fairies in the garden doesn't it?
By the way if you cannot see the tip of your screwdriver then you could put a mark on the handle to make it easier to count the turns.
Given that most people do not seem to be able to do this with a fuel screw, which is the easiest, then it may be an impossibility for many with an air screw. I think some people just have an ear for it. Give it a try and focus on the sounds not the screw.
Good luck
Blair
