1971 R5B needle jets pitted

[Greaser Greg]

Does pitting in needle jets, and worn needle affect idle? Seems to me that it will allow excess fuel past the needle, and cause rich mix. Plugs are wet. Timing is quadruple checked, no air leaks. Good battery, etc.

[teazer]

In theory the answer is that worn needle or jet should not have much impact on idle because the gap between the needle and jet is too small.  But if the jet is pitted and/or the needle is worn, the effective area through which fuel can pass may well be sufficient to allow fuel to pass through and make the bottom end too rich.

But before you replace either or both, are the slow jets NEW genuine Mikuni?  If they are old or non genuine, it is very possible that they may flow too much fuel.

Are the primary (pilot) air passages completely clean?  If they are blocked, it will run rich.

Are the pilot jet outlet and bypass orifices clean and looking more or less stock or are they blocked or oversize?

What is the FUEL level set to?  Forget about FLOAT settings.  They are just a way to get the right FUEL level on a new carb.  Set the fuel level to 3mm below the float bowl gasket surface.  Not the gasket, but the surface on the carb that it sticks to.  People set the float level and hope that it gives then right fuel level.  Sometimes it does but when in doubt check it.

Wet plugs could also be timing a long way out or a poor (weak) spark.  Low compression also makes it hard to light the fire and get a clean burn.  Old plugs or points also don't help.

Other things to look at include the air cleaner and exhaust system.  try running it without an air cleaner just to see if it makes a significant difference.

A shorter than stock piston skirt on the inlet side, or ports with too much duration on the inlet side will also be problematic.

[quocle603]

it impacts your mid-range throttle position. Check your pilot jets and the passage of the jet. CLEAN EVERYTHING and USE COMPRESSED AIR :)