That depends what bike it's on,and what you've done to the engine.
On a standard Triumph Bonneville ('71 onward) it would be no.1 position (top groove on needle).Position 2 often works OK.
On a standard single carb engine,it would be position 2.

 

Hi Mr.Pete,

The needles on my T140V were set at position 2 when I got the bike and everything I have read on here indicated that it was correct.

Plugs have always been a bit black.

I'm going to set them to position 1 and see what happens.

Cheers!

 

I've always understood that going to the higher groove weakens the mixture - so that'd be the way to go.

 

Would the second posiion on a 197o 650 Bonneville cause low speed roughness but strong at high rpm s?

 

The needle taper doesn't start coming out of the needle jet until about 3/8 throttle.Between 3/8 throttle and 3/4 throttle,the needle position will have a noticable effect on the mixture.
Using no. 2 needle position doesn't usually cause any problem.

The size of the needle-jet will have an effect,even before the straight part of the needle is out of the jet.From just above idle to about 3/4 throttle,a worn needle-jet will richen the mixture.

It sounds like your problems are at 1/4 throttle,or less.Your slides should be no.3 cutaway.Even 3-1/2 cutaway will work,but it takes longer to warm up and respond.

Check the pilot jets first.A blocked pilot jet will make it lean at low throttle opening.A #78 drill ,or 0.4 mm drill,with an extension,is the best thing to check it with.A guitar string 0.016" or less,or even the 0.014" wire from a plastic bag tie will do.Pull out the idle mixture screw and the wire or drill should push into the carb body at least 1-1/2",if the pilot jet is not blocked.

An air leak at the flange O-ring,or around the idle channel at the float bowl gasket,will also cause a lean mixture at low throttle.The O-ring should be 0.100" thick,and there should be about 0.045" gap between the flanges.Don't over-tighten the flange nuts.

On the other hand,you could have a rich mixture cause by a worn needle-jet.This can foul spark plugs.If it's done 10,000 miles,it's probably worn.If you replace a needle jet,check the float level at the same time.

The best float needles are the latest aluminium ones,and the float level should be 0.080" below the float bowl surface.

 

Thank you,sir for the help.

Thank you.

 

Sea level

Let's not forget the relation to sea level. I'm in Houston (just at sea level) and have my needles up all the way, 250 main jets and 2 1/2 slides. What works where you are is what's best. Also, don't forget the old saying about ninety percent of carb problems being electrical.