How to tell if your idle is rich or lean?

[trademe900]

hey guys, about to take my carbs off my 1989 to change out the 110 jets. while im at it, I want to set the pilot screws for the best possible low range and idle. How do you tell if your bike is running on the lean or rich side at idle?

some information that might be of use is if you crack/blip the throttle quickly from idle it bogs slightly before picking up, this is more pronounced when cold. It doesn't need any choke to start up.

[MexicanGringo]

Rev's usually bog at idle when its running rich, and hang when lean,rich should be worse when warm though. Idle it for a while then check the plugs, if black then lean them up a bit

[Cammo]

Idle it for a while then check the plugs, if black then lean them up a bit

The engine needs a rich condition to idle properly (a/f somewhere in the range of 12.5 - 13.5) so the plugs should be showing signs of running rich at idle.

Rev's usually bog at idle when its running rich, and hang when lean,rich should be worse when warm though.

Agreed. This is a good rule of thumb to determine which way they need adjusting.

[Malc]

The engine needs a rich condition to idle properly (a/f somewhere in the range of 12.5 - 13.5) so the plugs should be showing signs of running rich at idle.

I read somewhere in a book about carbs that idling the fuel/air ratio is way down nearer 7.0 - 1, but this was in a two-stroke biased book. Either way, unless something is disasterously wrong, idle is always going to be rich?

Cammo, do you subscribe to the theory that when the idle mix is correct and the bike is on full-choke cold it will rev at 4,000rpm? I did this with my old track NC30 - set the idle screws so it revs at 4,000 on full choke - and it was sweet on the throttle from idle... when set using the stop screw it would idle warm at 1,350 absolutely rock solid, but I did have the screw wound up to 2,500 as per the old racers trick

[trademe900]

What I don't get is one of my NC30's with aftermarket can runs at 4000rpm when cold on full choke and the 1989 stock exhaust kind of bogs and runs at 1500...

However they both do not need any choke to start.

what does that mean?

[Neosophist]

How cold it is? Maybe its rich?

Turn the pilot screws out to factory setting, these are usually on the lean side anyway.. turn each one out half a turn and see if the revs rise..

turn them out another half a turn and so on until the revs start to drop (too rich) then back them all in half a turn and your done.

This assumes carbs in good working order, float heights correct, valve clearances done etc.

Trying to sort carbs when everythign else is wrong will never work.