Low speed needle tuning.

[Malky]

I hear a lot of talk about the high speed needle being adjusted when tuning your engine. But what about the low speed needle. Is the low speed needle just for starting and idling. If it starts and idles does this mean that low speed needle is adjusted correctly?

 

[Rolex]

The LSN is tuned to provide a quick launch off the line, and the HSN takes it to top end.

 

[exmark93]

high speed- adjusts carb during high speed range, low speed- adjusts carb in the low speed range. if you lean your lsn enough you can raise your idle which is a bad thing cause its a pain to retune the engine from that point. the idle screw should be what you use to adjust the idle

 

[MotoGod]

The LSN really only affects idle to @ 1/4 throttle. If you look down your carb barrel....you will see the LSN inside. If you turn on the car and pull the trigger you will see the needle slide out.....when it's clear of the seat it sits in.....that's the point where the LSN does nothing and the HSN takes over ;-)

 

[Malky]

As I understand it the HSN is adjusted correctly when your engine temp is around 230 and you can see a streem of smoke coming from the exhaust. But how do you know if your LSN is adjusted correctly?

 

[Rolex]

Performance off the line. One thing to watch for....temperature. If you set the LSN too lean it will overheat quickly and nothing you do with the HSN will change that.
You need to pick the spot that gives you a good launch from a dead stop without going too far.

 

[2revo1maxx]

After making sure the idle gap is right and the hsn is set you can watch how long the idle 'hangs' after a speed run. It should only hang for a couple seconds at the most before idling down to a normal idle speed. It's very important to make sure you have the hsn set right. You want a good consistant idle and it shouldn't start to 'load up' for at least 3 seconds. It can be tricky at times so be patient and seal your engine if you can't get it right in 1 or 2 tanks of trying. Also use the 'pinch test' when in doubt.

 

[Malky]

The pinch test?

 

[Metalhead Matt]

After a high speed pass, pinch your fuel line as close to the carb as you can. Should take 2-3 seconds for the engine to rev slightly and die. Any more and your rich. Any less means lean.