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Are there 2 LSN tuning windows?

Are there 2 LSN tuning windows?

Old 11-17-2014, 03:25 AM
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Default Are there 2 LSN tuning windows?

I have an OS carb.
set the HSN TO 2 1/4 nice and clean

the temperature on the day was about 110 F / 40 C.

set the idle opening to about 1mm
started to rev and let the motor idle.
turned the LSN in until the high idle and subsequent drop disappeared.
turned the idle down. all was good.

problem was when i was racing the car had a rich bog just off idle.

I then went through and leaned out the LSN some more, lowered the idle etc and found a new idle window.

has anyone else experienced this and is there an explanation. thanks.
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Old 11-17-2014, 06:08 AM
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Sounds like you found a proper LSN tuning. Idle gap is underrated and it affects all needles. if people started to tune it less than 1mm most tuning woes wouldn't exist.
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Old 11-17-2014, 06:38 AM
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May have not been completely warm on the box and they will always tune different on the track. Now you can leave it alone, even if it sounds horrible when you start it up next time don't touch it till you get it on the track, then 1-2 hrs only
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Old 11-18-2014, 12:46 AM
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Thanks Frank.
I am relatively new to this tuning thing and I don't want to set it too lean.

If I set the LSN too lean how would the motor behave?
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Old 11-18-2014, 05:51 AM
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Originally Posted by pcook99 View Post
Thanks Frank.
I am relatively new to this tuning thing and I don't want to set it too lean.

If I set the LSN too lean how would the motor behave?
A lean LSN will show these 3 main symptoms:

- Idle raises (can be slight or drastic) after throttle released
- Idle 'pings' after a few seconds, as there is too little fuel in the crankcase for the engine to properly run
- You will hear a stutter (sounds kind of like a hiccup) when you pull the trigger after idling for a little while, as there is not enough fuel in the engine to have a crisp, clean acceleration. If the engine is extremely lean, your engine may even flame out on it's own or when you pull the trigger
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Old 11-18-2014, 07:13 AM
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There are more than 2 tuning windows for LSN. It's all depends on the idle gap. If you change idle gap you have to retune lsn and now you have a new window not each one is optimum. For fuel economy you should try tune LSN with least possible idle gap (0.6-0.7mm for most engines)
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Old 11-19-2014, 12:22 AM
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Thanks all, really good advice.

The "multiple tuning windows" has me intrigued.

Something to try next race day.
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Old 11-19-2014, 01:27 AM
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If the bottom is too lean, the idle will be erratic. You also have a chance of flame out at the end of the straight. One indicator is that when carrying your car from point A to point B and dip the nose down, revs go up almost immediately.

When you're not sure, the best thing to do is to tune from rich to lean. Even if that means if your tune is sorta there, but you're not sure which way to turn which needles you can richen hs and ls a few hours. The engine will be obviously rich top and bottom. Then keep leaning the top end needle until the car achieves the top end rpms. The bottom will be rich and it won't pull off well at this point. Once you have the hsn set, work on the bottom. Do a high speed pass down the straight. Let the car for about 8 seconds. Then gun it. The car will stumble. Lean the LSN an hour at a time continuosly repeating unitl it pulls off crisply and with out bogging. Adjust your idle accordingly through out. Then you're done.
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Old 11-19-2014, 07:43 AM
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Which OS carb are you using. The short needle J2 carb than came on he xzb tunes very different than the other J carbs.
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Old 11-19-2014, 01:57 PM
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short needle xzb carb j2.
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