LSN HSN please explain!

[trippinout]

OK, everything that I have read says that you tune the HSN then the LSN. WHY? It seems counter-intuitive to tune high speed first. Wouldn't you want the low speed to run good first? Then tune high speed?

 

[Chunky Monkey]

The HSN is also what helps to set you running temps...Tune the HSN until you get a steady trai lof smoke and it doesn't sound like your bogging or you exhaust is gargling at full throttle. Then you'll tune the LSN until you get the quick throttle response from a stop. If, at full throttle, your truck makes a wrapping sound where the motor sounds like it's fading in and out, then you're too lean. Easiest way to tune the HSN would be with a temp gun....

 

[trippinout]

Got motor tuned (for most part). Does great for a few passes then it bogs and barely runs from dead stop. Just couldnt understand the whole tune hsn first thing.

 

[jrstorm]

the hsn is what meters the fuel coming into the carb. the fuel comes in to the carb through the hsn first then the lsn meters that fuel so it idles correctly.so the overall runing of the engine is controled by the hsn. when you tune the lsn it is controling the ammount of fuel allowed in by the hsn[so the hsn needs to be set correctly before the lsn can be addjusted] so if the hsn is too lean then the lsn will cause the engine to run leaner until the carb is opened further. the lsn cant be set correctly til the hsn is set correctly .the way the carb works is the lsn is in the throttle barrel so it controls it til the engine throttle is opened past idle and part of mid. the lsn slides with the barrel and when the throttle is opened past idle and part of mid,once the needle slides out of the lsn jet it is now running only on the hsn . so tuning the hsn is more important and the lsn is a fine tune for throttle response at low end. if you try to tune the lsn with the hsn not set correctly then the balance of the engine will be off and the engine wont run properly. also sometimes when the lsn is too lean it can cause the engine to run too hot. the lsn is a small addjustment like 1/8 turns or less. when the engine needs to be tuned or is acting up always tune the hsn first then once it runs correctly then the lsn can be tuned to give a crisper take off.the best thing to do when an engine isnt running correctly is to reset the carb to factory settings and start over.[a temp gun is needed to correctly addjust the engine .] then you start leaning the hsn a 1/4 turn leaner at a time til it runs good and has a good smoke trail. once the engine is running good then you can do the fine tune addjustments on the lsn for throttle resopnse. i hope this helps...

 

[trippinout]

Thank you! That's exactly what I was looking for. Now I know how it works, it should help my tuning ability. It's been raining for past 2 days, so I can't work on the truck!!