H/L screw confusion

[RobRainford]

This thread is mainly for future reference, the screws on the carbs on stihl saws. Am I right that the h is for top end revs and the l is for rich/leanness? If I open up an exhaust on a saw and it needs richening, I turn the L screw up right?

 

I'm a bit foggy with the details. Cheers!

[Johny Walker]

go & read the stihl manual

[pgkevet]

go & read the stihl manual

 

That's mean:001_smile:

 

 

I don't know how much i can help.. not owning a chainsaw yet but I do fly r/c helicopters including a 2-stroke petrol one (as opposed to nitro) and they are modified strimmer-type engines Zenoah with walbro carbs and 2 needles.. low and high... albeit modded and ported fro the zenoah heli engine base.

 

Irrespective of all the different opinions one gets it is a case of both needles need to be right. Yes you can close down the high needle and get an idle going but as soon as you give any revs it's going to be lean as all heck.

 

The plug is a better indicator than exhaust (unless using exhaust gasses) and your most important consideration is the engine temperature. The oil you use is another argument..don't grab just any old 2-stroke oil. Ideally one uses the manufacturers recommendations but with our heli engines we're trying to get max performance and oil brands become a big issue as well as oil percentages when pushing the limits.

 

I actually have a thermocouple temp readout wrapped around the cooling head though most folk use an IR gun at second fin space from the bottom on the carb side. For our engines we avoid going above 260F and try to shoot for 230F - but we are running them pretty hard.

 

For the low needle we aim to get a good idle with comfortable transition and a little stumble about every 20 secs. And on the high needle lean it just so as it backs off on throttle it's a clean return without stumbling or over-revving.. the check the plug and adjust the high from that.

 

You often have to start with one needle change then play with the other needle then go back to the first and repeat.. messing with both at the same time leads to madness:001_smile:. Listening to the engine under load and with load removed...checking for over-reving as load comes off.. usually richen on the low for that.

 

Higher oil percentages on the right sort of oil can help one avoid cooking the engine by going a touch lean - obviously too high an oil of the wrong sort and it's a stuck ring.

 

We reckon it takes 2 gallons of synthetic oil mix fuel to run a gasser engine in before dropping that oil percentage and getting full power.

[geoff]

Its all to do with altitude...............if your up the top of the tree adjust your H screw,once you get lower down its the L screw, I thought everyone knew that:001_tongue:

 

actualy they affect the mixture at different throttle positions.L at small throttle/tickover,and H at wider /open ,which is obviously related to engine revs,as they are a response of what ya finger does.

[RobRainford]

Cheers. I've dropped on with my 260, runs brilliant and starts reliably. Just a little fiddle and it's been great for ages now.

 

Ive bought a dual port muffler for the 660 so I'll have to richen it up a bit I think, there's a place nearby I could nip to if I'm unsure at all.

[David Goss]

careful and not turn the high up too much or you can blow the engine, i know a guy we used to call cesar cos he ceased so many by doing just that.

[geoff]

Cheers. I've dropped on with my 260, runs brilliant and starts reliably. Just a little fiddle and it's been great for ages now.

 

Ive bought a dual port muffler for the 660 so I'll have to richen it up a bit I think, there's a place nearby I could nip to if I'm unsure at all.

 

Yep,you will want to go anticlockwise(richen) up the high screw,and maybe the low too,you dont want to sieze it up.

[RobRainford]

Thanks Geoff. I'll check it with the spark plug anyway. I'd rather have it running slightly rich so it gets more liquid in there!

[tree_beard]

I'll check it with the spark plug anyway.

 

check it with a tachometer and set the max rpms properly, checking the plug is not a reliable method, and a 660 with modded exhaust is not something you want to teach yourself tuning by ear with... could be a very expensive lesson

[spudulike]

check it with a tachometer and set the max rpms properly, checking the plug is not a reliable method, and a 660 with modded exhaust is not something you want to teach yourself tuning by ear with... could be a very expensive lesson

 

Beat me to it, I tuned by ear and found I was around 1KRPM down.

The L screw gives the mixture at idle to low mid revs and dictates stable idle and pickup, the H screw dictates maximum revs with the modern saws also having rev limiters on the coil.

 

Setting the H screw lean will make the saw over-rev, cause the piston to overheat and this will melt it with seizure following shortly afer.

 

Manufacturers give a maximum rev setting spec, a tach allows you to set a saw to a margin below maximum revs or to go over if the saw is modded to do so - porting and muffler mods alow the engine to shift more heated gas resulting in a cooler engine so a leaner higher revving engine tune can be achieved.

 

To get the best out of a saw, a tach tune is really the best tool but a decent engineer wil be able to tune by ear and not risk damaging the saw but it takes experience.

 

So - understand the facts