Understanding the engine terms “rich” and “lean”?

[Marc Lewis]

Cheers Hoggin. Can you tell by the exhaust fumes also?

[Wolfie]

Good thread.

Can we now go into how altering the H & L settings changes the rich/lean ratio.

I was once told that it was important not to set the L screw below factory settings, i.e. less than 1 turn out on a topper, so i'm presuming that this will make the mixed too lean and risk seizure.

 

Also, could it be simply explained whats happening when a two stroke machine is referred to as "four stroking".

[gensetsteve]

With out a tacho its difficult to set a carb on a saw with out some experience. For a complete novice I would suggest if you can rev your saw off the face of the earth its defo to lean and will damage its self. If its cutting the timber fantastically well better than ever before its probably too lean and about to cook the piston. When you rev the saw at full throttle it should make a 4 stroking noise almost like a slight miss fire. If your saw stinks when you are using it has no power and is difficult to start it is probably too rich. If this looks wrong and confusing can one of the mods delete and I will have another go. I have taught alot of youngsters this way and they seem to get it straight away, I give them a service book and I just get blank looks.

[Dean Lofthouse]

You should not have to turn the h jet any more than an 8th of a turn either way from standard setting. On older saws the standard setting was 1 1/4 turn out and on newer saws normally just a full turn out

[spudulike]

Right.....in for a penny, in for a........

 

The carb low setting is primarily for idle and low to mid range running - a saw will generally idle and rev up a little even with the H screw totally in.

 

Generally you take the L screw in until the engine revs up and starts to die and then undo it around 1/4 to 1/2 a turn. This will allow enough fuel/air in for the engine to idle and to pick up to mid revs well.

 

Failure to get this right will not seize an engine - it will either idle badly, not pick up or smoke like a tart on a street corner:thumbup:

 

The H screw - different can of worms - it is there to give the engine the correct level of fuel/air at mid - maximum revs. It also is used as a kind of fuel based rev limiter! Remember that as it is important.

 

You want the saw to peak at maximum manufactures revs, this is achieved by adjusting the saw carb H screw so it goes from firing on every stroke (Two stroking) to firing on every other stroke (Four stroking) at wide open throttle and at manufactures recommended revs - a tacho is a damn useful bit of kit on the smaller higher revving saws.

 

The way this is achieved is the carb throws in so much fuel that it starts to flood the spark every other stroke and stop the machine over revving, when you lean down the mixture, the saw fires every stroke and will also have less fuel/oil to lubricate it. In bad cases where the H screw is very lean, the piston will melt on to the cylinder causing lean seize through too little fuel oil being present in the engine and the extra friction between the cylinder and piston.

 

So - the H screws prime function is to control the fuel at high revs but also has a secondary function of stopping the engine over-revving. Limited coils are also fitted now but that DOES NOT stop an engine running lean - it is just another safeguard and works in conjunction with the carb!

 

Hope this makes sense!

Edited October 15, 2012 by spudulike

[gensetsteve]

Right.....in for a penny, in for a........

 

The carb low setting is primarily for idle and low to mid range running - a saw will generally idle and rev up a little even with the H screw totally in.

 

Generally you take the L screw in until the engine revs up and starts to die and then undo it around 1/4 to 1/2 a turn. This will allow enough fuel/air in for the engine to idle and to pick up to mid revs well.

 

Failure to get this right will not seize an engine - it will either idle badly, not pick up or smoke like a tart on a street corner:thumbup:

 

The H screw - different can of worms - it is there to give the engine the correct level of fuel/air at mid - maximum revs. It also is used as a kind of fuel based rev limiter! Remember that as it is important.

 

You want the saw to peak at maximum manufactures revs, this is achieved by adjusting the saw carb H screw so it goes from firing on every stroke (Two stroking) to firing on every other stroke (Four stroking) at wide open throttle and at manufactures recommended revs - a tacho is a damn useful bit of kit on the smaller higher revving saws.

 

The way this is achieved is the carb throws in so much fuel that it starts to flood the spark every other stroke and stop the machine over revving, when you lean down the mixture, the saw fires every stroke and will also have less fuel/oil to lubricate it. In bad cases where the H screw is very lean, the piston will melt on to the cylinder causing lean seize through too little fuel oil being present in the engine and the extra friction between the cylinder and piston.

 

So - the H screws prime function is to control the fuel at high revs but also has a secondary function of stopping the engine over-revving. Limited coils are also fitted now but that DOES NOT stop an engine running lean - it is just another safeguard and works in conjunction with the carb!

 

Hope this makes sense!

 

Yes much better put

[Rich2484]

I was wondering today the beat answer I could give to this thread about h l screws. Guess I don't need to now.

 

Well done spud.

[TimberCutterDartmoor]

Right.....in for a penny, in for a........

 

The carb low setting is primarily for idle and low to mid range running - a saw will generally idle and rev up a little even with the H screw totally in.

 

Generally you take the L screw in until the engine revs up and starts to die and then undo it around 1/4 to 1/2 a turn. This will allow enough fuel/air in for the engine to idle and to pick up to mid revs well.

 

Failure to get this right will not seize an engine - it will either idle badly, not pick up or smoke like a tart on a street corner:thumbup:

 

The H screw - different can of worms - it is there to give the engine the correct level of fuel/air at mid - maximum revs. It also is used as a kind of fuel based rev limiter! Remember that as it is important.

 

You want the saw to peak at maximum manufactures revs, this is achieved by adjusting the saw carb H screw so it goes from firing on every stroke (Two stroking) to firing on every other stroke (Four stroking) at wide open throttle and at manufactures recommended revs - a tacho is a damn useful bit of kit on the smaller higher revving saws.

 

The way this is achieved is the carb throws in so much fuel that it starts to flood the spark every other stroke and stop the machine over revving, when you lean down the mixture, the saw fires every stroke and will also have less fuel/oil to lubricate it. In bad cases where the H screw is very lean, the piston will melt on to the cylinder causing lean seize through too little fuel oil being present in the engine and the extra friction between the cylinder and piston.

 

So - the H screws prime function is to control the fuel at high revs but also has a secondary function of stopping the engine over-revving. Limited coils are also fitted now but that DOES NOT stop an engine running lean - it is just another safeguard and works in conjunction with the carb!

 

Hope this makes sense!

 

Quality