Setting H screw

[chainsawMal]

So yous have mentioned before about setting for 4 stroking and by ear.

 

But to give me a somewhat more of an idea, going by the book.

 

Book states;

Max power @ 8500 rpm

Max RPM @ 10000

 

My initial thoughts were out of the cut on WOT to set at 8500 cause if i hit 10000 then my engine will over heat and die.

 

But now i think that the engine will likely do as much as 12000 rpm. Is that correct??

 

So what i should do is gradually get to 10000 and set it at that, so that when my saw is cutting then it will be under load and cutting at 8500 at its best rpm for cutting.

 

Is that a good simple guide for Dummies??

[GardenKit]

Just like you say, you will tach it at 10k. At this point it should ideally just be on the edge of 4 stroking if you do it by ear. But, High speed equates to low torque, so as the load comes on and the revs drop the power will increase. In your case this is 8500 but you will not know this, or easily be able to test it. Just trust that it happens.

The 10k is a bit low for many saws though, what saw is it?

[chainsawMal]

Forgot to mention i would be using a tach!!

 

 

what saw is it?

 

Its a cheap Chinese one, yeah yeah, but it was dam dam cheap!!

[GardenKit]

Forgot to mention i would be using a tach!!

 

 

 

 

Its a cheap Chinese one, yeah yeah, but it was dam dam cheap!!

if it does what you want then who cares where it was made?

[chainsawMal]

So if i maxed out the H screw (which I'd be mad too), then my saw would probably scream out load at 12000-13000rpm??

[GardenKit]

So if i maxed out the H screw (which I'd be mad too), then my saw would probably scream out load at 12000-13000rpm??

Maybe, but not for long.

 

Why not try it? It is after all only a cheap saw:sneaky2:

[mikerecike]

Just like you say, you will tach it at 10k. At this point it should ideally just be on the edge of 4 stroking if you do it by ear. But, High speed equates to low torque, so as the load comes on and the revs drop the power will increase. In your case this is 8500 but you will not know this, or easily be able to test it. Just trust that it happens.

The 10k is a bit low for many saws though, what saw is it?

 

 

please remind me what 'fourstroking' actually is. I'm sure I'm tuning to it but have no idea what the term refers to or how to confirm it??

 

cheers

[spudulike]

please remind me what 'fourstroking' actually is. I'm sure I'm tuning to it but have no idea what the term refers to or how to confirm it??

 

cheers

 

Four stroking........a two stroke fires and creates a power stroke on each crank cycle - a fourstroke does it every other cycle - hence the terms four stroke and two stroke - a stroke is an upwards or downwards movement of the piston.

 

On chainsaws, they originally didn't have rev limiters to dictate maximum revs so manufacturers used the carb to set maximum revs. If the carb is leaned out too much, it over revs as it continues to fire on each stroke up to OMG revs:thumbdown: the result is a melted piston:thumbdown:

 

What we do is to set the H screw so the saw goes to max specified revs and then starts to almost flood the saw on every other stroke by putting that much fuel in to the cylinder, it stops the spark plug sparking.

 

When the saw is thumping when revved flat out, you are firing on every other stroke - the carb is acting as a rev limiter and the saw is acting like a fourstroke engine - as soon as you cut with the saw, the four stroking dies away due to the engine being under load and returns to firing on every stroke.

 

The modern rev limiter does part of this job but the carb still needs to be set correctly and just gives an extra level of protection!

[mikerecike]

Four stroking........a two stroke fires and creates a power stroke on each crank cycle - a fourstroke does it every other cycle - hence the terms four stroke and two stroke - a stroke is an upwards or downwards movement of the piston.

 

On chainsaws, they originally didn't have rev limiters to dictate maximum revs so manufacturers used the carb to set maximum revs. If the carb is leaned out too much, it over revs as it continues to fire on each stroke up to OMG revs:thumbdown: the result is a melted piston:thumbdown:

 

What we do is to set the H screw so the saw goes to max specified revs and then starts to almost flood the saw on every other stroke by putting that much fuel in to the cylinder, it stops the spark plug sparking.

 

When the saw is thumping when revved flat out, you are firing on every other stroke - the carb is acting as a rev limiter and the saw is acting like a fourstroke engine - as soon as you cut with the saw, the four stroking dies away due to the engine being under load and returns to firing on every stroke.

 

The modern rev limiter does part of this job but the carb still needs to be set correctly and just gives an extra level of protection!

 

I need to digest that and experiment with it - thanks a lot Spud - that's the first time i've had that explained - appreciated

[spudulike]

I need to digest that and experiment with it - thanks a lot Spud - that's the first time i've had that explained - appreciated

 

No worries bud, not many do know this - most just know what the effect is without knowing why it happens:lol: