High Speed Jet (Fixed Jet)

[sigafreddi]

I'm very surprised that the ms 460 standards jet size (#68) is larger than ms 660 jet (#64).

When i will put a dual port cover on my ms 660, i think i will try a #70 jet.

What do you think about it?

 

[Agoraphobia]

Make sure of what you already have. The BR jetting/carb are .74/WJ-76 carb the NZ/AUS is .74/WJ-67 carb. The US restricted carb is WJ-69A as noted in the original post. (I'm at sea level in AK and I had a .62 HS jet in mine, now a .74).

 

[sigafreddi]

It's a WJ69A on my 660, in France, like you.

Mine has a #64 jet.

 

[Tzed250]

and who told you that??

Well, uh, I dunno....maybe the fact that just about every Stihl made in decades before limited carbs had a base setting for the H needle of one turn out.

missed the 'other' thread about jets.

i realize that carb bore and venturi size create vacuum of fuel delivered through/from HS jet. the faster the intake air charge through carb, the more vacuum over the jet orifice to deliver more fuel. the original jet size was delivering enough fuel at 1-1/4 turns.....and, after larger jet 7/8 turns ....the end result was the exact same amount of fuel. although the original jet size amount of turns out, was more than the 'factory' recommended....can't see the difference in fuel the saw sees in WOT operation. the channel between the high speed need seat and the high speed jet remains the same size, and would seem to dictate the amount of vacuum of fuel in both jet sizes, with fuel flow being equal. if part throttle operation was poor, with the smaller jet size, (which couldn't be tuned out with the LS needle) i could see reason for change.

i'm missing the point, i guess.

Not uncommon. Did you read the whole thread? The point was not to change fuel flow. The point was to change the effectiveness of a small change of needle position. The EPA carbs have small jets because the saw is choked off. When the restriction is removed the carb works better when returned to the pre EPA configuration.

I'm very surprised that the ms 460 standards jet size (#68) is larger than ms 660 jet (#64).

When i will put a dual port cover on my ms 660, i think i will try a #70 jet.

What do you think about it?

Should work fine.

 

[David (saltas)]

The other thread sounds like a good read can some one post the name or link please

 

[Terry Syd]

Is there any difference in the fuel flow/power at lower air velocities (like when loading the saw) by using a larger jet?

It seems that if it is an EPA mandate, then there must be emissions issues involved - and that would suggest fuel flow differences.

 

[Tzed250]

Is there any difference in the fuel flow/power at lower air velocities (like when loading the saw) by using a larger jet?

It seems that if it is an EPA mandate, then there must be emissions issues involved - and that would suggest fuel flow differences.

The issue is that once the saw is choked by the EPA it needs less fuel at WOT. Instead of getting the correct mixture by setting the H needle at 3/4 turn, they install a smaller HSJ then obtain the correct mixture at 1 turn out.

 

[one.man.band]

gave up searching for the other thread. i'm all for better tuning. since it is working for you, i'm interested in knowing the 'why it works' part of it, which is still remains unclear to me.

from what i know, air bleed controls the transition point, at what rpm the vacuum pulls the main jet on. on most diaphragm carbs, the air bleed is a fixed size. by enlarging the air bleed, the main will come on at a higher rpm, and since its not possible, (except with epoxy and drilling) making it smaller would make the main come in at a lower rpm. on float bowl carbs, with a fixed air bleed, changing the main jet would definitely affect the low speed circuit and acceleration off idle response. creating a larger opening in the throttle plate over the idle progression holes would most likely do the same thing. too large a main jet would cause a flat spot in the transition phase (off idle to WOT), and too lean a jet would cause an richer adjustment necessary in the low speed jet setting in the transition setting. all that being due to vacuum.

the following points are why i remain puzzled:
1. somewhere, it was stated in this thread, that no adjustment was necessary for the low speed needle setting. 2. and the HS needle seat must flow more fuel than the jet size necessary for operation. (or the effect of having an adjustable need would be pointless). the needle adjustment setting you are using, is not all the way out in turns, to the point where the needle would be off it's seat far enough to provide full unobstructed fuel flow through the seat opening (at 1-1/4 turns out) with the smaller jet.

glad it works, still a mystery to me.

 

[Terry Syd]

If all the main jet does is act as a limiter to the High speed needle, then instead of messing around with jets, why not just remove the jet altogether? Seems like that would be the cheapest and easiest upgrade.

 

[Tzed250]

If all the main jet does is act as a limiter to the High speed needle, then instead of messing around with jets, why not just remove the jet altogether? Seems like that would be the cheapest and easiest upgrade.

Not so much a limiter, but a ranging device. When the needle is near the seat it changes flow rate very quickly with each movement. When far away from the seat it changes the flow rate much more slowly than it did when it was near the seat. The correct jet puts the needle in its sweet spot at one turn out.

 

[David (saltas)]

why the adjustment is linear but the right jet will make it seem more linear

needle valves are like control valves used in industry these are needle valves with a positioner on them

the most common valve trim is equal percentage.

this means if you open the valve 10% you will get ten percent of full flow

open it 60 % and you get 60% flow

this means the needle has a complex profile to give a linear out put.


the jet is described as a ranging device if you are turning the H just open or almost full open it will be much more sensitive to small adjustments and may run out of adjustment.

Putting a better size jet in to get it in the middle of is range means the adjustment will be more linear and you will not run out of adjustment.

 

[Terry Syd]

I understand the above. Now put two valves in series - which one controls the flow?

 

[rmh3481]

>>I understand the above. Now put two valves in series - which one controls the flow<<

The last valve controls the flow.

 

[Terry Syd]

And if the first valve is smaller, what then?

Going back to my point in post #46, perhaps there is an overlay of both fuel curves from both jets that gives a resulting fuel curve.

 

[Tzed250]

The above illustration does not show a tapered seat. A tapered seat has a non-linear rate change.

 

[ncfarmboy]

Well, uh, I dunno....maybe the fact that just about every Stihl made in decades before limited carbs had a base setting for the H needle of one turn out.



Not uncommon. Did you read the whole thread? The point was not to change fuel flow. The point was to change the effectiveness of a small change of needle position. The EPA carbs have small jets because the saw is choked off. When the restriction is removed the carb works better when returned to the pre EPA configuration.



Should work fine.

My 2003 Stihl IPL & WS disc says 1 turn out on both H/L jets for initial setting on every model I have checked.
Shep

 

[one.man.band]

only cutaway view i could find was for a walbro carb here:
http://www.tanaka-usa.com/pdf/serv/Walbro_Service_Manual.pdf

part throttle fuel delivered by both jets, and at WOT only the main.

WOT fuel path: diaphragm pulse, HS needle/seat, check valve, the finally main jet.... where vacuum (air velocity) dictates the amount of fuel volume and atomization. air bleed determines transition point rpm where main jet comes on solely. load on the engine must play some role as well.

if the air velocity is held constant at WOT over the main jet, you would get a proportional amount of fuel rate. whatever the limiting factor of fuel supply is, (in the WOT fuel path), would dictate final control of this volume rate. any of the WOT fuel path variables that control volume of flow, would have an effect, because each of them controls fuel volume flow. any adjustment has effect on all the others.

for tuning, a 1/4 turn of the screw, with a larger jet installed, would seem to result in a much more radical change than if compared to a same 1/4 turn of the screw, with a smaller jet size.

 

[Tzed250]

only cutaway view i could find was for a walbro carb here:
http://www.tanaka-usa.com/pdf/serv/Walbro_Service_Manual.pdf

part throttle fuel delivered by both jets, and at WOT only the main.

WOT fuel path: diaphragm pulse, HS needle/seat, check valve, the finally main jet.... where vacuum (air velocity) dictates the amount of fuel volume and atomization. air bleed determines transition point rpm where main jet comes on solely. load on the engine must play some role as well.

if the air velocity is held constant at WOT over the main jet, you would get a proportional amount of fuel rate. whatever the limiting factor of fuel supply is, (in the WOT fuel path), would dictate final control of this volume rate. any of the WOT fuel path variables that control volume of flow, would have an effect, because each of them controls fuel volume flow. any adjustment has effect on all the others.

for tuning, a 1/4 turn of the screw, with a larger jet installed, would seem to result in a much more radical change than if compared to a same 1/4 turn of the screw, with a smaller jet size.

That scenario does not apply to the WJ-69. You can see from my pictures that the HSJ lies next to the metering lever. The fuel must pass through it first.


Truthfully, this has become somewhat tedious. What I would like is for someone to explain why MS660s destined for different markets are delivered with different sizes of fixed jets. Since to many here the size of the fixed jet should not make a rats hind end, this ought to be somewhat entertaining.


Have at it...

 

[Terry Syd]

Obviously to change the fuel flow. If a smaller jet is installed to work better at higher altitudes or a smaller jet is installed to clear EPA emissions, it is because there was a desire to change the fuel flow.

If I was modding a saw with such a carb, I would have a selection of jets on hand to try out (like when you are modding a motorcycle, etc).

If the two different jets in series have different curves, then when one jet is changed, the resulting curve is changed. With the jets in series, the change in the curve can only go from one curve to a more lean curve. That may work well with a small carb that tends to get too rich as the revs rise.

If we wanted to make the curve go richer, we could use a slightly longer main nozzle. The longer nozzle extends out further from the sides of the venturi where the velocity is the highest. As the revs rise, the velocity in the center of the venturi will rise. Thus, the nozzle tends to lean the mixture at the lower revs.

Pop off Pressure is another big adjustment in a diaphragm carb. Allowing the metering needle to come off its seat at a lower pressure can eliminate the momentary lean condition when the throttle is cracked. It also allows the carb to flow fuel at much lower velocities (the HS needle will always correct the mixture on top end). A lower POP can extend the range of proper fuel metering so that the saw doesnt bog as easily when it gets loaded in a cut.

I wish my carbs had the extra jet, I also wish they had a transition jet and an air correction jet and a ... oh wait, the Auto tune carbs just eliminated all that.

 

[Tzed250]

Obviously to change the fuel flow. If a smaller jet is installed to work better at higher altitudes or a smaller jet is installed to clear EPA emissions, it is because there was a desire to change the fuel flow.

If I was modding a saw with such a carb, I would have a selection of jets on hand to try out (like when you are modding a motorcycle, etc).

If the two different jets in series have different curves, then when one jet is changed, the resulting curve is changed. With the jets in series, the change in the curve can only go from one curve to a more lean curve. That may work well with a small carb that tends to get too rich as the revs rise.

If we wanted to make the curve go richer, we could use a slightly longer main nozzle. The longer nozzle extends out further from the sides of the venturi where the velocity is the highest. As the revs rise, the velocity in the center of the venturi will rise. Thus, the nozzle tends to lean the mixture at the lower revs.

Pop off Pressure is another big adjustment in a diaphragm carb. Allowing the metering needle to come off its seat at a lower pressure can eliminate the momentary lean condition when the throttle is cracked. It also allows the carb to flow fuel at much lower velocities (the HS needle will always correct the mixture on top end). A lower POP can extend the range of proper fuel metering so that the saw doesnt bog as easily when it gets loaded in a cut.

I wish my carbs had the extra jet, I also wish they had a transition jet and an air correction jet and a ... oh wait, the Auto tune carbs just eliminated all that.

One look at the tuning possibilities for a Keihin FCR and it will be made clear how simple these little pumper carbs are.