Wot?

[Jonsered2095]

It is a good question

Re-phrased:


Does the vacuum caused by the shape of the barrel (Venturi) mysteriously disappear between idle (low speed jet) and WOT (high speed jet) or just the flow rate decelerate/accelerate?

What would cause any reduced flow through the venturi to off-set optimal tuning set at WOT and idle?

And should engines with two barrel+ carb.s on motor cars only be run at WOT also?

Mysteriously - well that is subjective. The throttle valve when closed does stem the flow across the Hi speed jet and causes the idle curcuit to be active (as previously said). The idle curcuit is metered by the Low speed jet and is activated by the vacuum passing over the idle jets - holes in the barrel adjacent to the nearly closed throttle valve which causes enough vacuum for the idle curcuit to work. There is more than one hole here and as the throttle valve opens, the other holes/jets in sequence provide a richer environment which would otherwise have led to a lean state (albeit momentarily) as the velocity of flow increases until the Hi speed jet takes over as the air flow through the venturi creates a vacuum across this (Hi speed) jet. The vacuum is then insufficient to maintain the Low speed curcuit active and it shuts down.

That leaves the question; over what range of rev's does the Hi speed jet work; and are the rev's (either above or below the optimum tuned point but still running through the Hi speed jet) richer or leaner or neither? Proper explanation please!


Oh, and should cars be run WOT? HELL YEAH!! RED HEARING

 

[teacherman]

POST 2000! And I did it running my chainsaw at WOT!!!!! :greenchainsaw:

Hoo Boy! Everybody get yer waders on, it's a gonna get deep!:greenchainsaw:

 

[teacherman]

Roger that

WOT for normal cutting. Partial throttle for those finesse situations will be fine. Just dont go with partial for regular long cutting .

Got it. Thanks gang. I am convinced. WFOT it is. Man the 084s'll wake up people in China.............lol

:chainsawguy: :chainsawguy: :hmm3grin2orange:

 

[Freakingstang]

Oh, and should cars be run WOT? HELL YEAH!! RED HEARING

That is how I used to be, but there is only so many rapid accelerations, failure to controls, etc you can get before they take your license.............

 

[Bowtie]

WOT for normal cutting. Partial throttle for those finesse situations will be fine. Just dont go with partial for regular long cutting .

Excellent post! It aint like using your saw at part throttle a little bit here and there will kill it immediatley. But using it as practice isnt good for it. i have done it and will do it again, its all in how long/often you do it. Common sense has to be involved.

 

[gink595]

That leaves the question; over what range of rev's does the Hi speed jet work; and are the rev's (either above or below the optimum tuned point but still running through the Hi speed jet) richer or leaner or neither? Proper explanation please!


Oh, and should cars be run WOT? HELL YEAH!! RED HEARING

Dude you have a dyno chart for a avatar, do a exhaust analysist. The high speed jet serves the WOT range and that is it, any thing under that is going to run richer until it is at an idle. Sure it will run and perform but it was never intended to run part throttle for great periods. If you had that same carb setup on a motorcycle it would load up, foul plugs and be a general PITA to ride. So to answer your question definatly richer.

 

[Freakingstang]

Dude you have a dyno chart for a avatar, do a exhaust analysist. The high speed jet serves the WOT range and that is it, any thing under that is going to run richer until it is at an idle. Sure it will run and perform but it was never intended to run part throttle for great periods. If you had that same carb setup on a motorcycle it would load up, foul plugs and be a general PITA to ride. So to answer your question definatly richer.

It all depends on the carb and if it is sized correctly. normally it is the opposite and runs lean for a couple seconds when the high circiut is closed off.

 

[gink595]

It all depends on the carb and if it is sized correctly. normally it is the opposite and runs lean for a couple seconds when the high circiut is closed off.

I'm talking chainsaw carb only. Anything else that has to have a cruising range is going to need a float system and mid range jetting of some sort. Yes letting off the throttle at a rapid pace is going to have a lean condition, but trying to cut, at say 1/2 or 3/4 throttle is going to be rich, agree'd? When I used to snowmobile up in Houghton lake Mi, we'd run our sleds on the lake at 115 mph and when letting out of the throttle with the cold temps I'd pump the throttle to create a rich condition to prevent the ultimate lean condition you stated.

 

[Freakingstang]

I'm talking chainsaw carb only. Anything else that has to have a cruising range is going to need a float system and mid range jetting of some sort. Yes letting off the throttle at a rapid pace is going to have a lean condition, but trying to cut, at say 1/2 or 3/4 throttle is going to be rich, agree'd? When I used to snowmobile up in Houghton lake Mi, we'd run our sleds on the lake at 115 mph and when letting out of the throttle with the cold temps I'd pump the throttle to create a rich condition to prevent the ultimate lean condition you stated.

yup.

The saw carbs are so simple they baffle everyone.... lol.

I'd love to be on a sled at a buck fifteen, well maybe without the cold amd white stuff.....lol

 

[gink595]

yup.

The saw carbs are so simple they baffle everyone.... lol.

I'd love to be on a sled at a buck fifteen, well maybe without the cold amd white stuff.....lol

I believe you are right the simplicity of these little buggers are confusing you just don't associate carbuerator with simplicity. Especially when you have worked on others like carters or rochester you just don't think they could cut that much stuff out!!

The sleds really aren't that much fun going that fast, seems like it is going to come unglued and the pressure cracks on the ice are killers.

 

[Freakingstang]

I believe you are right the simplicity of these little buggers are confusing you just don't associate carbuerator with simplicity. Especially when you have worked on others like carters or rochester you just don't think they could cut that much stuff out!!

The sleds really aren't that much fun going that fast, seems like it is going to come unglued and the pressure cracks on the ice are killers.

I prefer the carter AFB or edelbrock to a conventional holley vac secondaries any day of the week, although the newer demon (holley mech secondaries take off) are decent too.

I don't really know the mid range carbs (100-1500 cc) carbs, but they are still similar in theroies and design

 

[Lakeside53]

Many chainsaw carbs have a mid range jet. It's not adjustable, but supplies a reasonable amount of gas for partial power operations. Stihl added these to most pro saw carbs in the early 90s mainly for thining and limbing operations. Early 036 and 044 did not have the jet - it is possible to get the saw to suddenly lean out at mid-throttle.... just very slowly increase the throttle... it can suddenly move from 7-8000 to 15000.... futher slight increase of the throttle will drop it back to 10k... the jump to 15k was the "mid range" problem... not enough gas from the L jets and the H was ineffective.

In general, full throttle is correct, but use at low power partial throttle is find. Justs don't lug it down at higher power / low revs with part throttle.

 

[SawTroll]

Quick question: Is it true that a chainsaw should always be run at WOT when cutting? Just curious, since I will often use less than full throttle to give myself better saw control, esp. in tricky cut situations.:greenchainsaw:

Simple answer - YES - otherwise you loose control of the mix rate, and may fry the saw.

....better "blip" to full throttle for a small twig, than use partial throttle.

 

[SawTroll]

WOT for normal cutting. Partial throttle for those finesse situations will be fine. Just dont go with partial for regular long cutting .

yup.


Every now and again when I finish a cut on the ground, I let it idle down so I don't hit the dirt at WFO

Yup yup.......:greenchainsaw:

 

[ironray]

For arguments sake, This explains the chainsaw carbuerator's functions.
http://auto.howstuffworks.com/question377.htm/printable

Here is another link from Zama that explains how their carbs work.
http://www.zamacarb.com/pdfs/TechGuide_2007.pdf

 

[dancan]

As saw troll said in the previous post , yup and yup .

 

[stihl 440]

yup

Simple answer - YES - otherwise you loose control of the mix rate, and may fry the saw.

....better "blip" to full throttle for a small twig, than use partial throttle.

Yup.....but I don't blip 100 times a second.lol I probably blip it once or twice through a 2" limb.:greenchainsaw:

 

[Jonsered2095]

Mysteriously - well that is subjective. The throttle valve when closed does stem the flow across the Hi speed jet and causes the idle curcuit to be active (as previously said). The idle curcuit is metered by the Low speed jet and is activated by the vacuum passing over the idle jets - holes in the barrel adjacent to the nearly closed throttle valve which causes enough vacuum for the idle curcuit to work. There is more than one hole here and as the throttle valve opens, the other holes/jets in sequence provide a richer environment which would otherwise have led to a lean state (albeit momentarily) as the velocity of flow increases until the Hi speed jet takes over as the air flow through the venturi creates a vacuum across this (Hi speed) jet. The vacuum is then insufficient to maintain the Low speed curcuit active and it shuts down.

That leaves the question; over what range of rev's does the Hi speed jet work; and are the rev's (either above or below the optimum tuned point but still running through the Hi speed jet) richer or leaner or neither? Proper explanation please!


Oh, and should cars be run WOT? HELL YEAH!! RED HEARING

Here is another link from Zama that explains how their carbs work.
http://www.zamacarb.com/pdfs/TechGuide_2007.pdf

Nice one ironray.

This illustrates my point nicely (although my statement about the Low speed jet shutting down is wrong - I figured once the throttle valve was open so far the amount of vacuum acting on the idle jets was ineffective) :blush:

Point is; as the throttle valve opens more fuel is provided, effecting a midrange jet (or two) of sorts - variable fuel flow. The fact these jets are still operative at WOT only serves to illustrate my point further. And although the tuning between idle and WOT may be less than optimum I imagine the difference is negligible. Probably comes down to the tuner more...

Besides, why run it at less throttle? Tricky cuts such as boring should be run at high rev's BUT you must have sharp chain. Slow chain speed gives the cutter a real chance to bite, dull cutters give a real good bump, and both result in the force traveling back along the bar towards you.



Snapshot from above:

 

[scotclayshooter]

Many chainsaw carbs have a mid range jet. It's not adjustable, but supplies a reasonable amount of gas for partial power operations. Stihl added these to most pro saw carbs in the early 90s mainly for thining and limbing operations. Early 036 and 044 did not have the jet - it is possible to get the saw to suddenly lean out at mid-throttle.... just very slowly increase the throttle... it can suddenly move from 7-8000 to 15000.... futher slight increase of the throttle will drop it back to 10k... the jump to 15k was the "mid range" problem... not enough gas from the L jets and the H was ineffective.

In general, full throttle is correct, but use at low power partial throttle is find. Justs don't lug it down at higher power / low revs with part throttle.

Sorry guys but i would take the above advice!
In some of Timberwolfs posts when he was progressivly opening the hole in the muffler, He stated that the adjusting the low richer somtimes made such a difference that he would have to lean out the high a little, Im sure we all tought that we would need to richen the high when the muffler exit was made bigger!
Sort of shows the low is having a bigger effect on the overall ratios than most of us would think.
Timberwolfs research also showed that the conrod was under a lot of stress while blipping the throttle and could put the rod under tension instead of compression.

I would presume that strimmer carbs are much the same, Try strimming around a clothes pole or fence at WOT, See how much line you break off!
Its the same over stones or near windows you just have to back off the revs sometimes.

 

[Saw Bones]

I believe cutting at WOT is so the chain cuts cleanly through the wood. It is safer and actually gives you more control and less chace of being pulled forward by the chain "grabbing" I think it is a safety thing. There is less chance of the saw bogging down in the cut and damaging the clutch.