Simondo
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They do well , but like most things Dave..better if not abused and looked after.
I better hush and not muddy up the thread
Any Chinese chainsaw and chinese parts discussion
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Lol my saw's are run hard and get pressure washed once a month whether they need it or not.They do well , but like most things Dave..better if not abused and looked after.
I better hush and not muddy up the thread
I do clean the air filter after each use.
That's more upkeep than any saw i work on for customers had lol
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Thanks, Terry,
I really appreciated that story about your day with Jerry Branch. It reminds me that sometimes it isn't so much what you take out, but what you put in. The story of the air horn reminds me of another story; a concept in cars that makes no real sense, but makes the owners feel better about performance.
People often think bigger brakes are better for their cars, especially in regard to stopping power, but bigger brakes do not make you stop any faster. Stopping power is a function of the contact between the tires and the road, and the pads and your rotors. Bigger brakes allow you to fit larger rotors, which can hold more thermal mass before the pad starts to over-heat and slip. Therefore, larger brakes allow more frequent use between cool-down periods before experiencing brake fade. This would be useful in a track situation where you build maximum speed and are hitting the brakes hard in every corner as try to shed speed, but not so much in a daily driver. In a daily driver, larger brakes just add more unsprung weight, and aren't necessary to increase stopping power. However, many people spend extra money for the big brake kits when it isn't necessary or useful. Anyway, it just goes to show that what many intuit as somehow better can actually be wrong.
I was thinking about this question: is the amount of air an engine consumes dictated by the bore x stroke of the piston/cylinder, or by the carburetor's venturi? If the amount of air is dictated by the piston-cylinder, then the venturi simply controls the air-speed. If the air-speed is decreased, for a given jet size, would less fuel be pulled into the airstream? If the air-speed is increased, for a given jet size, would more fuel be pulled into the airstream? While some might think that the smaller venturi is a choke point, reducing airflow and therefore fuel, it might be that a smaller venturi would have greater air speed and could pull more fuel into the airstream.
I wanted to clarify my interpretation of what I think may be happening with the Zama carb modification.
I made up this chart, which may be wrong, but tries to explain what I'm thinking.
On the X-axis you have throttle position, but you can also think of this as RPMs. On the Y-axis you have air-fuel mix. The red dotted line represents the "ideal" stoichiometric mixture ratio, which may be between 14.1 to 15:1. Along the bottom of the graph are four jets that may be found in the Zama carb which contribute to fuel flow in different parts of the throttle range. In the low part of the thottle range (or at low RPM), the fuel is delivered by the idle jet. Although we can play with the choke to reduce airflow and richen the mixture, or adjust the throttle screw to allow more air, I'm showing that due to the fixed jet size, the fuel mixture is considered 'lean'.
At about 1/4 throttle the auxiliary jet opens up and contributes fuel, but because of our dear EPA , the jet is actually too small to supply very much fuel for the amount of air, and the mixture is still considered lean. The transition, or low-speed, jet has as ramp up in opening to add fuel (as I tried to show by a triangle shape), which is determined by the pop-off pressure of the spring. Changing the POP will change the shape of the ramp. At this point, we can also play with the low-speed needle to add more fuel to richen up the mixture or lean it out.
In the higher throttle position of the graph the fuel mix is rich, but is tunable by the high-speed jet. Most people will want to tune the saw to be close to the stoichiometric ratio for optimum efficiency.
If my image is correct, it should indicate that the low and high speed adjustment needles really affect the upper half of the throttle range, and therefore higher RPM values. While we have control over the air-fuel mixture here, we have little control over the mix at partial throttle and therefore lower RPMs. While cookie-cutting saws often sit at a specific RPM range, a work saw would benefit from having more control in the full throttle/RPM range. But come to think of it, a race saw cutting cants or cookies would benefit from having a faster spool-up time. So having more control over the full throttle-range would be beneficial.
If we drill the low jet, the auxiliary jet, and the mixing chamber jet, we can add more fuel at this lower throttle/RPM range.
I really appreciated that story about your day with Jerry Branch. It reminds me that sometimes it isn't so much what you take out, but what you put in. The story of the air horn reminds me of another story; a concept in cars that makes no real sense, but makes the owners feel better about performance.
People often think bigger brakes are better for their cars, especially in regard to stopping power, but bigger brakes do not make you stop any faster. Stopping power is a function of the contact between the tires and the road, and the pads and your rotors. Bigger brakes allow you to fit larger rotors, which can hold more thermal mass before the pad starts to over-heat and slip. Therefore, larger brakes allow more frequent use between cool-down periods before experiencing brake fade. This would be useful in a track situation where you build maximum speed and are hitting the brakes hard in every corner as try to shed speed, but not so much in a daily driver. In a daily driver, larger brakes just add more unsprung weight, and aren't necessary to increase stopping power. However, many people spend extra money for the big brake kits when it isn't necessary or useful. Anyway, it just goes to show that what many intuit as somehow better can actually be wrong.
I was thinking about this question: is the amount of air an engine consumes dictated by the bore x stroke of the piston/cylinder, or by the carburetor's venturi? If the amount of air is dictated by the piston-cylinder, then the venturi simply controls the air-speed. If the air-speed is decreased, for a given jet size, would less fuel be pulled into the airstream? If the air-speed is increased, for a given jet size, would more fuel be pulled into the airstream? While some might think that the smaller venturi is a choke point, reducing airflow and therefore fuel, it might be that a smaller venturi would have greater air speed and could pull more fuel into the airstream.
I wanted to clarify my interpretation of what I think may be happening with the Zama carb modification.
I made up this chart, which may be wrong, but tries to explain what I'm thinking.
On the X-axis you have throttle position, but you can also think of this as RPMs. On the Y-axis you have air-fuel mix. The red dotted line represents the "ideal" stoichiometric mixture ratio, which may be between 14.1 to 15:1. Along the bottom of the graph are four jets that may be found in the Zama carb which contribute to fuel flow in different parts of the throttle range. In the low part of the thottle range (or at low RPM), the fuel is delivered by the idle jet. Although we can play with the choke to reduce airflow and richen the mixture, or adjust the throttle screw to allow more air, I'm showing that due to the fixed jet size, the fuel mixture is considered 'lean'.
At about 1/4 throttle the auxiliary jet opens up and contributes fuel, but because of our dear EPA , the jet is actually too small to supply very much fuel for the amount of air, and the mixture is still considered lean. The transition, or low-speed, jet has as ramp up in opening to add fuel (as I tried to show by a triangle shape), which is determined by the pop-off pressure of the spring. Changing the POP will change the shape of the ramp. At this point, we can also play with the low-speed needle to add more fuel to richen up the mixture or lean it out.
In the higher throttle position of the graph the fuel mix is rich, but is tunable by the high-speed jet. Most people will want to tune the saw to be close to the stoichiometric ratio for optimum efficiency.
If my image is correct, it should indicate that the low and high speed adjustment needles really affect the upper half of the throttle range, and therefore higher RPM values. While we have control over the air-fuel mixture here, we have little control over the mix at partial throttle and therefore lower RPMs. While cookie-cutting saws often sit at a specific RPM range, a work saw would benefit from having more control in the full throttle/RPM range. But come to think of it, a race saw cutting cants or cookies would benefit from having a faster spool-up time. So having more control over the full throttle-range would be beneficial.
If we drill the low jet, the auxiliary jet, and the mixing chamber jet, we can add more fuel at this lower throttle/RPM range.
Terry Syd
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That's a pretty good description of what is happening. These carbs essentially have two fuel curves, the first being from the low speed circuit, the second from the venturi.
Those two curves do overlap. When you bump up the fuel flow in the low speed circuit you will often have to block off the extra jet on the back of the nozzle so that you can lean out the high speed needle.
The two curves are quite different. The low speed circuit is in the straight section of the carb body and would have a curve that was VERY rich initially if it wasn't for the metering spring and orifice ramping in the fuel. However, since the jet is a fixed orifice, the fuel flow will quickly flatten out.
The high speed circuit sitting in the venturi comes in slowly and then as the flow through the venturi increases, it also increases the suction on the high speed circuit. The result is that the high speed circuit actually increases in fuel flow as the RPM increases.
Getting the two curves matched to the requirements of the engine is the goal.
Those two curves do overlap. When you bump up the fuel flow in the low speed circuit you will often have to block off the extra jet on the back of the nozzle so that you can lean out the high speed needle.
The two curves are quite different. The low speed circuit is in the straight section of the carb body and would have a curve that was VERY rich initially if it wasn't for the metering spring and orifice ramping in the fuel. However, since the jet is a fixed orifice, the fuel flow will quickly flatten out.
The high speed circuit sitting in the venturi comes in slowly and then as the flow through the venturi increases, it also increases the suction on the high speed circuit. The result is that the high speed circuit actually increases in fuel flow as the RPM increases.
Getting the two curves matched to the requirements of the engine is the goal.
Terry Syd
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I should mention that there is one fault with the Zama that probably drove the designer nuts when he saw it. It is not a design fault, it is a manufacturing defect.
When the factory set up the castings they put the junction of the moulds right on the edge of the venturi. The edge of the venturi is rough, the roughest area in the carb. That is a problem as it is also where the air flow through the carb is the fastest.
If you are keen about setting up the Zama for the best air flow, you need to smooth the edge of the venturi. You can use a set of needle files to CAREFULLY trim off the rough edges without pulling the carb apart. However, to get the best surface you would have to pull the throttle and choke shafts and push out the nozzle. Then take a rubber hose and some fine valve grinding compound and by pulling it back and forth though the carb, smooth out the edge of the venturi without changing the overall shape.
When the factory set up the castings they put the junction of the moulds right on the edge of the venturi. The edge of the venturi is rough, the roughest area in the carb. That is a problem as it is also where the air flow through the carb is the fastest.
If you are keen about setting up the Zama for the best air flow, you need to smooth the edge of the venturi. You can use a set of needle files to CAREFULLY trim off the rough edges without pulling the carb apart. However, to get the best surface you would have to pull the throttle and choke shafts and push out the nozzle. Then take a rubber hose and some fine valve grinding compound and by pulling it back and forth though the carb, smooth out the edge of the venturi without changing the overall shape.
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Sleeper
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Terry, is there anything you don't know?
Terry Syd
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Heaps, just ask my wife...
Derf
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@Poleman Rich, do you pull that nozzle when you drill the jets? Or is all this separate from where you're working on the carb?
Poleman
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No, I don't pull the nozzle unless for other reasons. I just work/drill the Jets and bleed hole under the main jet.
I've opened carbs up similar to what Terry described with mixed results....you go to far and you've ruined a carb. Those experiments were on the AM carbs that were cheap and plentiful......now OEM, so I use more caution now as their not cheap.
I've opened carbs up similar to what Terry described with mixed results....you go to far and you've ruined a carb. Those experiments were on the AM carbs that were cheap and plentiful......now OEM, so I use more caution now as their not cheap.
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Carbs working perfectly great off idle torque and throttle response.
When blocking i don't have to keep the revs up as i get close to dirt/rocks
I just feather the throttle lightly and the torque finishes the cut.
Without risking my chain lol
I will try it on one of the big bores
It will be fun.
Thanks again.
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Glad it's working well for you!!! Your feedback is AWESOME Dave!!! I was wanting an opinion/test on the Chinese saws and your the go to guy on them!!
You see the direct results of a good carb on those saw compared to what come with them. I haven't had much luck with the non twin jet carbs. I can make them better but not consistent when put on another saw.
Thanks for your updates!!!!
You see the direct results of a good carb on those saw compared to what come with them. I haven't had much luck with the non twin jet carbs. I can make them better but not consistent when put on another saw.
Thanks for your updates!!!!
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I'm just a cutter/faller for my friends and neighbors now.
I've been rebuilding saw's since the 80s just the occasional dual port muffler was a common performance upgrade lol
I've always woods ported my own saw's to let them breathe.
Most around here don't want a ported saw lol
My next project after i finish 3 288s
Is to find somebody to cut down some 272xp pistons for my big bore saw's.
I'm not set up for it or to cut bases.
The guy that did it retired.
These china saw's are new to me but i would have gladly cut all day with one back when i cut a lot lol
Not much info on these saw's when i got mine now i get contacted by people everywhere wanting info.
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2 more my 288 lite is done lol
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Tore down my 268/372 saw went ahead and swaped the piston for a 372 huztl double ring.
I'm going to have some 268 and 272 pistons cut with pop ups soon.
Not broke in yet but better torque it feels like with the 372 piston.
The 268 revs higher it feels like.
I'm going to have some 268 and 272 pistons cut with pop ups soon.
Not broke in yet but better torque it feels like with the 372 piston.
The 268 revs higher it feels like.
Sty57
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That a little echo vent you got on there for the tank?
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.80 cent ebay jobbie i was running out of fuel on long cuts.
It was starving out this cured it new echo lines and a filter.
I carry my saw in my grand cherokee no fuel odor the original i could smell fuel occasionally.
I have to pack a saw with me here lol
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Which kit are you thinking about getting.
Sty57
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I don't really know yet.
Might be something that I never had before.
Was kinda thinking about a 044/440.
Just don't see many Stihl's here cheap to build.
It's mostly Husky/J-Red's around here.
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Same here mostly trashed logger saw's or poulans.
I can sell every stihl i fix lol clammys to pro.
But Husky's and jreds are harder to move.
It's funny though i looked for a 371/372 builder and couldn't find any that's why i bought the china saw's.
A clapped out pro husky 350-450 usually.
