4 stroking

[gmcman]

I don't think that the blubbering is a result of the rev limiter. I don't understand 2 strokes enough to explain, but doesn't the saw tach really, really high when you lean out?? I mean, when I get my engine sounding right, its running at a much lower rpm than if it is running lean. Just my .02. Perhaps someone can explain.

I don't know of many 2-strokes with rev-limiters with the respect of saws, trimmers, blowers, etc. When you get a 2-stroke at it's optimum mixture it will run strong and smooth. True, you can lean it out a bit for some more power but remember the oil is mixed with the fuel so when you turn the high needle in for more RPM's you also cut the oil as well so you need to ask yourself is it worth it. If you are in a situation where you need more power, step up to a larger motor.

When the atmospheric conditions change you have to remember the mixture screws don't automatically adjust....they stay right where you left them. When the air gets cold it is more dense and you need to richen the mix a little and vice-versa....more air means more fuel to keep the 9000:1 ratio required for proper combustion. I don't mean to start wrenching on your carb with a 2 deg drop but 90 deg summer heat and 50 deg fall/winter temps require minor adjustments.

When you hear the blubber it is usually a tad rich which won't hurt anything, when you have it too lean it will run up and seem to lose power because it is being starved of fuel. I usually start rich and open the throttle and listen for max RPM then richen the mix until the RPM's drop to the point of "blubbering", but not a constant blubber...just where the RPM's start to drop. Most times you run the saw wide-open and you want to have adequate lubrication so adjust for that speed. Granted a properly adjusted carb will give you proper lube at all RPM ranges but you don't want to burn up a piston for a tiny bit of power.

 

[Lakeside53]

9000:1?

 

[Elmore]

'4 stroke'...means it fires every other revolution.
On a 2 stroke engine the engine fires on every other stroke and on a 4 stroke it fires on every fourth stroke, what is meant by four stroke is that the engine sounds like it might be missing or only fireing on the the 4th stroke.
So rich that it misses a cycle.

 

[Elmore]

9000:1?

New EPA guidelines.

 

[Lakeside53]

New EPA guidelines.

 

[ShoerFast]

'4 stroke'...means it fires every other revolution.
On a 2 stroke engine the engine fires on every other stroke and on a 4 stroke it fires on every fourth stroke, what is meant by four stroke is that the engine sounds like it might be missing or only fireing on the the 4th stroke.
So rich that it misses a cycle.

If I may, that is a common misconception, but it is the lack of O2 that causes a partial firing or flame-out condition, just the opposite of a lean out condition, a rich out condition is what your hearing with the "burrlbe"

Gas will start to incompletely burn commonly at about 17 : 1 lean. And about 10:1 rich, for the other extreme. The mixture is too far apart lean, and runs out of O2 rich. best power is at about 11-12 : 1 and best economy at about 14-15 : 1 ,(economy is not for saws, as you want full-power) so just slightly in from a good solid burble (9.75 : 1) (?),, should be real close to 11-12:1,,,, Lakeside mentioneds it a lot that he likes the longevity running real close to the burble, gas is also engine coolant.

Better rich then lean, as plugs are cheeper then jugs!

 

[Lakeside53]

Better rich than lean, as plugs are cheaper then jugs!

I'm going to use that!

 

[ol'homey]

I adjust my old homie so that it's blubbering no load at about 8500 rpm. When it hits the wood the rpm drops to about 7000 and the blubber is gone. I have found it makes the most power adjusted this way. If I get it too rich it blubbers in the cut. But if it's right it will transition from blubbering no load to smooth power in the cut. Ok, now tell me what's going on to make it rich enough to blubber no load but not blubber under load? BTW this is a reed valve saw so this may not apply to the more common piston ported saws.

 

[ShoerFast]

I adjust my old homie so that it's blubbering no load at about 8500 rpm. When it hits the wood the rpm drops to about 7000 and the blubber is gone. I have found it makes the most power adjusted this way. If I get it too rich it blubbers in the cut. But if it's right it will transition from blubbering no load to smooth power in the cut. Ok, now tell me what's going on to make it rich enough to blubber no load but not blubber under load? BTW this is a reed valve saw so this may not apply to the more common piston ported saws.

Ol"Homey

Your saw makes a great point.

Picture if you would placing a very accurate pressure gauge just after the air-filter, air moving through the filter loses just a little pressure at 8500 RPM's , for a number, lets say that it loses 5% of it's pressure.

When that saw hits the wood and slows to 7000 RPM"s , just for a number, lets say that at the slower rate of air over the filter has now only a 3% loss of pressure, that alone would give you more O2 for the burn. Or perhaps be taking your saw from a 9.95 : 1 A/F ratio @ 8500 and giving a 10.5 : 1 ratio @ 7000 , if this makes sense to any one?

Of course this theory dose not work on "brillo-pad" filters!

 

[gmcman]

9000:1?

For gasoline it takes 9000 parts of air to burn 1 part of gas, diesel is around 20000:1. I don't know the specifics of 2-stroke oil w/ a 50/50 mix but that's what I was referring to.

 

[Paul61]

Ol"Homey

Your saw makes a great point.

Picture if you would placing a very accurate pressure gauge just after the air-filter, air moving through the filter loses just a little pressure at 8500 RPM's , for a number, lets say that it loses 5% of it's pressure.

When that saw hits the wood and slows to 7000 RPM"s , just for a number, lets say that at the slower rate of air over the filter has now only a 3% loss of pressure, that alone would give you more O2 for the burn. Or perhaps be taking your saw from a 9.95 : 1 A/F ratio @ 8500 and giving a 10.5 : 1 ratio @ 7000 , if this makes sense to any one?

Of course this theory dose not work on "brillo-pad" filters!

YOU DA MAN, Kevin

It's seems a cryin shame we don't have the input of the " the authority on all chain saws" to enlighten our junior members on the dynamics of 2 strokes engines, and of course......."How to save Mom's money on chain saw parts by replacing them with common house hold items"
:biggrinbounce2:

 

[Lakeside53]

For gasoline it takes 9000 parts of air to burn 1 part of gas, diesel is around 20000:1. I don't know the specifics of 2-stroke oil w/ a 50/50 mix but that's what I was referring to.

Maybe I'm missing something. Normally we refer to the stochiometric ratio and this is roughly (fuel dependant) 14.7:1 at sea level - the mass of air to the mass of fuel. What is your ratio refering to?

 

[blis]

Maybe I'm missing something. Normally we refer to the stochiometric ratio and this is roughly (fuel dependant) 14.7:1 at sea level - the mass of air to the mass of fuel. What is your ratio refering to?

thats what i´ve been told too... 14.,7:1...

 

[aquan8tor]

yep. I remember that from chemistry & physics. And turbocharging. Those ratios are given at 1 ATM--atmospheric pressure equivalent to roughly 14.7 psi, or 1 BAR--barometric pressure. I can't remember how many mm of mercury (Hg) that translates to . something around 760. maybe its 960. The important thing to remember is that a saw is going to lean out slightly as you go up in altitude. Also, Temperature is a factor. As someone wrote in a previous post, they thought husqvarna saws were "cold natured" This may mean that his saw was tuned for cold air and might be a little rich for regular temp. running--there's more oxygen in cold air. Hence why everyone wants to put a cold air intake on their econo-box honda civic--just that little temp. difference between the engine compartment & outside is enough to give a few HP. I know I'm a newbie and shouldn't get too technical, but I got an "A" in AP physics in high school.....over a decade ago.

 

[JUDGE1162]

OK, I now have a good handel on 4 stroking, but the replies have me asking another question?

How often do you guys adjust your carb??????

Every day? Every tank of gas? Once a week? Seasonally(ie spring, summer, fall, winter)? or just when it sounds out of tune?

How many of you use a tach to set your carbs or just tune by ear?

How may of you warm up your saw before cutting, it would seem a cool saw would have a diffrent carb ratio than a hot saw and if you do, how and how long.

 

[blis]

carb is adjusted when major changes in weather happen (from +10c to -20c) so, i would say 2 times a year, on spring and fall...

 

[Lakeside53]

OK, I now have a good handel on 4 stroking, but the replies have me asking another question?

How often do you guys adjust your carb??????

Every day? Every tank of gas? Once a week? Seasonally(ie spring, summer, fall, winter)? or just when it sounds out of tune?

How many of you use a tach to set your carbs or just tune by ear?

How may of you warm up your saw before cutting, it would seem a cool saw would have a diffrent carb ratio than a hot saw and if you do, how and how long.

I warm mine up for a few minutes - just idle on the ground. Carb adjustment - now and then, when it doesn't sound right.. Just a minor tweak usually "by ear". Tach check if somthing is way off or I've been working on it.

 

[BlueRidgeMark]

For gasoline it takes 9000 parts of air to burn 1 part of gas, diesel is around 20000:1. I don't know the specifics of 2-stroke oil w/ a 50/50 mix but that's what I was referring to.

Where are you getting those numbers? Obviously, since two of you are using them, they're coming from somewhere, but they are WAAAAAY far out of anything I've ever heard before.

ShoerFast's numbers are the usual numbers I've heard, and I know that other's reading this thread are thinking the same thing.

 

[Ombesb]

If I may flash in here a venture a guess from a far past memory... I believe one is in reference to WEIGHT and the other is in terms of VOLUME Quite different numbers

 

[ShoerFast]

Thanks Mark!

For a point, maybe some missed it, picture 2 balloons on a balance, one empty, one full of air, the balloon with air is just a little heaver then the empty one, air has a weight.

Picture if you would a lot more balloons full of air, a real lot more, enough for 1 pound of air. (one pound as in 16oz.) That 1 pound of air is enough to mix with 1/10 of a pound of mixture at a 10 : 1 for a rich burn. (to throw a wrench into the thought prosses, that 1 pound of air and fuel waights 17.6 oz's)

Gasoline will burn as lean as 17 pounds of air to 1 pound of gas, leaner then that the spark cant jump particles, picture a granary "dust-explosion" but not enough dust. Gas partly burning lean stinks , the fumes are raw to our nose and eye membrane.

Gasoline will start to soot-up (black soot is the carbon) at 10 : 1 as there is just too much fuel for the O2 in the air, nothing puts a fire out quicker then lack of O2 . My idea of what causes the "burble" is the incomplete burn and it's effect of the exhaust shock/sound wave, not sure as I never been in a class or proved this, maybe someone here has?