Small Engine Carb Adjustment 2-cycle

I just bought a one man Earth Auger at a local auction. (power post hole digger). The thing is old, but for $5, I decided to give it a shot since these things are over $350 new. It has a Tecumseh 2 cycle engine - power head. After spraying carb cleaner thru all the carb ports, I got the engine to run but it's runs very rough. I think much of this has to do with the carb adjustments. The problem is that I dont know which screw does what or what the initial settings should be (other than what they were, which I wrote down).
It's a 3HP Tecumseh 2 cycle engine. There is a large screw and a smaller screw. They are next to each other. Does anyone know anything about adjusting these? I repair lots of 4 cycle engines, but am not very familiar with 2 cycle engines.
The initial settings that it had was approx. 2 1/2 turns on each screw.
Is there a standard meaning for small screw and large screw, or is this just this particular model?
An additional mention. This thing has no name or model number. Just says Tecumseh 3hp on the engine, so I am not sure if the whole tool is made by Tecumseh, or just the engine. This will make it real hard to ever get any parts or a manual for it. From the looks of it, I suspect it was made in the 1970's or 80's.
Thanks
Mark
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One, probably the larger one is the high speed adjustment, the other the low speed. It gets a little more complicated than that, as they interact. Changing the high speed effects the low speed. You just have to get it running good wide open then try for a decent low speed/idle, then do it all over again till both seem about right. better to end up a little bit rich than a little bit lean.
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mixture control. If one screw is in the center of the carburetor, that is usually the high speed mixture control.
Be sure you have a good spark plug and that the exhaust ports and muffler are not clogged. These are very common causes of two stroke engine problems.
Don Young
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The instructor of our small engines course said this:
[Presuming idle set screw et. are reasonably correct.]
Start with both needles opened 1 1/4 turns.[3]
Start the engine and get it warmed up.
Rev the engine to full RPMs [1] and adjust the high screw - 1/8 turn at a time, and find the upper and lower limit of it running best. Set the needle halfway between the two points.
Let the engine idle, and do the same adjustment trick with the idle screw. Now, turn the idle screw 1/8 turn _rich_ from the center of the best running point[2].
Done.
He went to great pains to say "don't touch 'em again" ;-)
At this point you can adjust the idle set screw for what you want.
[1] You don't do this with chainsaws. High revs with no load can cause rather bad things to happen if it goes on long enough. You're supposed to do your high needle adjustment by full load cutting on a log, tweak the needle, and retest cutting again.
[2] The 1/8th turn rich on the idle screw is intended to give the engine a little bit of extra gas to ensure that the engine doesn't hesitate on quick throttle increases.
[3] He said that when set this way you virtually always only have to turn the needle one direction to find the best position. I forget which direction ;-)
--
Chris Lewis, Una confibula non set est
It\'s not just anyone who gets a Starship Cruiser class named after them.
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Chris,
Good advise. Especially the part about keeping your hands off the adjustments screws once they have been properly set.
Should the same advise be offered for snow blower adjustments that you offered for chains saws: Alwasys tune them under load?
=============== Chris Lewis wrote
The instructor of our small engines course said this:
[Presuming idle set screw et. are reasonably correct.]
Start with both needles opened 1 1/4 turns.[3]
Start the engine and get it warmed up.
Rev the engine to full RPMs [1] and adjust the high screw - 1/8 turn at a time, and find the upper and lower limit of it running best. Set the needle halfway between the two points.
Let the engine idle, and do the same adjustment trick with the idle screw. Now, turn the idle screw 1/8 turn _rich_ from the center of the best running point[2].
Done.
He went to great pains to say "don't touch 'em again" ;-)
At this point you can adjust the idle set screw for what you want.
[1] You don't do this with chainsaws. High revs with no load can cause rather bad things to happen if it goes on long enough. You're supposed to do your high needle adjustment by full load cutting on a log, tweak the needle, and retest cutting again.
[2] The 1/8th turn rich on the idle screw is intended to give the engine a little bit of extra gas to ensure that the engine doesn't hesitate on quick throttle increases.
[3] He said that when set this way you virtually always only have to turn the needle one direction to find the best position. I forget which direction ;-) -- Chris Lewis, Una confibula non set est It's not just anyone who gets a Starship Cruiser class named after them.
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Note that he specifically said "don't touch the high speed screw once you have it set right". In other words, you don't iterate between idle and high speed screws. Do HI first, then LO, don't go back to HI.
If you encounter run problems after doing the best job you can, something else is wrong.
[He identified that the chipper I was trying to clean up and was difficult to adjust had something wrong with the governor and it was overspeeding. Turned out a throttle limit screw was set ridiculously wrong, jamming the linkages and it was drifting to dangerously high RPMs. You don't want chipper blades coming off...]

No. The issue here is that you normally set the HI screw when the device is running full throttle, loaded or otherwise. The problem being with chain saws is that full throttle _without_ load means that the chain is moving dangerously fast and things may come apart. Most other devices run full throttle all the time, and don't need this precaution.
If you have high throttle low load -> high throttle high load issues, there's probably something wrong with the governor. Which shouldn't be mucked with unless you understand how it works, and follow a set procedure.
Our instructor really knows his stuff. Early on, he ran a small engine repair shop. Since that time he became a champion snowmobile racer (owing perhaps more to his skill with engines than his driving ;-), and now he's a senior scientist with NRC's wind tunnel in Ottawa. Testing all sorts of cool things like helicopters, F18s, cars, and things he couldn't talk about.
Instructing small engine theory and repair is his hobby.
His current job turned out to be quite helpful, after my son took the course and announced he was trying to make a windtunnel (to support his rocketry hobby) out of a souped up weed wacker engine.
--
Chris Lewis, Una confibula non set est
It\'s not just anyone who gets a Starship Cruiser class named after them.
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Thanks
This advice worked out real well on this engine. I figured out which screw is which too.
Now back to my clutch problem (another posting).
Mark
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On Wed, 09 Aug 2006 03:12:57 -0000, snipped-for-privacy@nortelnetworks.com (Chris Lewis) wrote:

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