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Diaphragm Carb Settings


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06-07-02, 05:17 AM   #1  
rev
H needle setting string trimmer

hi
what is the best way to set the h speed needle on two stroke trimmers?
thanks
rev

 
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06-07-02, 08:50 PM   #2  
Hello and Welcome rev to our Small Engine forum.

Best method is to turn the "H" screw out 2 turns and fire up the engine. With the engine running full speed, adjust the "H" screw outwards until the engine exhaust burns slightly black smoke.

Then turn the same screw inwards until the engine just starts to run too lean. This may be indicated by a slightly increased speed just before the engine appears to die slightly.

The center point between the 2 extremes is often times but not always, the best setting. One a 2 cycle engine, a slightly rich fuel mixture is best.

The reason is, the incoming fuel helps to cool the engine slightly, adds the fuel whichs is also the engine lubricant and the fuel mixture is also the engines speed governor.

A too lean fuel mixture will allow the engine to run at a higher top speed but also causes it to run hotter. A higher top speed will also drastically decrease engine performance under load and life.

The same proceedure is used for the "L" low speed fuel adjustment. When the carb is adjusted correctly, all speeds will run fine and there will not be any hesitation between idle and a quick jump into high speed.

All the above are based upon an engine in good mechanical condition & running condition, a carb that is correctly functioning, a correctly operating fuel flow system and ignition system.

Most likely I have not covered all of the potential problems. The other resident small engine service and repair professionals may offer additional suggestions, advice & help. Check back several times for additional replies.

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06-08-02, 02:55 AM   #3  
rev
trimmer h needle setting

thanks tom
i turn the H needle out to where the rpms drop then stop and close it to
where they drop again then set it halfway between the two drops is that your way?
rev

 
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06-08-02, 04:52 AM   #4  
Hello: Rev

The method you described Rev is correct and can be used.

For the benefit of all readers:

On some engines the top end speed may or may not decrease when the H screw is turned outwards. The engine may over speed to a point where it "screams."

This is much too fast for the engine to bare for too long a time. It's a top speed to avoid. However, if this is what the engine your working on is doing and most likely is doing because it is a 2 stroke, than consider it the high end max point.

When determining the low end point, your on track. The engine will want to die out. The half way point is between these two extremes. About an 1/8 turn outwards from this point should be the ideal fuel mixture for all speeds.

Much also depends upon the richness of the fuel blend and the age of the fuel. An incorrect fuel oil mixture can make carb adjusting difficult for the doityourself person. Each batch of fuel mixture can cause an engine to run slightly different then the prior batch without the user being aware.

Excessively leaning out the fuel mixture, by incorrect carb settings or an overly oil enrichened fuel mixture, is like leaning out a welders torch to obtain a HOT cutting flame.

Contrary to the popular misconception "More is Better," too much oil in a fuel mix causes an engine to run too hot, does not lube better and creates excessive carbon deposits.

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06-08-02, 05:35 AM   #5  
rev
trimmer h needle setting

thanks again tom
what about doing it a shorter way close to smooth then open to where it
runs rough then back of 1/8 of a turn?
rev

 
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06-09-02, 07:34 AM   #6  
Hi: rev

Basically, there is no shorter way or method.
Doing it as you described may or may not work under all engine circumstances and loads. But it may be possible and the results could be fine.

It's rather difficult to determine here, in a text only format, if what your describing will work. A lot of what a professional can determine from an engine and the carb settings that are perfectly adjusted for that engine, is based upon hearing the engine and noting it's operational condition.

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