F-340 Experts - Altitude Screw Adjustment Question

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TRUE or FALSE -

The altitude adjustment screw on my plastic carbed 1980 UTILITY model adjusts air volume, not fuel amounts.:biggrin:

Thanks for any input, Rocky
 

tom3

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From what I've gathered the screw allows air into the float bowl. Seems odd but I guess it's an emission thing. I do see that if you turn it in the motor runs fine until it just shuts down from lack of fuel. Adjust about 2 turns open. Really wish there was some mixture screws on these carbs, mine runs rich, misses and 4 strokes, sounds like crap but makes serious power in heavy grass.
 
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Your answer is exactly what instigated my original question.
The owner's manual states gently turn said screw CW until seated and then, back out 1/2 turn CCW.
There seems to be a fair amount of confusion on this very subject (i.e. amount of turns "out").
So "gas" or "air", which is being varied?

Rocky
 

Fishnuts2

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Mine does't seem to respond much to the altitude screw, except as previously noted, it runs out of fuel if I turn it too far. A bigger problem is it literally floods the engine if I am mowing downhill and pull the mower back towards me. The altitude screw has no effect on this behavior either. I'd like to refit an older carb to it and get rid of that plastic outfit.
 

tom3

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Same here. Mine is an F engine with a metal Walbro carb, worn throttle shaft bores, no mixture adjustments, tried everything to get it to smooth out. Even put a fine wire through the main jet, cut the jet size about 30%, no help. Got me beat, but it starts and mows great so I'll live with it, but I like a well tuned engine.
 

jp1961

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Hello,

The Altitude needle is used to adjust the engine from the distance from sea level. Obviously the higher from sea level, you live, the less air you have to work with. I'm assuming (maybe incorrectly), the less air you have the more fuel you'll need.

Obviously if the carb is worn, it may not respond to carb adjustments at all.

Regards

Jeff
 

Teds

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No, what happens at higher altitudes (less air) is the AFR becomes too rich, so the fuel mixture needs to be leaned out.

This "1.5 turn out" business as a general observation is simply a rough bench setting, if it were a permanent or one size fits all, then they would have done away with the parts & adjustment altogether and use a fixed orifice size. Think about it. They are actually VERY precise in their adjustment. You can see this in carbureted automotive engines by observing the AFR when making an idle mixture adjustment, just a 1/4 turn of will move the air fuel ratio a couple points.

It is also temperature dependent to some degree (heh) 2 stroke lawnmowers are different in that they should not run smooth at idle. Slightly ragged at idle, though settle in to the characteristic "LawnBoy hum" under load in thick heavy turf. Make sure there are no vacuum leaks and float height is correct or it will never run right. With the engine warmed up, turn the altitude screw in slowly clockwise. There should be a noticeable, definite point where the engine speed increases. This is the "spot" where it starts to be too lean (even at idle). Back it off slightly, just a smidge. Remember, these are precision cut needle screws. 1/4 turn out from this "spot" would probably leave it a little too rich, though safe for a wide range of temperatures i.e. it won't grenade the engine. I listen to the motor in operation and make a slight adjustment now and then through the season, on earlier models it only takes a split second, the screw is accessible by hand, no jiggery-pokery required. In the fall, during cold weather at the last mow(s) of the season it can need a pretty significant re-adjustment.
The power difference is very noticeable when they are dialed in "by the book".

"Slightly rich" is good and what the engines want, but this is still quite a bit leaner than many might think. "Slightly" is the operative word here, "1.5 turns out" is likely way, way off and an overrich condition leads to fouled plugs and excessive carbon buildup.

Another factor to consider is the 2 stroke oil to fuel mix. Adding more oil to the fuel than specified makes the air fuel mix leaner, not richer. Less fuel = leaner AFR. If you're using the modern synthetic 2-stroke oil mixes at 50-1 or more the engine will want a leaner setting.
 

Fishnuts2

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According to the 1977 manual, turning the altitude screw counter - clockwise richens the mixture, clockwise leans it out. I read the manual over 10X before it finally started to make sense. I'm kinda slow in some things, I know!! The screw regulates the amount of vent air to the carburetor float chamber. If the air is restricted, the motor cannot draw more fuel out of the bowl. It is like a gas tank that has a plugged vent. The slight vacuum created restricts the fuel from flowing out.

It seems like a strange way to regulate mixture, but it was probably a lot cheaper to produce than the two metal needles and jets of the older models. Which is more than likely the reason for the modular plastic carb in the first place.

Here's a link to the manual that I found

https://lookup3.toro.com/ttcGateway/acrobat/manuals/lball10.pdf
 
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Fishnuts2,
Thanks for the info. So it adjusts opposite of what conventional wisdom might suggest. Very counter intuitive, but effective once folks know.

CW = richer and CCW = leaner.

I'll stay on the slightly rich side to keep this 1980 UTILITY model happy.:smile:

Rocky
 

Teds

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According to the 1977 manual, turning the altitude screw counter - clockwise richens the mixture, clockwise leans it out. I read the manual over 10X before it finally started to make sense.


Here's a link to the manual that I found

https://lookup3.toro.com/ttcGateway/acrobat/manuals/lball10.pdf


Where does it say that? Here's what I found:


7. After carburetor adjustment is completed, shut off engine. IMMEDIATELY attempt to restart engine. It should start within 2 pulls on starter handle.

Check starting engine at both HIGH and LOW speed settings. If difficult to restart, turn altitude needle 1/8 to 1/4 turn counterclockwise to richen fuel mixture and obtain easy restarting.
------------

It is important to determine what carburetor we are talking about in any given discussion, get everybody on the same sheet of music.
 
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