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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

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.

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

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.

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.

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

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.

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

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

Fishnuts2 said:

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

Click to expand...

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.

Lawn-Boy Rocky said:

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

Click to expand...

No, clockwise leans it out. Counterclockwise richens the fuel mixture! It's not rocket surgery.

Teds said:

No, clockwise leans it out. Counterclockwise richens the fuel mixture! It's not rocket surgery.

Click to expand...

So, what I see here is two guys (fishnuts and Ted) with two opposite answers to my original post.
This is why I'm still hanging in here, trying to get to the bottom of this.
Not sure which carb Ted's info (7) relates to, but I do not have a high and low speed "setting" on my UTILITY plastic carb.
Rocky

Hi Rocky,

Are you referring to a small plastic lever on the back of shroud? If so, then that's actually adjusting the governor spring tension, not the carburetor.

The mixture needle valve would be on the carb itself.

Regards

Jeff

Probably a good idea to find out what's right for your actual carb type. Look through the Lawn Boy manual to find yours. Quite a few variations over the years.

https://lookup3.toro.com/ttcGateway/Acrobat/manuals/lball.html

Thanks to all for your input re: this screw adjustment.
I have seen various opinions on the various LB carbs.:wink:
Rocky

The beauty here guys, we don't need to rely on opinion, this isn't me talking, we can simply read the manual. It's right there in black & white, straight from the horses mouth.

I haven't pored over every single jot and tittle in the manual at least not recently, but I'm not aware of any carburetor altitude screws on any model that work contrary to CW=Leaner CCW=Richer AFR. The carburetors themselves are VERY simple construction, when you take them apart the next time take a look at the innards and it will make sense.

I don't know much about it but on my old F engine with metal Walbro carb I can take the screw clear out and it runs the same. Seat it and turn it out 3/4 turn and it runs the same - but dies in a half minute or so. I just can't see this as a normal mixture adjustment.

tom3 said:

I don't know much about it but on my old F engine with metal Walbro carb I can take the screw clear out and it runs the same. Seat it and turn it out 3/4 turn and it runs the same - but dies in a half minute or so. I just can't see this as a normal mixture adjustment.

Click to expand...

Well it would make more sense that probably something is defective with that particular carburetor versus the manual has it wrong?

I've suspected that since I got the mower, but a new carb is real hard on the bank account - when you can find one. And a 30 year old steel deck LB isn't worth a whole lot anyway. Wish the Chinese would get their act together and produce an $8 replacement.

Hm. They are so simple, it can't really be any weird stuff. Carburetors are designed and calibrated around a very specific fuel column height that is maintained in the bowl for siphon action through the jet. The fuel height is set indifectly through the float height adjustment. Too low of an average fuel level tends to run lean, too high it will run rich. There isn't really any way to check that on these. Automobile carburetors had some ingenious mechanisms, Holley float height for example was adjustable on the fly without disassembly, and resulting fuel level observed through a weephole in the side of the carb.

I was referring to the D-600 fuel system with the "Altitude adjustment screw" pictured on PP 4-15 and 4-16 of the shop manual that I mentioned. My carb is the modular plastic model as shown in the diagram. Please enlarge the carb diagram shown to about 200% and explain to me that if turning the screw CLOCKWISE to Lean the fuel mix, then how does the fuel even gets to the adjustment screw to be restricted?

The altitude adjustment screw is in the air path of the carb bowl vent, not in any fuel path. The two black arrows on the diagram lead from the air vent on the air filter side of the carb to the Altitude adjustment screw.

I agree on the rest of points made on the fuel levels of carbs, and wish that the float level on my carb could be adjusted. My 1973 Yamaha snowmobile had little clear pipes on the sides of the Kehin (Spelling?)
carbs to see and adjust the fuel levels. This modular carb is almost too basic, and it seems they either work well or not at all close to what I know LawnBoys can run like.

Fishnuts2,
Good to hear back from you. I do not pretend to know many details re: these plastic carbs, but I do have some recent experience to share.
Last week, I adjusted my altitude screw on my 1980 Utility model (~#4502) 1.5 turns OUT from seated. It ran best ever, even on lousey 10% ethanol pump gas.
It started immediately with only two primer pumps and stayed running. Also, no more idle "hunting" when hot.
I'm convinced I previously had it too lean (i.e. difficult cold starts and hunting/surging idle).
I'm all set, hopefully someone else can mimic these results w/their F-340 motor.
Lawn-Boy Rocky

Fishnuts2 said:

I was referring to the D-600 fuel system with the "Altitude adjustment screw" pictured on PP 4-15 and 4-16 of the shop manual that I mentioned. My carb is the modular plastic model as shown in the diagram. Please enlarge the carb diagram shown to about 200% and explain to me that if turning the screw CLOCKWISE to Lean the fuel mix, then how does the fuel even gets to the adjustment screw to be restricted?

Click to expand...

I have one of the modular plastic carbs, I don't like them either. I'll take a look at it, the diagram doesn't look too clear to me. The text right below it clearly states to turn altitude screw counterclockwise to richen fuel mixture. I guess it could be a typo, wouldn't be the first time in a service publication.

OK, a quick glance at the plastic modular carb shows the altitude screw doesn't factor into the venturi, there is a small passageway into the fuel bowl. So it works "as advertised" in the text near as I can tell.