Altitude Mixture Adjustment

I rebuilt this engine over the winter, replacing the utility modular carburetor among other things, with a new reed plate and rebuilt a used D600 metal carburetor. It was dirty with old gummed fuel but cleaned up nicely. I replaced the needle and seat on general principles. All the internal passages are clear.

In this picture the governor lever hadn't been adjusted yet so throttle arm points to the 11 o'clock position. It now runs great at 2800 on low and 3280 RPM "normal". I don't want to run it again till summer when I can get it under load in the grass (if I try to mow the snow, the neighbors are gonna talk) so the rings will seat.

But this altitude needle adjust business. The manual says to turn in clockwise from 2 turns out till it starts to "hunt" or slow down. I didn't really notice this, if anything the RPM increased and it ran smoother. On automotive carbs idle mixture screws are leaned out for highest engine manifold vacuum, or RPM. But this is a 2 stroke, so I don't know. It's not super critical, but I like things to be set accurately enough. It did restart easily without priming.

Should I adjust the needle for highest RPM or, richen it to run a little rough? I've heard people say "till it 4 strokes" but I don't know what that means. Thanks for any help.

by four stroking that means it runs a lil rough sounds like a 4 stroke engine comapared to the high pitch revving sound witch basically is where the term comes from when it's running in two cycle(aka the way a lawnboy typically sounds) that particular carb I always turned out 2 1/2 and then started it closed the needle till it started to die and then opened it back up to where the engine runs right and then maybe a 1/4 turn more.

Well I wondered about that. On many auto carburetors turn a mixture needle in far enough the engine will stall out. Seated the needle all the way in for a few seconds and it continued to run, so wonder if something isn't setup right. Didn't want to mess around with it too long till I can get it under load though, and get the rings seated.

OK a little update. I wasn't too clear on these 2 strokes but after mowing a few times it makes sense. The altitude screw is of course the only adjustment on these models.

The idea is, the fuel mixture is set slightly on the rich side at "idle", under no load. When the mower is actually under a heavy load, as in tall grass, it will be just right. Gasoline engines must never run a lean air/fuel mixture while under heavy load. I knew that already though 2 strokes are foreign to me. No power valves or accelerator pump circuits!

The engine in tall grass will have the classic LawnBoy "power hum" and provide maximum power when it's set just right, it will otherwise sound a little ragged at the turnarounds. Once the engine is warmed up you can hear a definite change in the idle quality at a certain spot turning the altitude screw. Can hear it lean out as it is turned clockwise (in). You don't want that. On the other hand you don't want an arbitrary 1.5 or 2 turns out from seated either. Black plugs and excessive carbon. Find the transition point and turn it out counter-clockwise just a smidge from there, it will sound a little staccato. Right there. Check plug after a run, it should be burning correctly at this point. Note that wide temperature or altitude swings and fuel quality may mean another adjustment is required.

Hope this helps anyone looking for this info, and thanks to the folks who 'splained this to us slower kids.

Hi Teds,

Regarding "till it 4 strokes", might be better explained like this. I got this info from an owners manual from an O.S. model airplane engine (a 2 stroke). "Close the needle valve gradually until a high pitched exhaust note begins to be super-imposed on the lower pitched sound". At this point the engine will almost sound like it's firing every other stroke, but of course it isn't. But, like you said, you may want to back it out a tad from there, so it doesn't run too lean.

Regards

Jeff

Well I think it's actually fairly tricky, I use a wideband O2 sensor on older carbureted vehicles. Looking at plugs for their color is difficult, and adjusting the idle mixture screws even slightly (less than a 1/4 turn) makes huge changes in the actual AFR, without being audible or perceptible in the way it runs. In the case of cars, the difference between passing a smog check.

I'm sure the general consensus was to run them well on the side of "rich" versus lean, since there is no chance of nuking a piston. It allows for fuel variability - Ethanol for example, runs much leaner, and temperature swings.

The operator's manual didn't even provide instructions other than "see your dealer". The more cynical among us would also suggest that these rich adjustments also meant bringing the mower in more often for service. With a 32-1 oil mix and a pig rich AFR, this was probably every few years.