Assuming the needle and seat are sealing and the float level is correct this is not unusual with a F engine.It happens from the combination of too much reed clearance and sustained high rpm. Reeds tend to get bent slightly open with long use allowing a tiny amount of reverse flow before closing on the piston down stroke. Now there is a fuel fog which is present in the carburetor of all reed engines, even when new and this tiny amount of reverse flow pushes this fog closer to the air cleaner end. The degree of this fog depends on the condition of the reed and how quick it closes. High rpm aggravates the condition creating a fog that will reach the air filter and soak it. Check the maximum rpm and adjust if necessary, but you probably will need to replace the reed. I think Lawn Boy made a plastic part that was made to fit in the carb to help control the fog, but I don't know the part number or if it will fit your carb. Also check the exhaust ports if you haven't done it in a while.

Lawn-Boy is known for the 2 cycle engine and built it's reputation on it for decades. Keep a new blade on it or resharpen it yourself often. You need to clean and re-oil the air filter more than once a year. Do it at least twice or three times per season. A new plug once a year is good advice and use gasoline stabilizer in your gas. Using a synthetic 2 cycle oil will eliminate 99% of the smoke and the carbon build up as well. Do not loan the mower to anyone without strict instructions on refueling with the proper gas/oil mix. Even then someone else could inadvertently add some straight gas and it's over.

"Curiouser and curiouser. We have read for years on here that the 16:1 was because of bushings rather than roller bearings, now it's no longer true. Okay, then what is the reason for 16:1 (I have no idea why I care, since I will never own one)?" You will note that I say the BOTTOM bushing. The D400 had needle bearings on the top end. The switch in mix spec came when they went to the 2 ring piston AND the needle bearings on the wrist pin end of the connecting rod, perhaps this was the bushing others were referring to which you may have misinterpreted. These parts can be interchanged and I have done it often only the connecting rod with needle bearings on the wrist pin end must be used with the 2 ring piston. As for never owning one, reminds me of someone I know who says he will never own a Ford. Surely most people know that the bottom bushing continued thru the D600 series which used 32:1 mix and all of these LBs had the mix written on them though in early years it was a decal which eventually became illegible. Some had the mix molded into the shroud. I would note that there is a difference between "needle bearings" and "roller bearings". LB, until DuraForce, used needle bearings unless it was the "M" with which I am not familiar. As I have said before on this and other lists, if you are uncertain what mix you should use, clean your exhaust ports and while you are at it, count how many rings are on the piston as you look thru the clean exhaust ports. Three rings = 16:1, 2 rings = 32:1. Walt Conner

"I respectfully disagree with Ted. Modern synthetic oils do not carbonize the rings, clog exhaust ports, or foul spark plugs." Who said anything about modern synthetics? In any case, I certainly stand by the statement that increasing the the oil & gas mix ratio over the manufacturers recommendation - is somehow beneficial. This has always been a myth, and though well meaning perhaps, a very misguided one. It's not really debateable. It is not a good idea, and anyone who has ever disassembled a 2 stroke subject to this abuse knows why! "Modern two stroke engines virtually never foul spark plugs from excessive oiling. Rich fuel mixtures and weak ignitions foul spark plugs." Well we're talking two strokes in general, though they certainly can get fouled if the oil ratio is high enough. It is also very common for people in general to use the gas & oil mix ratio terms interchangeably with AFR, they are not, although excess lubricating oil in the fuel actually leans out the AFR e.g. "I run a rich mixture". Bzzt. I don't believe we disagree on anything but we have to define our terms. Sorry if I wasn't clear enough.