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Discussion Starter #1 (Edited)
This hasn't been my week for my Lawn Boys.

I was mowing a neighbors lawn and I believe I adjusted the carburetor wrong on my 5273 when I rebuilt it last year. The engine starts and runs okay but it seems too smoky under load and like it's bogging. The "power hum" the mower gives when under heavy load seems very low and weak. I have the screw turned out to 1 1/4 turns. Is that too rich? Unlike the F series this adjustment seems way more tedious. I think I'm starting to understand why some people don't like Lawn Boys. Any help is again deeply appreciated :)
 

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Actually that has much less adjustment because that has an atmospheric adjustment because it is a fixed jet. Preset is 1 1/2 turns and then tune from there. Turn in a little at a time until it starts to falter and then back out 1/4 to 1/2 turn. I have a 5273 and it runs well with no extra adjustments and little to no smoking. Obvious question would be how are the ports? Clean? Pretty hard to clog up that big dinner plate muffler unless something would be in the round outlet. May be some other issue if it is running rich. Try manually lifting the throttle rod to speed up the engine and see what kind of power it has. Don't over rev it but it should speed up pretty well and have a lot of pep. If not, check ports. Any chance the needle is not seating after the cleaning and it is flooding out? And for the record, I personally love the Lawnboys, especially the older ones due to their simplicity and ease of operation and repair. Don't give up yet. Bill
 

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Discussion Starter #3
I had the adjustment at 1 1/2 turns but it didn't seem like it wanted to falter at any point even when I got close to closing the needle. That's where I'm stuck scratching my head. I allowed time for adjustments to stabilize. My ports are clear and throttle response is normal. The float was terribly loose before and I crimped the little hinge clip on the float to tighten it up a little. Needle and seat made a little clicking noise when opening and closing indicating they were working. Could the float still be at fault?

I love the old LBs the D400s are great reliable machines but the D600 seems to be finicky to adjustment.

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Discussion Starter #4
Update: I turned the needle adjustment out a little more after letting it sit a while and it starts good cold. Adjustment is maybe about 1 1/8 turns to but it doesn't seem to smoke bad anymore. It burbles and seems to tackle long grass okay. Thanks for the help! :)
 

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Discussion Starter #5
Another update: Turns out the oil seal is indeed leaking. The muffler was sealing correctly and had no blockages but I had a nasty oily film around the part where the crankshaft is. Guess that's why the mower wasn't responding to adjustment very well and running really rich at 1 1/2 turns out. Will pull the engine and give it a metal oil seal instead of the rubber one.
 

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Discussion Starter #6
Okay finally have an update. I've since replaced the seals and adjusted the altitude screw until the engine sounded okay. It still smokes but that's probably from all the oil residue in the muffler. What's a good area to have needle out? Is between 3/4 and 1 turn good for the metal D-601 car or is it too lean?

It seems any more than 1 turn will cause it to smoke more and rpms drop and the mower bogs in a load. Around the one turn spot it sounds like it gets up a little in RPM and burbles.

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Discussion Starter #7
Anyone?

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Discussion Starter #9
Everything checks out on this mower. Clean ports, new spark plug, new gaskets and seals. New needle and seat. Still runs like a dog. Adjustment seems to work but no faltering or hunting or surging. I was asking for general ballpark of where the needle rests after adjustment. My engine seems to run okay between 3/4 to 1 turn but I'm thinking that's lean. It runs rough still at 1 1/2.

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Okay a couple of points. According to the Lawnboy Service Manual, adjustments to the atmospheric needle are rarely needed unless the mower is operated at extremely high or low altitude so with the pre set at 1 1/2 it should run pretty well with minimal tweaking. Also, when the atmospheric needle is completely closed there will be no air flow through the carb vent passage therefore the float bowl pressure will have been eliminated and the fuel supply to the carb venturi will be cut off. Just curious, have you tried turning the needle all the way in and lightly seat it to see it the engine stops running? There shouldn't be any air flow if the needle is all the way in. Just looking for something out of the ordinary here. Bill
 

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Discussion Starter #11 (Edited)
Okay a couple of points. According to the Lawnboy Service Manual, adjustments to the atmospheric needle are rarely needed unless the mower is operated at extremely high or low altitude so with the pre set at 1 1/2 it should run pretty well with minimal tweaking. Also, when the atmospheric needle is completely closed there will be no air flow through the carb vent passage therefore the float bowl pressure will have been eliminated and the fuel supply to the carb venturi will be cut off. Just curious, have you tried turning the needle all the way in and lightly seat it to see it the engine stops running? There shouldn't be any air flow if the needle is all the way in. Just looking for something out of the ordinary here. Bill
Thanks for the insight on this! I checked and the engine does die at closed. I didn't run it long like that this time but usually around a minute the engine quits.. It seems to respond to 1/4 turn increments from preset. Is it best to adjust at high speed or light speed? The service manual states to test both speeds. I usually just adjust at top speed.

I forgot to factor I am actually fairly low elevation for Indiana.
 

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I never run my 2 speed Lawnboys on low but the manual does say to check ease of restarting on both high and low. If it restarts well on high then I wouldn't worry about it. Bill
 

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Discussion Starter #13
Will keep in mind for future tuning on my mowers! I unfortunately have come with bad news. I've had a sneaking suspicion that the bottom bushing in the crankcase was worn out of spec judging by the oil seal failure on both ends of the engine. I checked the bottom seal again today and once again seal failure. Found lots of play on the bottom end so I think that means the engine is toast. I don't really want to dump money on a new crankcase either so I don't know what's going to happen to the mower.
 
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