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Location: Ottawa Ontario Canada
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Lawn Boy 2-Cycle Carb Jet Settings

09/05/2008 10:39 PM

Have a hand me down Lawn Boy 2 stroke push mower approx. 20 years old .

Damned nice little mower , light wieght , strong , and up to this spring , very reliable .

Wouldnt start unless i sprayed fuel/oil into carb. Removed " plastic" carb. and cleaned etc. , noticed main jet had unscrewed itself downwards in the threaded plastic tube located in the center section of the carb. bowl. ??

There is a adjustment screw for this jet located on the top of the carb. " as usual ".

My question is ; do i screw the jet back into the tube until it seats or is there a factory setting regarding how far to screw the jet in ? I ask this because the jet had discoloration on it as if it had been set at a level that kept it in raw fuel for approx. three or four threads. If i screw it to seat it will be completetly inserted into the tube.

Thanks in advance.

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#1

Re: Lawn Boy 2 cycle carb. jet settings.

09/06/2008 12:11 AM

I believe that it needs to be seated. If it was left unseated, there would be nothing to stop it from vibrating out again.

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#2

Re: Lawn Boy 2 cycle carb. jet settings.

09/06/2008 12:34 AM

Check with the manufacturer. Normally this is the fuel mixture. A good start is to seat the screw all the way in but not real tight, then back off four to four and 1/2 turns.

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#3
In reply to #2

Re: Lawn Boy 2 cycle carb. jet settings.

09/06/2008 1:00 AM

I had interpreted his comments as meaning that he found the jet unscrewed inside the fuel bowl. You believe the jet is an external mixture jet?

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#4
In reply to #3

Re: Lawn Boy 2 cycle carb. jet settings.

09/06/2008 1:54 AM

Hey Bob ; You noticed the difference , .. Most would not .

Its nice to meet a tech. who knows his stuff.

Take Care bob c .

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#5

Re: Lawn Boy 2 cycle carb. jet settings.

09/06/2008 7:43 AM

Not seeing it sound like you are talking about the pick up tube drawing the gas from the bottom of the bowl. If so it may be intended to be screw out to that length. If it's loose in the threads it may have backed out some. I would run it in until the threads bind to insure its not sucking air around the threads or use a sealant on the threads. The end of the tube need to be far enough into the pool of gas to insure engine vibration does not disturb the the flow.

Picture would be nice

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#6

Re: Lawn Boy 2-Cycle Carb Jet Settings

09/07/2008 1:43 AM

Get the manual here

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#7

Re: Lawn Boy 2-Cycle Carb Jet Settings

09/07/2008 3:27 AM

Just guessing, comparing to other 2 stroke carbs of 20 years ago: The main jet you speak of is a metering orifice that allows a fixed amount of fuel per air volume draft to be drawn up into the tube it is screwed into. Consider this a fixed mixture 'first cut' limit of how much fuel can enter the air stream. It must be screwed in tight and not move. In the tube it is screwed in to should be a needle jet. There should be a built in slight loooong taper female part (hard to see it is tapered) in the tube, and a male needle that is fixed to a slide hovering above it in the carb body. The slide is a variable door opening to allow more/less air to rip across the top of the needle jet opening, and cause a pressure drop to suck fuel up the main jet, past the variable needle restriction. The needle itself is a variable flow 'cork' to attempt a rough mixture near 14.7:1 (Lambda 1.0) and the main jet is an overall limiter to how much fuel can enter the tube at full load. Any external adjustments are fine tuning points for this overall process for various positions of throttle (Idle, starting, etc).

One interesting misconception regarding owners of old machines is that they might think mixing a bit more oil in the gasoline will 'help' lubricate the engine parts more and help it last longer. In fact when you do this you are making the gas mixture more viscous and not as much will enter through the main jet making the overall mixture quite lean (Lambda of 1.4 or more), run hot, and melt the piston or seize it. Mixing less oil can actually put more oily mixture to the friction critical parts.

Immediately after a hard mow, stop the engine NOW, pull the spark plug and see if it is golden brown, black, or white-ish. Perfect / rich / lean. Run it to the rich side if given a choice as lean will eat pistons rather fast.

Careful with these tuning tips as you many triple the power and do a wheelie down the street chopping up cars and street light poles. If it works out you can enter your mower in a desert race or something more fun than mowing lawns.

And please, don't tell my ex-wife I used 'Lambda" in the description of setting up a lawn mower or it will just fuel her attorney with more ammunition. She's already listing things like me discussing power factor control on a blender whilst making margaritas. Maybe . . . I have . . . the Knack!

Cheers

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#8
In reply to #7

Re: Lawn Boy 2-Cycle Carb Jet Settings

09/07/2008 2:54 PM

GA

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#10
In reply to #7

Re: Lawn Boy 2-Cycle Carb Jet Settings

09/07/2008 9:12 PM

GA. That's just what I said, only you used less words than me.

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#12
In reply to #10

Re: Lawn Boy 2-Cycle Carb Jet Settings

09/08/2008 1:31 AM

Your post is where I got the basics and then I just asked my ex-wife about all the rest because she knows frickin everything ! I was going to post all her comments but I didn't want the world get 17 paragraphs about pilot jet set points, that I don't have a clue how to make love (with a partner), slide needle intermediate settings based on altitude and ambient, and that oooogie stuff between my toes that I thought was just toe cheese turns out to be webbing after all.

Cheers !

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#9

Re: Lawn Boy 2-Cycle Carb Jet Settings

09/07/2008 8:14 PM

I'm no expert(unless you take my soon-to-be son-in-law's description of an expert which is "anyone who has successfully done something once") but I have taken apart my share of small engines for repair or maintenence(you know how much a lawnmower repair shop charges today?) and those main jet thingys usually have an O-ring seal. If the seal seats above where you noticed the discoloration that may explain it. If that is the case, I would screw it all the way in. That's where I usually find them.

My two cents worth.

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#11

Re: Lawn Boy 2-Cycle Carb Jet Settings

09/07/2008 9:25 PM

If you send some info, I will try to find some carb adjustment settings. Did this use the Super Bee engine. I have seen them referred to as Chrysler Super Bee, and just Super Bee.

Can you find any numbers on the carb, engine, or the mower itself? There is a lot of stuff available for some of these old engines. Don't give up on it.

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#13

Re: Lawn Boy 2-Cycle Carb Jet Settings

09/11/2008 12:46 PM

Have you gotten enough information on the Lawn Boy engine yet. I found two sections in one of my small engine manuals covering Lawn Boy engines.

One is called the F series. The other is listed as the model M25.

If you want the book, it is Clymer #SES2-2. If you want me to send the pages ( about 25 ) send an email address.

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#14

Re: Lawn Boy 2-Cycle Carb Jet Settings

06/06/2009 2:14 PM

i have twenty five years working on small engines and have usually found if you have no manual to look up the setting you should turn the screw in all the way then back out one and a quarter turns ...if the mower starts after it warms up adjust the throttle setting to slow and turn the scew in until the engines flutters than back out one full turn...i also found that on an older non running engine of this type to pull the screw all the way out and use the straw on you wd40 to spray about one or two seconds worth in the hole to blast out any crap and then put the screw back in like i said...i just today did the same on a lawnboy that supposedly wouldnt run and after a half hour of cleaning the filter and new plug and the carb adj. it runs like a top

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#15

Re: Lawn Boy 2-Cycle Carb Jet Settings

06/29/2009 8:43 AM

i would assume by now, no one cares, but the jet needs to be seated. (old lawnboy "master" mechanic - got the tee shirt to prove it)

those plastic carbs were a pain, but they were a fixed mixture, with a small screw located on the side of carb for air bypass

hard starting is usually due to old fuel or more likely, leaking crankshaft seals

shmear (I'm from NY, deal with it) a bit of grease around crankshaft seals, and see if it starts easier.

(back in the day, I made my boss quite a bit of money because of old gas!)

plus, the old fuel would work fine in my 76 honda... recycling!

PS, my lawnboy is 1950's era, still going!

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#16
In reply to #15

Re: Lawn Boy 2-Cycle Carb Jet Settings

04/16/2010 9:58 AM

Johnny, I have an F model engine on my Lawn Boy. Cleaned everything and can't get it to run ... even though it was running when I did my spring cleaning on it. Have rebuilt the carb (everything replaceable) and have it assembled correctly but can't get it to run. It will half-heartedly start but not run. I think it has to do with the screw on the side of the carb. Where should it be set? Should it also be covered by tape ... as it was when new? I have 125+ psi of compression, great spark, but can't seem to get fuel. Cleaned the gas tank and put in fresh fuel too. I used to be a Lawn-Boy dealer/mechanic back in the day ... and this one REALLY has me confused why it refuses to run. Got any ideas? TexanInFlorida@yahoo.com

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