I have a 2 stroke lawn-boy that won,t start

mic742005October 20, 2006

I am working on a lawn-boy 2-stroke lawn mower for my neighbor I believe it mid 70,s model #5273. I tore it all the way down to the piston took the carb apart and cleaned it.It has 70-80 psi of compression, good spark its getting gas.But I can,t even get it to fire with starting fluid. Does any one have any suggestions. Thanks Mike.

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Don't ever use starting fluid in a 2-cycle, since it washes off any trace of oil in the cylinder. The rings could stick if it doesn't start or if no fuel mix is getting to the cylinder. Just use regular fuel mix if you want to prime it. If it has compression and you introduce fuel through the spark plug hole and it doesn't at least fire, the problem is in the ignition.

    Bookmark   October 20, 2006 at 9:47AM
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wise_guy(NW MN)

Saxman makes 2 good points.

I would like to add that you may have a crankcase leak. If the case halves leak or you have a bad crankshaft seal, it will not create proper vacuum and compression in the crankcase to draw fuel in and push it up in the cylinder properly.

Remember: two-strokes have primary and secondary compression. Just because the top-end secondary compression is good, doesn't mean the bottom-end primary compression is.

Also, don't forget to rule out a plugged or restricted exhaust port. Any restriction of the exhaust greatly reduces the volumetric efficiency of the engine.

two-stroke operation
Above link is in reference to a rotary-disk-valve ported induction system, but the basic principals are the same.

    Bookmark   October 20, 2006 at 1:03PM
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Nice link, illustrating how the 2-cycle is mechanically simple but complex in theory. Although not technically correct, I like to call them 'self-supercharging'.

    Bookmark   October 21, 2006 at 9:30AM
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Lawnboy is notorius for clogged mufflers. New replacements are somewhat expensive. But I see you have the capability to do the overhaul, so if you want to save some bucks. Remove the muffler clean the outer edge (upper flange) you will see were it is spot welded togather drill out the spot welds, seperate the halves and clean it out (no special baffles inside)then reassemble and plug weld the old spot welds. Or if it has rust through holes repair them, 2strokes don't run with plugged mufflers but do need some back pressure which is gone with perferater rust or no muffler at all.

    Bookmark   October 22, 2006 at 4:39AM
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While a plugged muffler or plugged exhaust ports will keep a LB from running right, it will not keep it from at least firing a few shots. Engine couldn't possibly have run to that point.

Cleaning the muffler depends on WHAT muffler your LB has. The one pc. steel ones can be thrown in your charcoal grill when you have finished cooking and left. Tap/blow out when cooled.

The muffler for the model number given is a 2 pc. cast aluminum one which easily comes apart for cleaning.

Use some SeaFoam in your fuel for a few tanks full.

Your trouble could be a lot of things. That model is the first one with CDI which continues to be a weak spot with LB. However, you say you have good spark so shouldn't be that. Could be bad crankcase seals or bad reed valves.

Walt Conner

    Bookmark   October 22, 2006 at 12:22PM
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I just rebuilt a Lawn Boy 10201 Silver Series (1993). It was brought to me with no shroud/tank because the old one leaked and he threw it out. I found another one but it leaked too around the slot tabs where it slides onto the cowling. I managed to seal it up some till I can find a new one. I'm getting good compression but have no way of checking leak down. I work on so many machines as a hobby so I can't remember if I made a new gasket for this one or not. Hum, one thing to check. I haven't checked for spark yet.

    Bookmark   July 29, 2014 at 5:48PM
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Toro/Lawn-Boy had a recall on the gas tanks and replaced them for free. I have no idea if that is still in effect, but you might ask a Toro Dealer.

On the original question, I still say if you put some mix in the spark plug hole it should at least fire unless the coil or plug is bad. This bypasses the reed valve, crankcase compression and everything else. The CD coil pack has two stages of spark advance; 800 RPM and below is the starting circuit and over 800 RPM is the run circuit. If the low range circuit is bad, you won't start it unless you can spin it at over 800 RPM.

    Bookmark   July 31, 2014 at 10:52PM
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rosemallow(z7 Md)

After all the replies I see nothing mentioned about the reed valve part# # 1856710 .
Also one of the things that made Lawn Boys run so well was they used a stuffer plate to get better vacuum, which meant more gas intake.
I haven't worked on one in many years, but reeds do go bad. I remember if a engine missed or backfired it could bend a reed. I also remember turning them over.
You rebuilt the carb , but did you check the reed??

    Bookmark   August 3, 2014 at 12:51PM
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