Lawn Boy Not Starting Back Fire through Air Filter

15 years ago

Hi,

I have a Lawn Boy self-propelled push mower that is about 2 years old. All was going well, first pull start, until last week when it didn't start.

At first, I thought it was the spark as I really didn't see one when I held the plug to the head and pulled the cord. I checked the safety switches, replaced the plug and today I replaced the coil. Still no change.

I did notice that if I take the air-filter off and hit the prime button a couple of times I get a backfire or two coming out the air filter side.

Any ideas or anthing I may of missed would be appreciated!

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Comments (7)

bill_kapaun
15 years ago
I'd check for a sheared flywheel key.
concretehole
Original Author
15 years ago
How would I check for the sheared key? I am not much of a mechanic and I am wondering if I should bite the bullet and haul this mower down to the local lawn mower shop for a $30 dianogtics test or start lookimg for a new mower?
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Tarheelman, I did have the cd pack replaced some 10 years ago--when it could not be restarted when hot. I don't really know how to test for spark--have to take the spark plug out, right? I've never done that. fordtech, I guess that would mean the repair shop also for me! Thanks for both of your suggestions. I was hoping for something simple--like, you know, siphon out all the gas, do the sheboygan ritual foozie dance around the container, put the gas back in and voila! working mower!
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I'll stand by my previous post. Having worked with 2 strokes for quite some time now and small engines in general, two stroke small engines make their power at full throttle or close to it. Its detrimental to a small 2 stroke engine to run it at half throttle for extended periods of time, and burns off more carbon, makes more power, and is most efficient at wot or close to it. Think of it this way, You can lug a 4 stroke more because it has a wider power band starting at a lower rpm range up to full throttle. 2 strokes dont lug well at all, and make their torque at high rpm, aka full throttle. More torque, more cutting power. Lawnboys are governed as to not run as fast as string trimmers or chainsaws but still achieve optimum cutting efficiency when running at peak. I'm basing my post on experience in working with and rebuilding/tuning 2 stroke jetskis, lawn mowers, trimmers, chainsaws, etc. All engines have sweet spots, 2 strokes are made to rev and run at higher rpms constantly due to their much smaller power band.
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I know - its the kind of thing I like to do too. Sounds like the deck is okay. I would keep on using it. CD pack failures are irritating, but when you consider that most people have to buy a new mower long before 9 years, it seems very cheap to operate. Even if you have put new wheels and axles on at about the same frequency as the CD packs, you really are not spending any money on it. And, you can still find the rear-bagging kits and deck plates (blank for discharging, blocked for mulching). Other things you may encounter after such long service are crankshaft end seals popping out or just leaking, the need to rebuild the carb and carbon build-up in the exhaust ports and muffler. I always recommend adding Marvel Mystery oil to the gas in addition to your regular oil mix as well as the use of a fuel stabilizer. If you use Ethanol-blended fuel, get the Stabil Marine for Ethanol Fuels. All engines like MMO, in particular the 2-cycles.
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bill_kapaun
15 years ago
You have to remove the flywheel.
I'm not an LB guy, but I expect one will show up to help.
1saxman
15 years ago
Do you know if it's a Tecumseh engine or a Honda? Tecumseh has fins on the head, Honda has a cam cover with OHC on it. Assuming it's the Tecumseh, you will have to take out the brass bolt that holds the fuel bowl on the carb and clean the three orifices in it. One big one across the bottom, one small one down in the center and another small one at the top of the threads. Use the soft wire out of a bread bag tie, then blow them out with carb spray. If you turn the mower up on the right-side wheels you can do it without spilling gas.
Your problem really doesn't sound like restricted fuel but it's so easy you may as well try it. Most likely it's an electrical or valve issue because it would run at least for a few seconds on the primer if the jet was stopped up. You could have a stuck exhaust valve, or it could be old or bad gas. I personally don't think it's the flywheel key since it was running when you put it away, then it wouldn't run next time. I'm assuming you have the bag or mulch plug correctly installed, and the channel for the mulch plug handle is clean? There is a magnetic switch there that cuts off the ignition if the bag or plug is not sensed.
doo_hauler
15 years ago
I can with 99% certainty tell you your flywheel key has sheared. The key is less than $2 and flat rate time is .5 hours to install. Unless you have the knowledge and ability to pull the flywheel off, you may want to seek professional help.

My .02
zoomie
15 years ago
If you changed the coil your were about 2/3 of the way there. Next would be to remove a large nut and have a wheel puller handy. I was able to change a key in the parking lot of my work in 20 minutes. Look at an engine illustrated breakdown and you will see how close you were. Good luck...
concretehole
Original Author
15 years ago
Hi, Just a follow up.

I started getting over my head so I took it into a lawn mower repair shop. It turn out that I had a stuck valve and the repair guy asked if I had used old gas as the cylinder was heavly varnished. I use only hight octane gas with a stablizier as we use this same gas in our motorcycles.

Anyway, I paid $30 for the diagnostics and the repir was $70 which the repair guy subtracted the $30 from, so my final for both the repair and diagnostics was $70 total. Beats paying $250 for a new mower.

Hope this helps someone in the future....

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Lawn Boy Not Starting Back Fire through Air Filter

Comments (7)

bill_kapaun

concretehole
Original Author

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Lawn Boy Not Starting Back Fire through Air Filter

Comments (7)

bill_kapaun

concreteholeOriginal Author

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bill_kapaun

1saxman

doo_hauler

zoomie

concreteholeOriginal Author

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concretehole
Original Author

concretehole
Original Author