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B&S 18HP stopped running

3014 Views 16 Replies 5 Participants Last post by  T-Trim
I'll try the points to check on the tiller. Thanks.

Next patient...I have a Sears 18hp lawn tractor given to me by a friend six months ago. I had to replace the steering gear on it.

It's been running great and was idling while I was taking the trash cans out of the trailer two weeks ago. Suddenly it just stopped running. I thought it ran out of gas but that wasn't the problem.

It cranks but won't fire. I checked and it does have spark on both plugs but I'm not sure how strong it should be with one of these engines. It's not a zap just a small blue arc. When I crank it over it actually spits little droplets of fuel out the carb throat into the air with the air cleaner off. I can see the throttle plate is wet with fuel. I've tried starting it several times in case it was just flooding but no go.

I don't understand how the carb works on it. There's a black diaphram with several springsinside. There's the main fuel line that runs from the tank to the carb and then a line that runs from the carb down to the bottom of the engine. I assume this somehow works off a fuel pump down there? The fuel pump is not in-line between the tank and the carb? New territory here for me but I'd like to learn.

I went to the B&S site but found nothing of any help.

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that is strange that its getting fire and gas, but won't crank, especially when it worked right before that. perhaps the spark isnt strong enough? could be a bad coil.
It will crank it just won't fire over. It was strange because it was idling just fine while I was working beside it and then it died. The seat switch had been overridden by the previous owner. Thinking about it, I guess I need to check other ones. But still, I was getting some spark in it's current state. What should the spark look like, a solid bang, bang, bang or what I have which is a faint blue glow? Where is the coil located on an engine like this and can you explain how the fuel pump works?

Thank you.
since, im not familiar with the model you have, i couldnt tell you a thing bout it other than what the spark should look like. it should pulsate (as you pull the rope) with a little more than a "glow", more like a spark. as for the coil, just trace the plug wires back.
The black diaphram you describe is your fuel pump, which is powered by the pulses (changes in internal pressure) created by the piston's up and down movement. The pulses are transmitted to the pump via the line that goes from the engine's crank case to the fuel pump. Through the use of one-way valves, the diaphram translates the engine pulses into suction which in turn pulls fuel from the tank into the carb.

As far as your problem goes, and this may sound dumb, have you tried some new spark plugs? Sometimes plugs can get fuel fouled and show a spark when NOT under compression but no spark under compression. Considering that the machine was idleing, perhaps it loaded up and fouled the plugs.
I haven't had a chance to even look at it again yet. Things have been too busy. I may get to it this weekend and I'll check all the suggestions I've received so far. Thanks to everyone.
Check your fuel! it sounds by your description that you are getting what u believe to be fuel and spark, your blue spark sounds good to me, but your fuel concerns me. Make sure you have good fuel. You may have gotten water into the fuel. In that case you would appear to have fuel but it would not burn if it was water.Please check this before you tear into the engine.
Ok, I finally found the time to look at things. It had stopped cranking the last time I was checking it so I had to deal with that first yesterday. The starter gear had several teeth chewed up and was wedged up in the flywheel. Both the the starter and the engine were bound up because of this. I put on a new pinion gear and spring set. It works fabulous now.

Back to the original problem. I do not see any spark now that I've got it cranking again. Before I had a small blue spark but nothing now. I also checked it against a direct ground and got nothing. My question is what is the most likely cause and how do I check it.

This engine has a dual circuit alternator stator under the flywheel and a Magnetron ignition armature located beside the flywheel.

Should I be getting voltage from the alternator when cranking? How do I check that and what are the voltage specs if so? Is that power used only to charge the battery and run accessories or does it also have anything to do with generating spark?

My next question is that if I'm not getting spark is the ignition armature the sole problem? How do I test it or do you just replace it if no spark is the problem?

Thanks for the help and the lessons.

The grass is beginning to grow for the season so I need to get this guy running again.
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Erik, the alternator has nothing to do with the starting of the engine, so don't worry about that part. So, here is what I would do next, locate the black wire coming out from under the flywheel, it should exit the engine just to the left of the carb if you are looking at it with the carb right in front of you (front of engine).Follow the black wire (it will be a small one, not the coil wires) you should find where it unplugs. Either a spade connector on the engine or a connector in line. Unplug it, see if you have spark, if you do, it is a safety switch somewhere giving u fits, if you still have no spark remove the coil and make sure it is getting GOOD ground by pulling it off and cleaning all the mating surfaces. Then reinstall, and reset gap, then try it again. If you still do not have spark your coil has most likely failed and you will have to replace it. By the way when you are finished don't forget to plug in the black wire again, otherwise you wont be able to turn your engine off once it starts.
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I know exactly what wire you're talking about. I was looking at it yesterday. I figured it was the ignition wire but hadn't thought of the safety switches. Does that wire go to ground when any one of the safety switches are closed? Does it also go through the ignition switch? I picture a long series of switches that have to form a closed curcuit or else several branches all going off to a ground. Is that how it interrupts the coil? I like to learn and understand how this rig functions for future troubleshooting and repairs. Like I said in my first post - I'm just learning about small engines and everyone here is the teacher.

Do you know what the gap should be? If not I'll measure the existing gap and reset it there.

Thanks much.
the black wire hooks to the kill switch in the coil and is activated by the ignition switch or the safety switches. Just use a business card for your gap that should be sufficient.
If your really interested in how the engine works Briggs & Stratton has several different repair manuels available at your local dealer. If you don't see any, ask him, they are easy to order. Just tell him which engine you have.
Checked it out today. No spark with or without the wire on. Cleaned the ground coil contact with the block - still nothing. I'm going to pick up another coil and new plugs tomorrow and try that.
I think you found your problem...Good Luck
Changed the coil and it fired right up. After idling for a short while it starting running a little rough. Not really missing, just rough - idles up and down a little. I'll check the air gap again. The coil comes with a card to use for the gap. The old one was set at .009 and the card said it was .010.
With the engine you have (L head twin) they do not always idle particularly smooth most the time. If your model is an older one, there will be a idle mixture screw just below the fuel pump which is bolted on the carb. You may be able to adjust this a little and smooth it out but it will also affect how it runs at high speed, so adjust to get the best for both. A engine under no load will quite often run off the idle circuit.
Last night I found the owners manual that I got when the unit was given to me. It's the basic one that comes with it but it does describe several carb adjustments. I figured I'd check them Saturday. Thanks for all the help!
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