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PK., Small Engine Technician
Category: Small Engine
Satisfied Customers: 1052
Experience:  Retired Owner of a full service shop and national parts sales website.
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Briggs and Stratton 11 hp vertical shaft motor has no spark

Customer Question

Briggs and Stratton 11 hp vertical shaft motor has no spark I replaced the armature-magneto and get a very weak spark, but the the motor won't start. I pulled the flywheel and couldn't find see anything wrong with the wrappings under the flywheel. There are two wires coming from the magnet ring under the flywheel (I don't know what it's called) that aren't hooked to anything. This is an old riding mower that has the deck removed and is just used as a small tractor. All of the safety switches have been removed, and a stop switch is the only electrical control. Over the summer it would occasionally fail to start after it had been run for a while, but would eventually start after sitting for a while. The last time I ran it, it wouldn't stop, even when I grounded the stop wire directly to the frame. I think it's haunted.
Submitted: 8 years ago.
Category: Small Engine
Expert:  PK. replied 8 years ago.

I've had a few that were haunted.


The windings and magnets under the flywheel are your alternator and won't have anything to do with the ignition so you can rule that out for that problem. The two wires are for charging the battery and running lights.


When you installed the new magneto, did you gap it from the flywheel properly? Also, take a screwdriver or some other ferrous object and test that the magnets on the outside of the flywheel are strong.


What color is the spark? When you replaced the flywheel, did you use a new key? What is the stop switch hooked to?

Customer: replied 8 years ago.

Magnets seem strong.

I didn't use a feeler guage to set the magneto as I don't know the specs, but I did use a thin calling card to set the space between the magneto and the magnet. May not be exact but I'd think it would be close enough to fire it.

I didn't use a new key when I put th flywheel back on, but a new one came with the magneto. The old one didn't appear to be sloppy.

The spark is weak and bluish.

The stop switch is just a ground.

Expert:  PK. replied 8 years ago.

The spark should be blue. It may not be very long, but if it's blue, it's pretty good. The ground that the switch is hooked to, is that the ground wire from the magneto?


Will the engine hit on ether?

Customer: replied 8 years ago.

When the stop switch is hooked up, it is to the ground wire on the magneto, but I've disconnedted the wire so it can't possibly be grounding out.


I have tried starting ether with no success.


BTW, I also put a new spark plug in and cleaned the magnet surface.



Expert:  PK. replied 8 years ago.

I think you have something else going on. The first thing to be sure about is that flywheel key. The recommendation is anytime you pull the flywheel you replace the key.


Since it won't hit on ether, then you know that there is a problem either with ignition or compression. You have established that the spark is sufficient. Were it orange, that would be bad, but blue is just fine. What I would rule out next is the timing of the spark. The ignition timing is controlled by two things on this engine. The flywheel key and the air gap of the coil. Check that the flywheel key is in place and that the coil is spaced properly. You can use a dollar bill folded in half to gap the coil. Just spin the flywheel all the way around afterwards and make sure there's no rubbing.


If these things don't help, then you should use a compression gauge to establish the sealing ability of the engine. It won't give you a true picture because of the compression release mechanism, but it will point to the next step.


If you can give me the model and type numbers of the engine that may be of some help. Thanks, PK.

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Customer: replied 8 years ago.



I set the magneto gap with the $bill, and put the new key in the flywheel. Compression is 75psi. Spark looks good but still won't start.


Model 252707, type 0620-01, code 86080111


I'll go ahead and accept the answer and I appreciate your help, but if you can offer any more suggestions I'll appreciate it.

Expert:  PK. replied 8 years ago.

We can keep working on this as long as you like.


I just want to recap first so we can make sure that we're going in a reasonable direction.


The engine won't start on ether with the air filter removed. There is a blue spark at the plug. The flywheel key is in place and the magneto is closely gapped. This is an older, 1986, L-head engine. It was running prior to the current problem and didn't seem to have any catastrophic failure.


75 psi is a little low for this engine, so what I would do is pull the head and give it a good cleaning. Rotate the flywheel so that both valves are closed and the piston is at the top. You may have a valve that's not sealing fully. This is normal maintenance for an L-head engine anyway so even if it doesn't fix things, it should be done every hundred or so hours for an occasional use machine. It's pretty easy, just remove the bolts, use some carb cleaner and whatever you can to scrape the carbon off. If you can find a product called Valve-Tect, that does very well at dissolving carbon but carb cleaner works pretty well.


It's best to replace the head gasket, but I'll admit that I've done literally hundreds and reused the gasket if it looked good. 165 INCH pounds is what the head bolts should be torqued to. The pattern to torque them is usually in an X then in + but a lot of people just start in the middle and work outwards.


Other things to do would be to give the muffler the once-over and see that it's clear and when you have the head off, rotate the engine over a few times and see that the oil is being wiped from the cylinder and the valves are fully opening and closing.


Try these things and let me know what you get.

Customer: replied 8 years ago.

The mystery continues, or maybe not.


I pulled the head and cleaned both surfaces and the gasket with carb cleaner. The gasket doesn't look all that good to me, but it's not broken anywhere. After I put it back together the compression was down to 73. The valves and valve seats look good, and open and close properly. If the compression is low enough to keep it from firing, maybe I need a new head gasket, because it still won't start on ether. I hate to keep throwing parts at it without knowing the problem, but we know it has spark, fuel and (some) compression.

Expert:  PK. replied 8 years ago.

I'd call around to small engine shops and see about getting a leakdown test done. On a single we charge a half-hour shop time. That's about average around here. This will give you a complete picture about whether the engine is sealing well enough to run and will isolate any trouble spots.


You can try one other thing if you like and that's pouring about two tablespoons of gear oil into the cylinder and then taking another compression reading. I expect you'll see it go up by 20 psi or more. If it does, then you can put the plug in and see if it will hit on gas. Then try it on ether if it doesn't hit on gas. Don't use much ether when you're doing this, you don't really want the engine to run, you just want to see if it will hit and turn over a few times.