Briggs & Stratton 31C707 won’t start after flywheel magnet failure

Hi everyone,


I’m looking for some advice on a Briggs & Stratton 31C707 Type 3346 G5 engine.


The problem started when the flywheel magnets came loose and struck the stator while I was cranking the engine with the starter motor, causing the engine to lock up.


Since then I have:

• Installed a new flywheel.
• Installed a new stator (alternator).
• Installed a new flywheel key.
• Verified strong blue spark.
• Confirmed the spark plug gets wet with fuel after cranking.
• Confirmed the engine has compression.
• Verified the valves are moving normally.
• Found an air leak at the crankcase breather cover because the cover was warped. I flattened the cover on glass, cleaned both mating surfaces thoroughly, and resealed it with Permatex Black RTV. I’m currently waiting for the sealant to cure before testing again.

The engine still cranks but will not fire or even attempt to start.


My question is: Could something inside the engine have been damaged when the flywheel magnets came loose and the engine locked up during cranking? What would you check next?

Briggs & Stratton, 31C707, no start, flywheel

Could the ignition timing still be incorrect even with a new flywheel and flywheel key?



Any advice would be greatly appreciated. Thank you!

You say you have strong blue spark but no fire ?
You have
Compression
Spark
Fuel
The last thing is timing ,
Give it a shot of starting fluid next if you still have No engine response , it's fire or timing

How do you know you have the “correct” replacement flywheel?

It seems your trouble shooting skills are excellent and IF everything is correct it SHOULD run perfectly.

I’m thinking the flywheel is incorrect. It could be the magnets are oriented differently North South instead of South North. It could be the keyway is different. If you have the old flywheel you might be able to find out

Others have given you guidance on the big 3: fuel, compression, and spark at the correct time. I think you have an issue with the latter.

Please let us know your findings?

Thank you for your reply. I still have the original flywheel, and I also have the new one that I installed. I’ve attached photos of both for comparison. Please let me know if you notice any differences that could affect the ignition timing or if the replacement flywheel appears to be incorrect. I appreciate your time and assistance.
Do both flywheels appear to be identical to you?


Thank you!

Just at first glance. I do not see a keyway in the top flywheel but I definitely see a keyway in the new flywheel?

I had one with the keyway off by 15 degrees and it was the wrong one.
You set them upright with the trigger coil magnet down and they should be the same angle. This is the “timing” of the spark.

I see they both appear to have 12 charging magnets but it is the orientation of the keyway to the outside coil trigger magnet that I cannot determine?

When you installed the coil did you happen to put a business card between it and the flywheel to set the “country boy” gap? One business card thickness has always done the trick for me .

Just want to help you get it running, then we can look at the voltage output of the 12 magnets

The photos help but need to confirm more please?

Thank you. I took another comparison photo with both flywheels positioned so the keyways are in the same orientation. I noticed that, with the keyways aligned, the magnets do not appear to be in the same position. Does this indicate that the original flywheel is different from the replacement flywheel, even though they have the same part number stamped inside? Or am I comparing the wrong flywell? Thank you for your help.

They are not the same flywheel. The timing is significantly different. Jack's shows 698281.This is from Reddit.

Be careful the newer 591759 flywheel has the keyway machined about 90degees off from the 698281. Will not work, throws ignition timing way off and all it will do is backfire

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Lixsán said:

Thank you. I took another comparison photo with both flywheels positioned so the keyways are in the same orientation. I noticed that, with the keyways aligned, the magnets do not appear to be in the same position. Does this indicate that the original flywheel is different from the replacement flywheel, even though they have the same part number stamped inside? Or am I comparing the wrong flywell? Thank you for your help.


View attachment 73828

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These are not the same flywheels.
I see the keyway is 12 o’clock on both.
The ignition coil magnet on the top one is at about 1 o’clock.
The ignition coil magnet on the bottom one is at about 10 o’clock

The magnet on the flywheel is where the spark originates and the keyway is critical to the proper “timing”

I believe you have found a reason why it will not run :-(

It’s not going to fire at the correct time until you get a good replacement flywheel. The good news is you now know how to identify the replacement flywheel better by looking at the keyway and the coil magnet.

I think there may be some confusion. I’m not referring to the charging magnets inside the flywheel. I’m referring to the outer ignition trigger magnet that passes in front of the ignition coil. With both flywheels aligned using the keyway, the outer magnet appears to be in a different position. Does this indicate that the replacement flywheel is different from the original, even though both have the same part number? Or am I still comparing the wrong feature? I’ve attached another photo to better show what I’m referring to.

yes. The timing is off with the new flywheel. Not just off but way off.

The simplest solution until you can find one, and there are none on ebay that I could find. New is NLA and no one is posting a superceded number. You can pull the rest of the magnets off, use a rotary sander to remove the epoxy that held them on, and run it without charging. The other alternative until you can find a replacement is to take both to a machine shop, have them measure and weigh the two to be sure they are similar in size and weight, then have a new key way cut in the right location. Neither may be optimal and frankly, all bets are off as to whether a new keyway would eventually cause a crack on the old key way, or the new one. Finding a shop that can cut a key may be hard, and even then getting the timing right may not work.

A mower bone yard may have one. There is one on ebay but it is probably as much $$ as a new one.