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I cannot find a model 421707 Briggs listed anywhere on-line??? Does anyone else see such?????

I was attempting to look at a parts lists for both of his engines, such as flywheels, mags, etc and compare.
flywheels do shown different part numbers for the two engines.

Magneto for both shows a part number of 590781 and this amazon link indicates the mag should work ok on either engine.

https://www.amazon.com/HIPA-Ignition-Armature-Stratton-Horizontal/dp/B012OEQDK0/ref=sr_1_2_sspa?hvadid=78408975543882&hvbmt=be&hvdev=c&hvqmt=e&keywords=590781+magneto&qid=1566818523&s=gateway&sr=8-2-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEzMzZCWVVMSEdSNDI3JmVuY3J5cHRlZElkPUEwNTc1OTE4NVNQWFgyTktIOUI4JmVuY3J5cHRlZEFkSWQ9QTAyMzQ5MjAzUVIyWExSMDRRM1pMJndpZGdldE5hbWU9c3BfYXRmJmFjdGlvbj1jbGlja1JlZGlyZWN0JmRvTm90TG9nQ2xpY2s9dHJ1ZQ==
 

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Ok so the basics
•flywheel turns/
•coil charges/
•charge travels down wire's/
•plugs recieve and transfer charge to tip and combustion happens in chamber/
•ground(kill switch) disrupts charge at coil
 

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Is there an armature ground wire with the diodes between the key switch and the coils? I picked up a 16hp vanguard v-twin that would not start and had several modifications to the key switch and ground. Once I got to the heart of the matter, the two diodes were toast. $13 and a new OEM wire later it fired up!
 

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Magnetron ignition engines are timed that every time the flywheel passes the coil the magnets fire the coil. It is a waste spark engine. No points needed. Roger
 

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Well, as long as no one still has the answer, I guess I'm allowed my silly thoughts.

You mentioned a 2-business card gap. I'm wondering if the flywheel is not completely round and setting in the opposite side (opposite the magnets) is setting the magneto pickups too far out. (Could just be me, but it looked like the magnets were not completely flush.)

Before giving up, I'd try setting to much narrower gap as a test.

Baring this, it leaves only the quality of the magnets as the only variable.
 

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You said it has good compression,I’ve had the Briggs motors valves seize in the guides,you might check if both valves are free but if you have the compression you posted most likely that’s not it but a check would eliminate that.
 

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Is there an armature ground wire with the diodes between the key switch and the coils? I picked up a 16hp vanguard v-twin that would not start and had several modifications to the key switch and ground. Once I got to the heart of the matter, the two diodes were toast. $13 and a new OEM wire later it fired up!
OP has opposed twin with only one coil.
 

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Is there an armature ground wire with the diodes between the key switch and the coils? I picked up a 16hp vanguard v-twin that would not start and had several modifications to the key switch and ground. Once I got to the heart of the matter, the two diodes were toast. $13 and a new OEM wire later it fired up!
The diodes are there to convert AC current from the alternator, to pulsating DC for the 12 volt DC motors.

On my Simplicity, the diodes convert the AC to DC for the deck lift motor.

Nothing to do with engine ignition.
 

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The diodes are there to convert AC current from the alternator, to pulsating DC for the 12 volt DC motors.

On my Simplicity, the diodes convert the AC to DC for the deck lift motor.

Nothing to do with engine ignition.
V-Twin Briggs, which have two magnetos, do have diodes on the kill wires, where they are spliced together, to prevent the coils from shorting each other out. It was a good thought but not applicable to opposed twin Briggs with a single magneto.
 

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Hi everyone.

I have here a B&S 18.5 HP opposed twin (model 42A707) that has been sitting since 3-4 years maybe less maybe more. I'm trying to bring it back to life, the starter motor was seized up so I changed it. The engine is cranking, and have very good compression. The fuel is reaching the spark plugs in the cylinders. It doesn't make fire. So I have here a brand new ignition coil so I installed it on the engine. No spark on both cables (tested with a spark tester). I thought maybe the new coil was bad (things like that may happen) so I took another one from another engine. It's the same. No fire.

I reinstalled it on the engine it took it and it fired right up. My question here what am I missing? All the wiring is good, the grounds are good to (nothing in excessive rust), I've checked the safety switches ( brake and seat), I have a tractor here of the same model so I have compared the wiring and everyting is there.

Can the ignition switch cause a problem? It's the only thing I don't have a spare it has more connectors than the ones I have on all my other tractors...

Thanks in advance for your help
These engines have some unique issues with spark as follows: 1) the spark circuit is a series circuit through both cylinders plugs at same time, ie out one wire, thru that gap to ground and ground to other plug and thru that gap to high tension lead back to coil. 2) Because of this, both must be tested together. If not tested together, damaging high high high voltage is trying to go somewhere and it may damge the coil by a turn to turn failure. 3) the cranking speed of these engines is often to low to create sufficient spark. The best way to test this circuit is to leave plugs in and use a neon bulb tester that lights if current flows. If no external shorts to allow current flow, then the bulb tells if spark exists.
This also tests that cranking speed is good enough for spark.
 

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Discussion Starter #54
The best way to test this circuit is to leave plugs in and use a neon bulb tester that lights if current flows. If no external shorts to allow current flow, then the bulb tells if spark exists.
This also tests that cranking speed is good enough for spark.
This is the tool I have. Both plugs in the heads. No light. :tango_face_sad:
 

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Discussion Starter #55
I didnt see where you stated you changed the plugs. Did you install new plugs?

Yes brand new out-of-the-box spark plugs tested and working on other engine.

I don't know what to do.. The two things I didn't do is swap flywheel (because I don't have any that fits on that engine) and swap the engine on another frame...

This the setup:

- Full charged battery
- Brand new ignition coil that works on other engine (1 business card air gap)
- Ignition coil grounded on negative post of battery (to eliminate bad ground issue as suggested in replies)
- Brand new spark plugs
- Engine is rotating a the same cranking speed, like the 5 other opposed twins I have here (and all of them are running engines)

And thank you for the help guys. :tango_face_wink:
 

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This is a long shot but I recently worked on a mower that had a time delay built into the seat safety. This would allow you to bounce up out of the seat for split second without having the motor shut down. Time delay module. Not sure you would have one on your mower but on this unit the motor would turn over but would not fire.
 

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FWIW, the magneto doesn't give a fig about where the keyway is in the flywheel, as it will fire whenever the magnets have done their things. Generate enough juice to create a field in the coil, and then drop that juice(voltage) to ground and collapse the field. That is all they do.
So, if you pulled both spark plugs to allow for faster cranking, and to supply the high voltage terminals with something to work on(lay the plugs on metal while cranking), you should see sparkage at both plugs are just about the same time. As noted above, the same electrical voltage is applied, and the current flows roundabout to finally get to ground.
I bet a nickel that you have grounded the coil somehow, or mounted it somehow, that is causing the spark to go right to the block, and bypass that rigorous route through coil-plug-block-plug-coil.(more or less).
To put it in different words, this particular coil setup grounds differently than a single plug coil, even though all it does is create one jolt of juice that flows across two plugs, and the block before getting back to the coil.
I am trying to some mental gymnastics to figure out if the coil itself needs to be grounded, or just one side of the high-volt secondary.(along with the 'generate' winding that creates the low-volt primary).
Either way, with two plugs in series, the ground is totally different from that of a single cylinder application.
I started yapping about the magneto not caring about keyway. That was the start point of my thought about the flywheels, one working, the other not. The layout would have to be the same as far as the permanent magnets around the periphery of the flywheel to be able to generate base voltage and signal voltage. The NSNSNS pattern of magnetic fields around the flywheels on the outside would have to be the same to operate the magnet the same. I'd take a hacksaw blade and move it around both the working and non-working to determine if I get the same pattern of magnetism. Perhaps draw a picture/schematic relative to the keyway on both, show where magnetism starts/ends and see if they are similar. To my thinking, for the magneto to work, it needs a particular setup of fields, or it won't work. While poking around with a hacksaw blade, you can mentally compare the strength of the fields on each flywheel.(if they are totally different in strength, maybe one flywheel got dropped and lost magnetism) ... I understand that B&S used to offer a re-magnetizing service for the occasion when the electronic ignition was taking the place of the 'points' ignition. Along those lines, if a field is weak, perhaps it can be strengthened by B&S or by cooking up an electromagnet that induces stronger permanent magnetic field in your weak flywheel. I *think* an electromagnet with an iron/steel core to 'concentrate' the field could be placed on/near the flywheel magnetic spot, and have the winding energized such that it is in accord with the flywheel magnets, and make them stronger. Dunno, never tried it. I do know you can magnetize screwdrivers by winding around them and applying DC(quick/short bursts or it gets HOT). I think the same can be done...
tom
 

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Discussion Starter #58
This is a long shot but I recently worked on a mower that had a time delay built into the seat safety. This would allow you to bounce up out of the seat for split second without having the motor shut down. Time delay module. Not sure you would have one on your mower but on this unit the motor would turn over but would not fire.
I tried with the seat switch plugged and unplugged. The blades and brakes switches must be operational because they let current pass through them, unlike the seat and reverse switch when leaving the seat or shift in reverse they ground the coil and the engine stops.

The tractor is done, it's rusted and the mower deck is all seized up. Can a rusted frame somehow interact with the running of the engine?

Later this week I'll remove the engine from the tractor I want to put that troublesome engine on and see... Maybe some wire somewhere... The engine runs on the other tractor so with the same wiring maybe a miracle will happpen....
 

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At this point I would use a multimeter and ohm the coil(s). One probe on the spark plug socket, the other on engine as a ground. A reading between 3k and 15k is normal. Higher and it's a poor ground. 0 ohms means the coil is bad.

If all good, it can only be the flywheel magnets. I know on old engines (I'm dating myself) the magnets could actually break off and get lost. But enen in modern engines, magnets can get weak.

Theres a used one on Ebay for $15 (inc shipping).

https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.com/ulk/itm/113749920449
 

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That was the start point of my thought about the flywheels, one working, the other not. The layout would have to be the same as far as the permanent magnets around the periphery of the flywheel to be able to generate base voltage and signal voltage. The NSNSNS pattern of magnetic fields around the flywheels on the outside would have to be the same to operate the magnet the same. I'd take a hacksaw blade and move it around both the working and non-working to determine if I get the same pattern of magnetism. Perhaps draw a picture/schematic relative to the keyway on both, show where magnetism starts/ends and see if they are similar.
Excellent suggestion!
 
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