Magneto Mower Produces Shower of Sparks

Hold on..... so you are saying there are no points?

How on earth does it 'know' when to fire the spark plug then?

There must be some trigger signal for the Atom thingie to fire the magneto coil, to produce a spark... Or maybe its one of these diesel engines that just need a spark to get it going like a glow plug???

Is it diesel?

John.

Its a Magneto, points are not required.

I believe Magnetos even predate points.....Aircraft used them for years....

Its a Magneto, points are not required.

The term "magneto" has nothing to do with the presence or absence of points.

Ordinary old-style magnetos, as used in aircraft, lawn mowers, older motorcycles, etc used points to time the ignition event. The term "magneto" simply means the the energy for producing the spark comes from a coil past which a magnet moves. The alternative is to use a battery to supply EMF to the secondary coil. Thus magneto-equipped engines do not required a battery to run.

More modern magneto ignition systems replace the switching function of the points with a transistor (and the supporting circuitry). A small trigger coil generates a pulse at the appropriate time, and that pulse causes the transistor to conduct. A simple but huge improvement over points, resulting in no need to set and clean points, and in practical terms, to set ignition timing routinely. This triggering method is the basis for modern ignition systems on cars (which, of course, use a battery rather than a magneto to supply ignition current), and is it is what allows 100,000 mile tuneup intervals.

A timing light would help the poster determine if the spark is really occurring at something other than the correct time.

Ohhhhh I've just had a look at the patent and its obvious that you should be getting multiple sparks... its a multiple sparker type wotsit!!!

But it will only spark when the magnets in the flywheel pass by the maneto coil so building up the flux in the magneto and then the ATom shorts or closes the primary of the magneto coil after a time delay which is built into the Atom, thus creating a high voltage for the spark plug... This will happen every revolution even when its not needed to fire the plug!!

It sounds like its working fine! maybe the flywheel is not fitted correctly, causing the sparking to occur at the wrong time?

John.

it is indeed a multiple spark wotsit. I think it is more useful to have a multiple spark to fire, it has multiple sparks all the time. This is evident by backfiring through the carb when cranking or puffs of smoke from the exhaust, it is igniting while the valves are still open.

The atom guys have several similar patents for this style of device, they have all expired now and there is no actual patent number on the device, it simply says its patented. The patent i linked too was the last one and is the only one to mention multiple sparks, it stands to reason that this device is of that style though.

by eye, i can not see a visible pause in the sparking when it is cranking over. I am going to try and video it tonight if I get home before dark so i can play it in slo-mo.

I'll put the vid up somewhere if anyone is interested.

I think the atom thingy if it acts like points will be closed for most of the flywheel rotation and open when a spark is required.

This is why i figured an intermittent coil (going open during high current) could cause the sparking.

Not sure how to test the hypothesis the best way yet.

First, I would suggest you check the valve timing. Are you sure you have got the valve timing correct? If that is wrong it will backfire too. And if it is wrong it may have broken the rod in the first place by the valves interfering with the piston at TDC.

As well as checking the valve timing marks, check the timing visually by rotating the engine by hand while checking that the valve overlap is roughly uniform at TDC on the exhaust TDC. A valve gear key could be sheared somewhere giving you the false impression that the valve timing is correct, whereas if a key is sheared on a timing gear then the valve timing would be wrong and that indeed may have been the original problem that broke the rod in the first place.

Also, make sure the valve clearances are sufficient so that a valve is not being held open when it's supposed to be closed as this will also cause leaking and misfiring.

Also do a compression check. There should be a compression figure somewhere on the internet and make sure yours is within 10 or 12 percent of that figure.

Doesn't it have any ignition timing marks on the flywheel and on the stator? If it does, could you get someone else to do the pulling on the starter cord while you strobe it? With such a good spark you should be able to get a strobe to work and let you see if the ignition timing is somewhere near correct.

If it doesn't have timing marks on the flywheel and the stator, is it possible to put a mark on the outside of the flywheel where the magnet that operates the coils is, and onother on the stator where the coil pickup is, and then see if they roughly coincide when the strobe is lit?

Is the stator plate rotatable so that the timing is adjustable or is the stator fixed in place? If it's rotatable then try moving it small increments at a time to see if it shows signs of wanting to start at any point.

Is the fuel correct and fresh? Could the broken rod have been caused by the previous owners using polluted fuel or diesel or water infested fuel?

All the basics such as the above need checking first when you don't yet know where the problem is.

Because a hard hit at the mower blade it is possible that the flywheel is not as his place, check the key (take it out an check).

On the briggs site you can find good service advice and spre parts list.

Remove head, check valve timing. Correct valve timing by removing & rotating camshaft into correct alignment. 4 stroke engine = 1 intake stroke, intake valve open, 1 compression stroke, valves closed, 1 combustion stroke, valves closed & 1 exhaust stroke, exhaust valve open. Fuel + Compression + Spark = Running Engine!

Shade trees & apple trees

I thought all of the aforementioned where good, I replaced the rod,piston and rings, rehoned. checked all the internal keys are fine. Valve timing was easy to set up and I double checked the valve timing by eye before putting the head on.

I put a new flywheel key in it, since the old one was torn in half lengthwise when removing the flywheel. I expect it was pretty much sheared when the rod broke.

Replaced the fuel, partially stripped the carb, checked the float needle and seat functioned ok, set idle and main jet positions to 1 1/2 turns from the seat.

Its an L head so the valves are in the block and run parallel to the piston and off to one side, there is no way a valve can hit the piston. I didn't spend much time on them though .. I think I should have.

I was thinking the only thing stopping it from running is this timing issue .. now I'm not so sure and I wish I had given the valves a good going over while it was all in bits.

I was thrown off track by the multiple sparks, just not used to that many, It didn't look right.

I just went out and made a video of the spark. its nighttime so I illuminated with a big torch and hit the starter, results are inconclusive so far since the flywheel is spinning to quick to catch the (blurry) mark on every rotation. I think i am getting about 8 frames per rotation and catching a spark on maybe 3 of them. Several times I can see a spark well before TDC though. it looks on the video to be at least 20 degrees before. Its probably correct.

BTW its an electric start model. Pinched the battery from my FJ1100 temporarily.

The timing mark appears to be pretty much lined with the flywheel magnets. TDC feels like its when the timing mark is lining up with the magneto.

I haven't managed to get the lighting well enough set up to illuminate the flywheel but not the spark plug yet.

Stupidly, I hadn't even considered getting hold of a decent timing light. I thought it would show an erratic spark. Now I'm not so sure, I think I was on the wrong track. I will get a light and check what is happening.

I guess I will have to go back to square one and check the valves.

I'll post the video somewhere if I can edit the size somewhat. I've never done any vid editing before.

Stoney, I use Windows Movie maker to edit videos and stuff...

Its free to download from Microsoft and its a useful tool to have...

You can search the Microsoft web site for windows movie maker or search for windows power tools...

John.

what about the distance between the ignition coil and flywheel ? if it is about 0.5mm (check with normal postal card if special tool not available)

give us the type numbers of the engine

Everything you are stating is pointing to a valve timing issue. Assuming it actually has all the right parts for that engine, carb, camshaft, magneto, crankshaft, check for crank timing and cam timing. Use a stop plate and a degree wheel, to verify that the crank and camshaft are not twisted out of true. It is not too uncommon that a catastrophic engine failure can cause this parts to twist and go out of reference timing to each other, doesn't always matter what the marks show.

If you replaced the cam or the crank then you need to make sure the right parts were sold to you. Another problem is the cam reference mark may be wrong, it is not rare for these to be 6 degrees off, if both are off 6 degrees the same direction all of sudden everything is "lined up" and off by 12 degrees. Big problem and one I have seen more than once.

Good luck!

One other thing I forgot to mention is with a broken crank you can bounce the valves off the cam or the head, even on a flat head like you have. If this happens the valve can be bent causing a slight sticky valve and this will also cause all the problems you are having.

One more thing are the valve springs any good? If they let the engine run out of oil all the internals will get very hot, taking the temper out the springs. They will seem okay until you put an actual test on them to see what their current rating is. If you have soft valve springs you will also get the problems you are experiencing due to valve float at very low RPM.

armature clearance set to 0.05mm with the plastic shim supplied with the Atom module. I have tried it further away with only a reduction in spark to prove anything.

some good info on the valve side of things I hadn't considered, thanks for all that.

I will post any results here just so anyone else with this problem may stumble across it via google, I have seen mention of exactly the same symptoms but people do not seem to come back and post the actual solution.

the last post trail i read ended after a mention of a worn intake valve guide possibly causing the intake to not close fully. i think i still have a compression tester aroun somewhere that could be well worth the effort in finding.

motor is a 281707-0148, its the IC (industrial commercial) version of a 280700 series.

12.5 HP L-head.

briggs and stratton manuals have got to be one of the most woefully inadequate things I have ever come across.

I have Briggs and stratton manual CD that has a search function, it comes up with all the searchable words including useless ones and these can be selected or type in your own. The word 'flywheel' is not mentioned in this list even though there is an entire chapter on it. plain stupid.

stoney, the ignition system we built was had a coil with many turns generate a hi voltage pulse which was rectified and charged a capacitor. a second coil was pulse coil to trigger a scr, discharging the capacitor thru the primary winding , generating a spark from the secondary winding. no points

You will get two sparks per four stroke cycle,i.e. one per revolution,one at the correct point and one,well you work out where in the cycle the dummy spark comes.

There is also a centrifugally controlled gizmo on the camshaft which lifts the exhaust valve for starting which sometimes malfunctions and causes bangs and flames,but usually back thro the carb. (was it the exhaust valve?,maybe it was the inlet)

The key between flywheel and mainshaft often shears or part shears due to the mower blade hitting a stone,stopping suddenly and the crankshaft inertia doing the damage.This obviously alters the spark timing drastically.

The ignition modules I have seen are triggered by the trailing edge of the flywheel magnet passing a trigger coil incorporated (hidden) in the module.

You can actually get keys 'ready part sheared' to tune engines for go-cart use.

The B&S manual and the info from the ignition module makers is no help,and can be disastrous.The instructions may tell you to remove the flywheel to take off a pushrod which was used on some engines to operate points,I did this and not having the manuf puller I used a three legged puller which cracked the flywheel just as it separated,only to find there was no pushrod anyway,I had to buy a new flywheel.Cost £55!

Hope these ramblings help.

Dear Stoney, how are you getting on? Or are you still "Stoney Broke"?

a combination of overwork and house drainage problems have taken over at the moment. Drains fixed up yesterday, no longer is my washing machine pumping out through my shower drain. picking up my new V8 tonight too I hope. just too much going on.

I did manage to find my $10 garage sale compression tester, this is indicating a solid 75psi with the needle peaking at about 95psi. I would assume it should grab this peak if the valve in the compression tester was perfect, so i am taking this 95psi as the actual result. considering this is a freshly honed bore i think it should go up maybe 10% once run in.

from this i would be led to believe that compression is adequate and my valve timing is ok.

I am going to check what the actual mark ( a wide white line) on the flywheel indicate and what it is checked against. it is offset from the flywheel magnets by small amount, i suspect that when this mark is compared to the armature it is indicating TDC.

Without pulling the head again it is tricky to properly identify TDC since the plughole is in the squish chamber adjacent to the piston, it is however directly above the intake valve so I could check the valve timing if I knew exactly where TDC was now the motor is back together. I think a bent paperclip or similar and i should be able to get a feel for the piston position.

regarding ...

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here is also a centrifugally controlled gizmo on the camshaft which lifts the exhaust valve for starting which sometimes malfunctions and causes bangs and flames,but usually back thro the carb. (was it the exhaust valve?,maybe it was the inlet)

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i don't think this motor has a decompressor at all, I had the camshaft out and there is nothing on there at all, it is one piece.also turning it over by hand slowly there is full compression.

I can not identify any parts that look like a decompressor on the exploded parts list, I have not had the valves out at all, so i may be missing something.

I can only use the electric start at the moment as the rip cord pulley is shattered and I had no real intention of replacing it.

on ..

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The B&S manual and the info from the ignition module makers is no help,and can be disastrous.The instructions may tell you to remove the flywheel to take off a pushrod which was used on some engines to operate points,I did this and not having the manuf puller I used a three legged puller which cracked the flywheel just as it separated,only to find there was no pushrod anyway,I had to buy a new flywheel.Cost £55!

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I am not entirely sure what this motor must have used originally. no mention of any points anywhere, no pushrod for a points. no trigger magnet. I suspect it was an electronic ignition built in to the armature. Working on a similar method to what is now being used. (the atom doohickey)

I am also thinking that the offset key might be a solution, i have been tempted to remove the keyway and tighten the flywheel on so the igntion is quite retarded from what it is now just to see if it fires properly.

anyways .. next jobs here.

identify TDC exactly.

beg/borrow or buy a decent Xenon timing light.

Many thanks for the update.

Good work and well done. I gave you a GA for your methodology.

As with many problems, once you KNOW what is wrong, its suddenly simple, but getting there shore ain't!!!

See you again in another blog sometime.