No matter what I did, I could not start my engine last weekend.
Although BME 110 is a temperamental engine never have had starting issues.

Since I have 2 BME 110 engines at home (one mounted on my Edge the other one in stand by to be used for another project):

1) Swapped carburetors, to rule out any carburetor issue, same results.
2) Swapped batteries, to rule out a battery issue.

Once in the shop I removed the spark plugs and tested the spark to check the ignition. It seemed to work OK however after flipping the propeller to verify the timing, after couple of flips both spark plugs stop giving up!

This fact paid my attention since this engine flame out apparently due to a lose tank more than 6 months ago.
However the sparkles event woke up some suspicious, then I started troubleshooting that seemingly lack of spark.

a) I started by swapping the ignition unit, after flipping the propeller couple of times I did not observe any spark.
b) Then I swapped the hall sensor, same results.
3) Then I tested the ignition on the other engine and this one seemed to work fine.
At that point the only think left was the small permanent magnet attached to the hub of the engine.
One thing I noticed is that the magnet could not hold a small needle pin. While the other engine’s magnet could easily hold the needle (check the attached picture).

Clearly the failing engine has mounted a weaker magnet.
I am not sure if these observations make sense to you guys that have seen more gas engine failures.

A possible theory:

To me the weaker magnet induces a smaller electrical impulse into the hall sensor, (Faraday´s law induction E=dB/dt) otherwise will not induce any pulse at all, therefore without no input pulse or too week amplitude, it will not trigger the electronics inside the electronic ignition box. Could this explain why I could not start this engine, having everything else ruled out?

Would that be a valid conclusion?
Has anyone else seen such problem before?
Do these small magnets “decay” or lose magnetism?
Can this explain the spontaneous engine flameouts that I have seen on this engine?

I am going to perform additional tests, I am kind of reluctant to accept the above outcome, however the evidence is telling me that there is not other why to explain this problem.

Heating or hitting a magnet will cause it to lose it's magnetic properties.

Makes sens to me, temperature will affect permanent magnets.
I guess I am tracking the events, long time ago I removed this engine hub:

[link=http://www.rcuniverse.com/forum/m_6319985/anchors_6346853/mpage_1/key_/anchor/tm.htm#6346853]removing the BME hub[/link]

I guess this is how I eventually overheat that section, however the engine worked seemingly fine after I removed the hub using the methode described in the above link.
If you see the date that was long time ago.

That can explain a lot of things happening on this engine. I am sure I did not overheat that area, however enough to expand the diameter of the hub to remove it.
The pulling forze was not enough that day to extract the hub I recall.

Now I face the dilema, how can I remove the hub of the new engine without using some heat to remove the hubs?

I dont know about BME but with mine I remove the prop shaft from the crank and use a DA prop boring jig. It is center threaded for 1/4 20 threads, I mount it to prop hub with short bolts and the run the 1/4 20 bolt down to the crank and it pops off the hub.

Milton

Remove the hub with the weak magnet and get a new magnet, don't screw with the good engine.
A new magnet is only a couple of $$.

You are right, looks like removing the magnet will be the way to go.

Who is taking care of BME parts?
Other wise where can I order these magnets?

Depending on magnet material, magnetism decays more or less at elevated temperatures. Not all Hall sensors are very sensitive, so it can well be that a weak magnet will fail to provide sufficient voltage switching for ignition actuation.

The original CH ignition can be manually trigered by swiming a screw driver with a magnetic tip right in front of the hall sensor.
These screw drivers have a relatively strong magnetic tips.

I am not familiar with the BME ignition system............... BUT,

On some ignitions, if you flip it over without grounding the plug to the cylinder or engine in some fashion, it can blow the hall sensor out. The plug MUST be grounded or you will blow the hall sensor if you rotate the engine with the ignition unit turned ON.

Like I said, I'm not sure if the BME works that way. I've gotten into the habit of NOT turning my engine over unless I KNOW my spark plug and plug lead are POSITIVELY grounded when that hub magnet passes the hall sensor.

Ask me how I know that............. Thanks Ralph Cunningham, for sending me a new hall sensor after I blew it up by being an idiot. [sm=thumbs_up.gif]

mm... I guess is a possibility. ESD protection is what damage the semiconductors, once is gone is gone for good.
Otherwise a high voltage arc can for sure damage any semiconductor as well.

The paths for both electrical arcs are the spark plug gaps by themselves. Developing a differential voltage is still possible unless you take measures against that possibility, I guess thanks for the advice will take care of that.

Nevertheless, the fact that I still can trigger the ignition with the magnetic tip screwdriver tells me that the hall sensor is still functional.
So I am sure that these hall sensors are in good conditions.

AlbatrossCall up the new BME and see if they have any magnets laying around, if not, start calling local supply businesses and see what you can find from them. You could either pein/glue it in as 3w or just pressure fit/glue it with JB weld.

You say that the dodgy magnet is weak compared to your other - it certainly sounds like you've found the problem:- Magnets go-off over their Curie point. For the small Neo magnets it isn't that hot, 80C I think. You can get high temperature versions, but the one fitted in my DL sure looks like the bog-stock Ebay one I fitted to my chainsaw motor! The hub never gets that hot.

Drilling might be awkward, probably more like a Sweeney Todd barber job (butcher!). But a tight fit and epoxy holds them fine. Rotate 180 degrees and re-drill if possible.

At glance removing the magnet seems to be difficult unless I manage to remove the hub + timing collar. I still can use "my patented methode" described above only to remove the hub and the collar timing, the old magnet is damaged anyway.

However once I got the new magnet in place, I wont use any heat at all and will re-insert the hub + collar timing by using a large bench press or ruberized hammer head.

I will try to get the right substitute from the BME folks in Kansas or Kriss if I can find a local shop I will take it.

Thanks

I remove the timing collar form the other engine, and mounted on the old one.
This time the ignition worked flawlessly.

For a minute I thought was not necessary to proceeded with the transplanting procedure, since the ignition system seemed to start working out of the blue.

However the device was triggering double spark; one at 28 or so and another one at 34 degrees. That was weird!
The CH and FALCON + battery and hall sensor worked fine on the other engine.

Removing the prop hub requires a fair amount of pulling force.
The technique I used still works, however this time I paid attention to apply the flame only at the opposite side of the collar and for a short bursts allowing cooling off.
For those interested I can send you details explaining how you can do this job without harming the magnet.

The attached photo shows how the new magnet collar is holding an allen wrench; the damaged one is too weak to hold anything.

I guess the fact of still having some kind of triggering (double spark sometimes nothing) that could confirm comments from other users in regards that the magnet should not be that strong to be able to trigger the CH or FALCON ignition. However double triggering and/or another potential dead stick due to ignition failure are things I don’t want to see happening next time I go flying this airplane.

So by now the ignition is fully functional I hope to do a test on the ground before going to the flying field next weekend.

Just a thought on the double spark. The manufacturer of my aftermarket ign system was very specific about which magnetic pole faced the sensor. If it triggers on the north pole and you have the south pole exposed, it may try to trigger on both sides of the inverted magnet. Additionally, if a magnet is cracked both parts have poles. This too could give a double spark.

I made a quick search on hall sensor, in effect depending on how your ignition brand designer conceived the system; they will use different types of sensors.
The magnet position definitely makes a difference.

The hall sensor may be divided in three different categories:

Bipolar Hall Effect sw sensor
Unipolar Hall Effect sw sensor
Omni polar Hall Effect sensor

[link=http://www.allegromicro.com/en/Products/Design/an/an27705.pdf]Hall Sensor categories[/link]

Otherwise depending on whether or not BME used the same system as 3W or DA, the observations about hall sensor and magnets may change.
I think they are still useful.

As far as having a broken or cracked magnet I can’t tell for sure. I may need to extract the magnet to know, however the sole fact of extracting the magnet may breake it anyway.

The halls sensor is an electronic device, it could very well triggering by the effect of a hysterisis effect or so. Noise can trigger these circuits as well.

A magnet supplier once gave me what looked like a credit card. Instead, it is a laminated transparent sleeve containing iron particles so fine they aren't readily descernable. If you hold this card near a magnetic field, it displays the flux. I have found it invaluable in mounting magnets in outrunners. I'm not sure where these are available if they still are, but it would be interesting to hold it over your magnets just to see what the field looks like. It can spot a cracked magnet in an instant.

The hall sensor paper is well done, I'll keep it as a reference.