Others will give you a more technical description but basically your engine is firing before the piston reaches top dead center.
If your having this problem then either reduce the nitro or use a colder plug
Dennis

Hi Robin,
  I don't want to be awkward, but most engines fire before top dead center, unless they are very slow revving. The flame front takes a significant time to travel through the mixture, so the maximum pressure on the piston is exerted after TDC, even though ignition started before TDC.
  Pre ignition is a problem when ignition starts too early and there is a significant pressure exerted on the rising piston from the fuel combustion. Dennis' suggestions for a cure are on the ball. I have had a problem with engines that have a lot of carbon build up on the piston / cylinder head. I don't know if the pre ignition was because the engine was then overcompressed (less combustion volume, than one would expect because of the carbon build up) or whether bits of the carbon became red hot and became the source of ignition, rather than the glow plug! A decoke solved the problem!

Regards,

Andrew.

Andrew nailed it.

This is why your car has ignition advance units. Time: the faster it runs the more ignition advance needed so maximum cylinder pressure happens when the rod jurnal is in the right place. To early and the pressure slows piston down on upward travel, to late and the pressure lags behind the piston movement. I think the point of max pressure is well after TDC. I just don't remember any more.
Good combustion is a smooth steady burn not an explosion.

Good chamber shapes allow more advance to be used over bad shapes.
Sharp edges and thin section can glow and cause it and carbon also cause it through glowing and compression increase.
The multiple ignition sources and colliding flame fronts would move the max pressure up to a to early situation.
 COMPRESSION RATIO is what sets the timing on Glow engines and diesels.

My best understanding on our glow/ compression ignition engines is that As RPM goes up Volumetric Efficiency(VE) does also.
OR- RPM increases and cylinder pressure rises due to higher boost pressure the ignition point advances(earlier)

Squish band heads narrow the window where ignition happens so timing can be controlled better.
Mirror polished chambers reduce carbon build up. does not stick as good.

If you run Castor fuel and carbon build up becomes a problem sometimes a few good lean hot runs on synthetic oil will remove the worst of it.
This helps ringed engines with carbon build up in ring land. Works to restore compression almost every time.
Balky starting and down on RPM is good indication the ring is sticking due to carbon.

The is a book written by  Dr. Schnurle(spell?) on this and other 2 stroke subjects that I hope to read one day.

Hope this made sense, best I can do this early in the AM

Yes
Air density temp mix all falls under compression changes.

Have to puzzle awhile on the plug heat range thing.

Spark plugs
Cold transfers more heat to head= race engine
Hot plug transfers less= farm tractor? Grandma's car?

Glow plugs I would assume use same reference direction? I don't know.
My best guess
 Hot plug----- less flame out
Cold plug----Less element burn out
Timing shouldn't be affected?

I wouldn't mind a good education on glow plugs.

All I know of them.
K&B 1L & S and the Fox Std. is all I ever used from sport planes to Combat/ Rat and all in between. 5% - 50% nitro.
All I know about glow plus is to put new one in for each race or match, run em till they blow in practice and Fireballs blow the seal if you make any HP.

David

Pre-ignition is caused by something other than the needed ignition source be it glow plug or spark plug where ignition occurs from an unwanted hot spot, usually glowing carbon which I very much doubt can happen in our two strokes unless you're hammering the snot out of it . Pre-ignition shouldn't be confused with a too advanced ignition. Pre-ignition or an overly advanced ignition can lead to detonation which is a very very bad thing. The "pinging" sound you sometimes hear from a car that's being lugged in too high a gear is detonation (and makes me shudder every time I hear it).

Heat range of a spark plug is to ensure the electrodes run at a high enough temperature to burn off carbon deposits but not so hot that they promote pre-ignition. Heat range of glow plugs is entirely different in that it basically indicates how readily it'll ignite a charge. A hot plug ignites the charge more easily than a cold plug which is how the ignition point can be altered for different fuels. Compression also plays a part in when the mixture ignites too so you need a balance of heat range and compression for the correct timing. But as was mentioned earlier, volumetric efficiency changes with revs which means that the effective (actual) compression also changes with revs, peaking at the point of maximum torque. For a glow engine that's run peaked out with any prop/fuel/compression the correct heat range is the one giving the most revs (optimum timing).

The problem with glow plugs and heat range is that it's a bit of a misnomer. It's not just the actual temperature that the coil glows at but how easily it can start the catalytic reaction between the methanol molecules and the oxygen. The more molecules of methanol in contact with the coil the more easily it can initiate the catalytic reaction. The reaction heats the coil keeping it ready for the next ignition cycle. A thick coil with large surface area acts hotter than a thin coil but the disadvantage is that the thick coil conducts heat away from itself after ignition. Depending on the manufacturer, sometimes both a hot and cold plug can have identical coils but use different sized cavities. The cold plug will have a smaller diameter cavity which restricts the ability of fresh mixture getting to the coil so delaying the reaction and vice versa for the hot plug with the larger cavity. The cold plug can be made to act as a hotter plug by pulling the coil slightly further out of the body if you're too cheap to go buy another plug .

To some extent, plugs give a variable ignition timing which is handy for those on the dark side (RC ) because at idle or low revs the plug conducts some heat from the coil into the body so with the coil then having a lower temperature it makes it harder to ignite the mixture which retards the timing making it more suitable for the low revs.

As far as compression goes, if you raise the compression then the methanol molecules are more tightly packed against the coil making the reaction easier to initiate (advances the timing point), effectively making the plug a hotter heat range.

Twin plugs don't alter the timing but nowadays are usually only used in larger engines run at very high speeds such as boat racing to get a more even spead of flame across the combustion chamber because of the quite slow flame rate of the fuels and the distance from the ignition point to either side of the combustion chamber.

"Pinging" in a full size engine, while it can sand blast a piston, is actually just the shock wave caused by detonation which makes the cylinder ring like a bell. This ringing has a certain frequency, which remains the same no matter if it's a cast iron or alloy block, and is what the detonation sensor on modern engines is tuned to. The sensor only needs to detect one ping and it automatically backs off the timing before the engine can fire again on any cylinder. Very clever .

Charlie
That's kind of funny on making plugs.
When young and flying was supported by News paper routes a steel wool stuffed plug would get us going!
Needed fresh steel wool every time and never really thought about all the places the last load went, Young an innocent.
WD-40 will also light off with out glow.
We do fill the fuel filters of diesel engines during servicing with ATF to clean and lube the inj pump.
The slight change in RPM as the ATF clears out indicates the ATF has a little less BTU per unit.

The Youngest boy has made off with my text and tech books on the subject so not much reference available at the moment.

Boundary layer cooling effect', also known as the 'law of the wall' is what happens to the mix where it is in close proximity to the metal surfaces in the combustion chamber. Right up next to the metal, a layer forms that is about .020 thick. This boundary layer is cooled by its proximity to the metal surface: it is cooler than the rest of the air further away from the metal. The coolness of this layer makes it less capable of igniting and burning.

This layer of cooler air, or air/fuel mix, DOES NOT WANT TO BURN. One could look at the boundary layer as being inert and it does not want to burn and it actually acts like an insulator because it does not want to burn.

In actual fact, it is the boundary layer's cooling effect and barrier effect that enables a piston made out of aluminum to be exposed to combustion temperatures that are more than twice the melting point of aluminum...and NOT melt.

...for something so thin, to be able to keep temperatures approaching 2500F [or more] from melting the aluminum that is ONLY 20 THOU AWAY from that heat, you would think that that boundary layer was more like a layer of asbestos that was 1/4 of an inch thick.

That boundary layer IS pretty tough.

it is so tough, that the only thing that can actually penetrate it is the pressure waves that occur when detonation (very rapid uncontrolled combustion...as in 'explosion') occurs. When detonation occurs, the boundary layer protecting the head and the piston can be ruptured; which is why you can see particles of aluminum on spark plugs, or on the pistons, after detonation occurs. Those detonation pressure waves blast through the boundary layer, and melt some of the aluminum, and splash it around.

Dual glow plug heads. The only thing that comes to mind for this to ever be needed is when RPMs reach a high enough level so that combustion speed will not have time for maximum cylinder pressure to be reached at the place where it can be used.
2 flame fronts would speed up the combustion for this condition and allow max cyl press to be reached in time.

When my info returns I hope to find something covering Glow plug heat VS timing (in a proper mixture)
I do not ever remember reading anything that indicated this.

Brian is correct noting that "pre-ignition" cannot be cured by spark timing.  On my model spark engines running on gas & oil, I adjust the timer so the points "open" at exactly TDC.  That is the "idle" setting.  All pressure buildup occurs after the piston has already started down.  High speed setting can be as much as 30 degrees before TDC.  The exact amount depends on "high speed" RPM desired, and engine design.

I don't notice any different spark setting between gasoline fuel and methanol fuel.  There should be some difference in pressure build-up between the two, but I haven't seen it.

Floyd

Still thinking on the 'hotter plug giving sooner ignition due to more heat.'
Need more info.

This link http://www.modeltechnics.com/glowplug/firepower.html gives the best info I've found as to the affect of different heat range plugs on revs with various engines and fuels. Fairly obviously the plug that gives the highest revs is the one that gives the optimum ignition timing. Remember though that it's not so much the actual temp of the plug so much as the ease with which it can fire the mixture.