I have a magnum .15 xls that I have been flynig for about a year I am running a fuel mix of 10%nitro 10% caster and 10% synthetic the engine has been running great until the last few times to the field it has stated eating glow plugs I have richend the highend fuel mix and checked the thead bolts. I can not find any air leaks. I have been running the os hot plug. sometimes I can get a flight and then the plug will be dead when I get ready for another flight some times the plug will go bad after a few seconds of running. any sugestions or ideas would be great.
thanks.

I dont think its rich enough or there are air bubbles going through leaning the mixture. I'd suggest pointing the nose of the plane straight up while tuning the main needle. I pinch the fuel line so I get a slight rise in RPM. If using muffler pressure, I'd also suggest setting the needle at 1/2 tank since the mixture will lean out as the tank runs down. Keep an eye on that fuel line looking for air bubbles. In my experience, air bubbles account for half of the tuning troubles I've ran into.

What do the plug elements look like after they croak? If they're distorted or misshapen, lean mixture is almost guaranteed to be the cause. Just my 2 cents.

I have checked for air going through the fuel line and not seen any bubbles, I also use the nose up method of setting the engine, the glow plug element is toast up inside the plug it will still be clean with no discoloring. the engine is on an ssc plane. thanks for your suggestions what else could it be.

If it has being doing fine on the same fuel and prop before, then it could be the ballbearings that are about to get worn out. Any foreign metal can alloy with the platinum and lower its melting temperature.

What prop are you running and how much does it rev?

I am running the master airscrew combat 8x3 prop it tachs in at 17,500. If the bearing was going out wouldn't it cause the plug to discolor which I am not getting?

I haven't tried the 8x3 prop on this engine, but your numbers might be might be a little low(?) indicating some other problem too. With a 7x4 APC (which would be a similar load) I'm getting about 19500rpm on 10% nitro.

I haven't had this problem myself, but if the ballbearings are letting off metal, you would not see any change in color of the glow plug. Only noticeable effect would be a lowering of the filament melting temp. The exhaust oil could get a bit discolored though.

I think magnum recommends a 7x6 or 8x4 prop with this engine with the highest practical rpm being 18,000 so you folks are pushing them pretty hard. My guess would be metal getting loose and killing the plug.

The 8x4 is a little high load to me (gives around 15000rpm) but a 7x5 is a fairly good match. It is a two-stroke engine so over-revving isn't really an issue as I see it (within limits of coarse).

Hi!
It could be a worn cylinder /piston!
Have you taken the engine apart and looked at how the piston to cylinder fit is? Any carbon build up?
How is the piston to cylinder fit? If you try to push the piston through the cylinder, does it go straight through?The piston should never go trough completely with just pushing it with your fingers, if it does...the piston is worn out! The piston should stop a few millimeters from the upper edge of the cylinder if all is well.

Thanks every body, It sounds like it is time to dismantle the engine and do a check of the internal parts. I talked to another combat flier from our club and he told me that he had the problem once before and found that the bearings were going out, which id in line with what most every body here on the forum said the engine parts are going south. Thanks to everyone for your great help.