Spark plug problems
 

I have got a DJ 72 V-2 single cylinder, side exhaust engine. It is just the usual engineering like DA, DLE, etc.. except for the stroke/bore ratio being slightly greater than its counterparts. Performance wise I am really satisfied with it. However there is something that makes the spark plugs (NGK CM-6) go bad very fast. At first you notice that starting the engine is a bit more troublesome. A couple of gallons of fuel later it gradually starts to get flooded, then it back fires and at the end it just refuses to fire up. It has happened four times in a row; 2 NGK, 1 Champion and 1 DLE CM-6, at least that is what was printed on the plugs. I do not believe in coincidences. By now I learned the lesson of course and I always keep two spare brand new NGK's in my flying box.
Additional clues: Unlike the new ones the "exhausted plugs" show no or erratic conductivity between the two ends of the electrode. Fuel mixture 35:1 (Klotz), RCEXL high voltage and factory set ignition timing.
Does anyone have an idea of what can cause such a thing?

Too much oil or too rich a mixture would be my guess. I go several flying seasons w/o replacing a plug in my DLE's

Actually the porcelain isolator of all the plugs that I replaced was dark brown. Even after I set it leaner, the colour didn't change much but I personally can't see some extra fuel destroy the plug electrode. I mean that the plugs I removed didn't spark at all, it wasn't like brush them clean and fly again; to the point that at first I thought the ignition was defective.
I would rather blame a too high temperature at that vey point of the combustion chamber but I haven't seen any evident sign of overheating when I disassembled it (to replace the original gasket) either.

https://cimg7.ibsrv.net/gimg/www.rcu...d8af911a38.jpg

Even after several seasons of flying the porcelain section on my CM-6 plugs is still quite white with just a slight discoloring at the base.

I would say too slow fuel burn at a low temperature. Possibly crappy fuel / rich mixture and/or retarded ignition timing .

I think you are right CK1. Crappy fuel was also my doubt but I couldn't really figure out how to improve it. 35:1 oil ratio seems fine to me and the premium gasoline down here is supposed to be 98 octane or so. Would one of those octane boosters (STP for example) benefit the combustion diagram in a two stroke engine as well?
Then I started to focus on the ignition timing, which I didn't touch yet because I am not sure how to tune it. Should I measure 28 - 30 degrees prior the TDC from the sensor axis position or from the point at which the plug sparks? Moreover, due to the tolerance gap between piston pin, connecting rod, etc. at the very TDC point the piston neither travels upward nor downward for a couple of degrees. So, which is the true TDC point to be considered in this instance? Well, not much of a deal, I reckon that but it is always better to know how things work before taking an action.
Thanks for your help.

Unless your engine has an unusually high compression ratio I don't think increasing the octane is the answer as increased octane actually slows combustion . Unless the engine mfg requires high octane using 87 to 89 is best as long as it is methanol free and it's fresh . Fuel age is the biggest issue to impacts volatility . There are several good videos out there on setting the timing using a protractor to indicate TDC Set the timing to 28 to 30 BTDC the spark should occur within that range. The slight free play feeling at TDC is normal and on these engines does not seriously impact the ability to get timing set within the proper range . 2 or 3 degrees either way is not an engine killer .

My engine doesn't have much of compression, judging by the force required to flip the prop. Therefore I will try regular gasoline instead of premium and check the ignition timing by the way you say. Then I will let you know if my "spark plug eater" had enough. CK1, thanks again.

If you are in Italy you probably have alcohol in your gas here in California I get my gas at the marina or airport no alcohol If you have exhaust getting on the plug then the spark will short around make sure of your plug seal and divert the exhaust out of the engine compartment