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Message: [QUOTE="cb23, post: 1230290, member: 52611"] I have a two cylinder two stroke 500 cc race engine (make unimportant as you have never heard of it). Timing is clearly/easily adjustable by changing the hole in which the pin is placed in the hole in the disk, magneto driven, holes clearly marked. There are two timing disks available, and timing is (theoretically) adjustable from 12 degrees to 22 degrees. The company discontinued the 17-22 degree disk because of "engine reliability" issues. The standard disk now is 12 to 17 degrees. I KNOW that earlier users ran at higher spark advance. My engine will only run well at 12 degrees advance (the minimum). And it runs very well there. At 13 degrees, the engines idles OK but sputters and coughs at more throttle. At 14 degrees it will not start, and I get an occasional backfire. I use medium-high octane race fuel (pre-mix, 20:1, synthetic, metal cans only), somewhat higher octane than recommended. I used to run 93 pump gas (recommended), same problem. I tried VP's Oxygenated 2-stroke fuel, high octane, re-jetted with my oxygen sensor, same issue. Spark plug is normal-ish (right plug, just a bit hot), gap is appropriate, squish is slightly large (for engine longevity). Rebuilt engine top end twice so far. Any ideas why I can't advance the timing? I know why I don't want too much, but this issue just confuses the dickens out of me. :| Thanks! [/QUOTE]