Can't Advance Timing

[cb23]

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!

 

[Ol'89r]

You are probably 'out of phase'.

The timing of a magnito has to be phased in with the electrical pulse that a mag produces. Or in other words, the mag only produces voltage during part of the engine revolution. The timing has to be set during that pulse or within those parameters. Those parameters usually exist when the magnets are passing over the coils. Once the magnets have moved away from the coils, the mag is no longer producing enough energy to produce a spark.

The timing disc with the higher advance is most likely moving the timing beyond that range. In order to advance the timing you would have to re-phase the mag by moving the plate that the coils mount to and re-align them with the magnets. Not sure if this is even possible with your engine since we don't know what kind of engine you are working on.

Food for thought anyway. :cool:

 

[cb23]

Thanks for the info, Ol'89r. I see what you are saying. But the two magnetos screw into two extended stubs from the block (magneto holes slotted to allow clearance for the spinning magnet). And I can't have the magnet 180 degrees off, there are pins. Hmmm.

 

[hot125mod]

is it a banshee motor or a twin road race motor

 

[cb23]

It is an AMW 500cc twin. A near custom made motor.

Told you the make wasn't gonna be helpful. . . :)