Can't Advance Timing on a 2 Stroke

[smalldriver]

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 BTDC. 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 advances.

My engine will only run well at 12 degrees advance (the minimum). And I admit that it runs very well there. At 13 degrees, the engine 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 oil, 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, no difference.

Carbs tuned to oxygen sensor calculated numbers.

Spark plug is normal-ish (right plug, just a bit hot), gap is appropriate, squish is slightly large (for engine longevity) and adjustable (base gaskets). Reed valve engine. No engine mods allowed. Rebuilt engine top end twice so far.

I have tried different magnetos and plugs with the same results.

I know a bit about "phase change" issues, but the magnetos/magnet are not changeable rotationally.

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. Puzzled for nearly four years.

Thanks!

 

[sreid]

Hve you "degreed" the crankshaft so you can compare the "real" timing vs. what you think it is set to (via timing light)? I'll bet that somehow the timing marks are off.

 

[smalldriver]

No, I have not. That is on my list of Winter projects, though. . . (Nat Champs over)

Never used a timing light on the engine - not sure how I would. I have a disk with holes marked 12 - 17. Nothing in the manual about timing lights. . .

 

[patprimmer]

Before you try to correct a problem, you need to know what it is.

Check your timing mark for accuracy and put a known reliable timing light on it.

Test both cylinders.

When you have reliable data, then try to make sense of it.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[MikeHalloran]

The pin/hole sets the static timing. There's probably an advance 'mechanism' in the magneto. I used quotes because it may be mechanically simple but magnetically less so, e.g. stepped or tapered rotor/stator magnetic gaps and calibrated magnets so the total timing changes with rpm.

Measuring the total timing vs. rpm on a simulated engine with a real magneto, and learning how to make the magneto's behavior change might be a productive winter project.

It's time to buy an inductively coupled timing light.





Mike Halloran
Pembroke Pines, FL, USA

 

[Fabrico]

What is it you are trying to do? Many 2-strokes will generate more top-end power with less advance.