I have a Stihl 62cc engine that I converted. It was a labor intensive conversion, but it turned out very well and runs great. I was perusing the technical data and specifications for the Sthil 62cc, and the timing was listed as "X" millimeters before Top Dead Center. Just for grins, I got my degree wheel and dial indicator out and checked the numbers. Guess how many degrees it turned out to be.

28 degrees, just as Bill and TKG and Ralph have been telling us for years!

Of course it never hurts to experiment. There is an engine I build that uses an Echo 25cc crankshaft in an Echo 23.6cc engine. It very definitely prefers 24 degrees timing, while the stock Echo 23.6cc engine likes the more normal 28 degrees. An engine needing less advance is generally believed to be burning the fuel more efficiently all else being equal, and my Echo stroker engines do run very well. Who knows exactly why they like less timing..... Probably something to do with the increased piston dwell time at TDC with the longer stroke.

AV8TOR

The combustion chamber shape (squish etc.) and EFFECTIVE compression ratio + chamber size + mixture ratio define the combustion speed. The ignition and spark plug have a certain amount of flame introduction delay. These factors define the required ignition timing.
With tuned pipes the timing most of the time is later due to very high effective compression ratios.
At high rpm, with associated high squish turbulence action, ignition should be later as well, sometimes as late as 15°.

Low compression, lousy design engines can get away with timing as early as 35°.

I agree. What is odd about my Echo "stroker" engines is that they want the later timing, even though the combustion chamber, squish/quench distance, etc. is all the same as the non stroker versions of the engine. (I mechanically set the squish distance to be the same with a custom cylinder spacer.) The increase in swept volume via the stroker crank is only 1.4cc. That of course would slightly raise the static compression ratio, but so little that I can't believe it would be enough of a raise to warrant needing four degrees less timing advance. I think that there is more at work in this case, and it is interesting.

AV8TOR

by running methanol and cranking up the compression ratio the correct timing can be all over the place...the simple timing curve of most ignitions is no longer optimal. since i'm totally inept at electronics i'm trying out another type of "programmable" timing: connecting the mechanical advance to its own servo that gets slaved to the throttle channel with a multipoint curve. only tried it so far on the bench but it does work nice....and because it's not "wagging the dog" like most electronic timing controls the throttle transition can be razor sharp.

I don't think any of these engines need auto advance but ( IT sure does hurt your hand)

Posted in the other forum, so I might as well repeat it here:
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There really is not a general timing because engines are so different.
Low compression, low specific power engines on pitts muffler 28-30°
high compression low specific power, 26°-30°
high specific power engines, high compression, 24° - 26° (18° - 24° on tuned pipe)

In general, with tuned pipe or canister use, time 4° later than with pitts mufflers
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What is the copper tube coming from the exhaust?
Thanks,
-Gary

The copper tube is for smoke oil

This engine didn't have auto advance in it's original use. A few newer chainsaws do have auto advance but not many.

Thanks for answering for me W8YE. Yes, the copper tube coming from the muffler is for the smoke system. Inside there is a spray bar that I made that shoots several tiny streams of smoke oil into the exhaust near the exhaust port exit.

AV8TOR

We have a volvo Penta petrol engine 3 L 4 cylinder. it's states on the rocker cover to set the timing ignition at 2 degrees after top dead centre but the engine doesn't like to run at this setting as soon as we advance it to about 7 degrees before top dead centre the engine smooth out the engine likes to run at that setting. would you say this is the correct setting or should i leave it at or am I missing out on something ?