I realize this is an old thread, but it has my curiosity, so I will share some of my past experience and thoughts regarding ignition timing and maybe it might help someone. Please bare with me as I explain the engine build and thought process as it might shed some light on the ignition timing.
In 1981, just out of high school, I purchased a go-kart with a 100cc McCulloch 91-B1 that mainly ran on methanol. Eventually, I was ready to build an engine from scratch and asked "Is it better to build a top end engine or bottom end?" A driver told me he didn't think it mattered as long as whatever I built it needed to have a lot of it. I purchased a new engine block from the local kart shop that was cheap. Later I learned nobody wanted it because the exhaust port height was low on the cylinder wall and the transfer ports were high resulting in less blowdown time before the transfer ports opened. Basically, this engine was not going to rev. I decided to press on and build it with as much bottom end as possible.
I purchased the longest legal stroke crankshaft and longest connecting rod I could find. I then reworked the ports all "wrong". I did not raise the exhaust port to gain blowdown time, instead I left it low so I had a longer power stroke than my competitors and more compression. I did widen the exhaust port to the maximum legal dimension. I decked the block and reworked the cylinder head for more compression. I used the thickest stuffer plate gasket I could find when everyone else was running the thinnest. Ignition timing was set to 28 degrees BTDC, 2 degrees above stock setting due to methanol fuel.
Once assembled I found the engine difficult to snap over due to high compression. When I checked the compression with a top of the line Snap-On 0-250 psi gauge tester it pegged the needle and broke the schrader valve, never learned what the actual compression was.
I took the engine to a fast 1/5 mile dirt oval, installed a rear axle sprocket 1 tooth smaller than usual and the engine was a dog, barely mid pack. Wondering if I was past the powerband I dropped a tooth on the rear sprocket, then another, this did nothing. Out of desperation I dropped 2 more teeth and it started to come alive.
Over the next few weeks I focused on ignition timing and surprisingly every time I backed the timing off it got faster. The improvements flattened out at 22 degrees BTDC so I left it there, about 6 degrees less than everyone else. As I experimented with ignition timing and expansion chamber length I was able to drop teeth on the rear axle sprocket until I stopped about 7 teeth smaller than was typical. By now the engine was unbeatable, nobody was even close.
Back to ignition timing, my thoughts are your goal should always be to have an engine with as little ignition advance as possible. The more spark advance you have the more the piston is being fought against as it approaches TDC during compression. I think you want to improve the fuel burn rate by having a good port design that gives a well mixed fuel/air ratio which needs less spark advance. Increasing the compression will result in a tighter packed fuel/air charge which will burn faster and need less spark advance. Optimizing the expansion chamber length for your powerband will also give a denser fuel/air charge. I just think the more variables that you can optimize in your engine the less spark advance it should need, resulting in less opposition as the piston approaches TDC on the compression stroke and more power.
As a side note I also built another 100cc McCulloch for road courses that had a higher exhaust port and lower transfer ports, thus more blowdown time. It revved higher and had a broader powerband and won a lot of big races. It ran best with about 24 - 25 degrees spark advance.
I now find myself rebuilding Stihl chainsaws and being asked for a little more power, so I've been experimenting gradually with muffler mods that have shown improvements. This thread leaves me thinking it's time I looked at ignition timing because it sounds like the factory isn't putting enough advance in.
I hope my experience and thoughts might help someone think their way thru their next project and I welcome any input as I have a lot to learn.
