Originally Posted by Kerry W
my experience that some/many octane improvers don't achieve much, especially where they don't contain lead. As they don't, if they want to be acceptable in street engines in most of the western world these days.
Right. I'm generally skeptical of snake oil, although overall I'm a believer in Monsanto's old slogan of "better living through chemistry." That's why I've tried to qualify my comments about the octane booster I used by writing 'if it worked', and the stuff I have on order with 'if it works'.

Originally Posted by Kerry W
high performance competition 2 strokes..easy to get the fuel/compression wrong and you will have an expensive seizure..
Another important factor is temperature. I reported my jetting results as having been made at an ambient temperature of 95 oF because that matters. My friend whose shop burned down was an active AHRMA competitor in the '90s and '00s on his now-destroyed Yamaha 350. Since in principle he might race one weekend at Willow Springs, California at 95oF and the next weekend at Steamboat Springs, Colorado at 65 oF, I realized he would get more practice time at the second higher, colder track if he needed to spend less time sorting out the new jetting.

It turns out that fuel flow depends on the ratio of density to viscosity, and both of these properties change with temperature. So, using a Cannon-Femke viscosimeter and Ertco hydrometer I determined these to 0.2% between 32 oF and 120 oF for VP C12 racing fuel. Making a long story short, I then created a kit containing the instruments for measuring the necessary parameters (relative air density, temperature, humidity, etc.) and devised a program for a portable calculator that used all of these inputs to convert the presumably-perfect main jet determined at one race to the required main jet for the current track.

Returning to my Competition, fuel flow at 95 oF is ~35% greater than at 65 oF which, of course, will affect the jetting. Unfortunately, it's more complicated than simply measuring the ambient temperature since what matters is the temperature of the fuel in the float bowl (which can be measured, but with more effort). But, without being quantitative, it's safe to say that, all other things being equal, more fuel will flow on a hot day than on a cold day. It's also safe to say the air density will be lower on a hot day than a cold day, which means that both the total amount of air ingested by the engine, and the "vacuum signal" at the top of the spray tube that draws fuel in, depend on temperature. As I said, it's complicated.

Originally Posted by Kerry W
Fuel quality, as a function of time and handling, is also a factor: that tin of gas that goes 'whoosh' when you take the lid off is less of a fuel than you might think.
I was interested in 'the whoosh effect' in the context of determining whether or not fuel stabilizers made any difference. So, in the '00s one long-term test I conducted was of the volume of fuel lost as a function of time. In an open container at 72 oF fully 50% was gone in just three days. While that's not too surprising, 1.9 years later -- hey, I said it was a long-term test -- a small quantity still remained of a very viscous liquid.

Originally Posted by Kerry W
the spark plug (and presumably any air-fuel sampling device) sees rich, though the combustion is actually lean...
Bosch wideband sensors are sensitive only to the oxygen and they produce a voltage proportional to the difference between the percentage of oxygen in the exhaust pipe and that in the outside air. All the sensor "knows" is how much oxygen is present, not how it got there, so there are circumstances when interpretation of that result as rich/lean mixtures can go awry. For example, if an engine misses, oxygen doesn't get combined with the hydrocarbon so the sensor sees too much 02 which the meter incorrectly displays as a lean mixture.

No matter how bad the fuel is, if it burns, the oxygen sensor contains information about the rich/lean mixture strength. That said, "classic" gasoline combines with all the oxygen if the AFR is 14.7, but I don't know if that same 14.7 applies if, say, a lot of the 'whoosh' was gone from the fuel that burned. If the stoichometric ratio of de-whooshed classic gasoline is different than 14.7 then interpretation of the mixture strength from the output of the sensor will be affected.

Octane, compression ratio, ethanol content, timing, temperature, density, air/fuel ratio,... It's remarkable carburetors and engines work at all, let alone that they work so well.