CLEMTOG said:
Since the weather may be in the low 30's (c) I cannot risk a jetting change that is bang on the ideal for (say) 21c.
Blimey. You don't think you're getting a bit too fussy Clem? Laverda engines are not highly tuned, so a bit of variation in air/fuel ratio can be tolerated.
My Jota's jetting stays the same all year. Temperature can vary from high 30s to low 10s but the bike always runs OK.
Applying a bit of science (universal gas law): if the pressure remains the same, air density difference between 20C and 30C is 3.4%. To put that into meaningful terms as far as your engine is concerned. Air density reduces with increasing temperature. So the volume that gets sucked into the engine has a lower mass of air in it. Air/fuel ratios are based on mass, so less air with the same fuel means the mixture gets richer as the air gets warmer. If your air/fuel ratio is 14.0 at 20C, it'll be 13.76 at 30C - ie. richer, but only marginally.
The above assumes constant air pressure. Changes in air pressure also affect air/fuel ratios. High pressure (higher mass of air per volume) gives a leaner mixture, low pressure richer.
Let's consider the weather map: wherever you are in the world, warm weather is generally associated with high pressure areas. So if you have a high pressure day (leaner) with warm weather (richer) the effects on air/fuel mixture pretty much cancel out.
Changes in elevation work in a similar way. It gets cooler (leaner mixture) as you go up into the mountains, and the air gets thinner (richer mixture) while your ears are popping with the lower pressure. So again, the effects tend to cancel each other.
The bottom line is that carbies are smarter than we think and can cope very well with changing conditions. If you try to think for them you'll more than likely get it wrong
