Whups. Sorry azman. I was buried in cam spec sheets, and that blurted out.
IVC = Intake Valve Close, the number of degrees past Bottom Dead Center when the intake valve closes.
The compression stroke is a funny guy. And compression ratio, CR, is another character.
You've got your mechanical, or static CR, based on the ratio (stroke volume + combustion chamber volume) / (combustion chamber volume).
Then you've got your dynamic CRs. Occurring at various rpms with their various volumetric efficiencies (VE).
For Jay's kickstart scenario, a strong influence is when the intake valve actually seats, and seals the cylinder. The cam grinders publish their valve events with some pre-defined valve openings, like 0.050", 0.040", 0.012" (Yamaha value). Those give you a fair idea of engine performance at higher rpms, but don't help much when trying to find the actual valve closure.
Long, slow ramps will hold the valve off the seat well beyond the published spec, making the kickstart stroke short, reducing effort. Influenced by the valve lash setting. Modern fast ramps will have the valve closure closer to the published spec, increasing kickstart CR and effort.
Hot, high-rpm cams usually have longer durations, and will leave the intake valve open quite a ways past BDC, reducing the kickstart effort.
For example, on the XS650, here's the approximate distances the piston travels from BDC for various IVC (Intake Valve Closure) values.
50° ABDC - 10mm
60° ABDC - 15mm
70° ABDC - 20mm
80° ABDC - 26mm
Take those values away from the stock 74mm stroke, and you can see how the residual stroke reduces the effective CR during kickstart. With reduced valve lash, holding the valve off the seat even longer, the kickstart CR really comes down.
Hopefully this helps Jay figure out his kickstart issue. To be sure, need to get the actual IVC from his cams.
Other things that can help reduce kickstart efforts:
- The "256" transmission, with its taller 1st gear, and matching kickstart gear.
- The "high-ratio primary" gearset.
- Idle, low-speed ignition timing set back aways.