RE: YS 170 DZ CDI
Hi Sleeping,
The discharge form the pump is/can be high pressure. The clear tube running from the just in front of the cam gear housing up the the regulator allows the diaphram to "see" crankcase pressure which varies largely on throttle position and rpm. Don't forget, the engine uses a lot of it's internal volume as a plenum chamber for the supercharging process. I'd bet Will Crossman has some really intersting numbers in this area. I beleive he stated somewhere that fuel pressure can be up to 7psi, (or it may have been the pressure regulator sensing line). This would allow the fuel to maintain the correct pressure differential from pump discharge to the end of the injection needle.
Modern (I still drive a car with a carby, so any car with fuel injection is modern) fuel injected cars do the same thing, they have a fuel pressure regulator that senses the manifold pressure and adjusts the fuel pressure to maintain a constant pressure differential across the injector.
If you remove the intake tube from the head you'll see the shiny needle/tube pointing at the intake valve. All fuel goes in here, there's no fuel/air mix traveling up the intake pipe.
I haven't a clue what system the OS uses, the YS is my first 4-stroke and to be honest I know too much about it, much more than I ever wanted to know. I just wanted to pull it out of the box, put it in a plane, run it in and use it. I never wanted to have to strip it completely twice after bearing failures and fuelpump/ignition problems. Chasing down some obscure, intermittent fault over 5 months, isn't my idea of flying model airplanes at all....I also know too much about my car
As for diagnosing issues?? This is why it took me 5 months to sort mine out because I was wasting my time trying to figure out why my engine wasn't running right instead of just coughing up the cash and replacing parts until it worked again. If I had just dived into a magic toolbox and pulled out a new fuel pump, sparkplug, cdi unit and sensor, I'd have been back in the air trouble free in about 45 minutes...