A/F ratio and open vacuum hose question.

[69427]

Just curious about an item I'm not totally understanding. I had the engine all warmed up and was adjusting the idle mixture/bleed? screws trying to smooth out the idle. Got them set as good as it was gonna get when I noticed the vacuum line (for the wiper cover/headlights, etc) wasn't locked on as good as it should be. I tugged on it and it slipped off, and of course the RPM jumped up noticeably. Nothing new here. But I'm trying to figure out the physics here. Given that opening the hose would lean out the intake manifold mixture, why did the engine speed go UP? I would assume that when you adjust the mixture screws that you are getting the mixture close to what the engine wants at that RPM. Even when the screws are at the point of maximum idle speed, is it still running that pig rich that the open hose gets you back closer to stochiometric? Or, is it that a lot of air, and perhaps a lean ratio of 17:1, makes more idle power than a smaller amount of air and a ratio close to 14:1?
Just curious what the engine and A/F ratio is doing before and after the hose uncoupling. (My LM-1 is unavailable as it's currently hooked up to the other car.) I understand when the RPM goes up that the centrifigal advance goes up, compounding the issue, but it's the chemistry that's got me curious.

Thanks,
Mike

 

[mrvette]

Only way I ever understood it, was there was less load on the engine from acting less like a vacuum cleaner...less work, higher rpm....

other than that, I dunno.....I do know that too much free air and it dies, I assume it goes beyond 17-1 whatever can power the engine to run at all....

funny how these thing can still run at such a wide variation in a/f ratio....:amazed:

 

[TimAT]

You're correct, it does lean the mixture out. But the induction of more air acts just like you had opened the throttle plates so the speed goes up. It might get a little more gas, but since it's not pulling thru the carb, I doubt it.

Just pull the PCV with it running and stick your finger over it- Idle will go down due to the slight loss of air.
No idea about the mixture, I've never had the luxury of being able to actually measure the A/F ratio.