THEN we have the vacuum advance.. This advance depends on how wide open the throttle is.. IF the throttle is WIDE OPEN the spark will start the fire and the fire will JUMP OUT of the carburetor if this timing occurs.. SO this amount of timing is connected to the vacuum which drops off as the throttle is opened. As the vacuum increases the timing begins to advance with the increased rpm's. So at certain rpm's ALL the timing is working as the vac is sufficient to keep the timing increased.
<img src="/forums/images/graemlins/confused.gif" alt="" /> Okay, Don't know 'bout RAL, but I haven't got my noodle around it. I get the mechanical advance. But if throttle is wide open, rpms go up, would have thought you'd need even more advance. Or is that what lower vac does? So if vac didn't lower, timing wouldn't be fast enough, valve wouldn't be closed, and it would backfire through the carb? Have I kinda sorta understood? <img src="/forums/images/graemlins/baby.gif" alt="" />
I'll try to help with the noodle problem..
Ok so we are driving at say 1900 rpm..steadily along..we have most mechanical and full vac advance of perhaps 30* or so..
We punch it! Opening the throttle to WIDE.. if the timing stays where it is the advance will cause fire to jump out the carb top.. making backfire.. BUT HERE is what happens.. as the throttle suddenly opens the vac suddenly DROPS and the vac advance goes away..QUICKLY!! Now we have an engine that wants to gain RPM and the advance is prolly around 16/18* and at that advance the fire is contained inside the cylinder...
Now as the engine gains rpm the velocity inside the venturi (carb or TB) gets greater and greater causing MORE vacuum which in turn causes the vac advance to start opening and getting more and more advanced timing.. At around say...3,000 the vac is high enough and the velocity is high enough that the vac advance is full again..
The above numbers change from camshaft to camshaft but are representative of what happens with the advance..
If the engine will be run..say on a track and most of it's time will be spent at 5,000 and above it is not uncommon to have 42* of timing in the thing..
But our little engines can only use 35* or thereabouts and then only at 4,000..
And yes the timing changes from any movement of the skinny pedal at normal driving rpm..
When the throttle is suddenly OPENED the mixture try's to LEAN out from the low velocity.. It is this lean condition that the FLAME likes as the flame can travel thru it faster than it can the proper mixture.. So if the timing doesn't get retarded by the vac advance bad things happen..
Big Jim <img src="/forums/images/graemlins/pfft.gif" alt="" />
