I've got my 4.3 V8 (carbed on Eidelbrocks) running on MJ. Drived fairly ok, a little missfire under heavy load at times but mostly ok (there IS a rythimic pulsing of the engine in the mid rev range but that must be something else...)
I'm analysing my spark table which is basically a safe map from this site, and Im a touch confused as I don't totally inderstand how the carb vacuum relates to engine load.
My Map is: (that was taken with the engine at tickover)
Now, on the MJ instructions PDF it shows an example map with, on the left, an arrow going up with "Vacuum increases or throttle closes), then an arrow down with "Pressure increases or throttle opens". Advance increases with rpm and increases as Vacuum increases or throttle closes (their words).
So that tutorial seems to tell me that the higher KPA readings - at the bottom of my map, represent.the engine "under load", and the lower KPA readings higher up in the map represent the engine "coasting" ("Vacuum increases or throttle closes"). This tallies with what I've read about engines needing less advance under load and more when coasting.
So from this I assume that the KPA figure is high when the "pressure increases or throttle opens" and drops off when you back off the throttle.
This is where I get confused. On my car, at tickover, the green square floats around the bottom left of the map. Should it not be the top left as that is where it should be for "closed throttle/vacuum increase". Or is it because, revving the engine at standstill is not a "load" and you have to actually strain the engine for teh figures to be right?
When I blip the throttle (standstill) the green square starts bottom left and then , naturally goes right as the revs increase, and also it climbs up the screen (to lower KPA figues)
So it's doing kind of the opposite of what I'd expect from the MJ tutorial diagram. It's also confusing as the MJ diagram talks about both vacuum increasing and pressur eincreasing. Which is it? Do you start with a vacuum that turns to positive pressure under load?
(p.s. I'm using the non timed vac port on my carb - I've tested it, and the timed port to be safe)