[Ducati] Retarding Ignition Advance On Carby Models (2V) By Modifying The Flywheel

[Kerry and Brad Black]

this is a cut and paste from a thread i started on the dml - easier than typing it out again.  just a fyi if appropriate.

brad

>i got to have a play day today with minnie the '97 600m/750 and tried something i've been wanting to for years.  a local bike electrics/ignition shop told me years ago that the kokusan boxes use the leading edge of the flywheel lump for max advance, and the trailing edge for idle advance.  so i figured if you wanted to retard the max advance, but not the idle you could shorten the lump at the leading edge.

so when i had drained the oil i pulled the lh cover (got to love greased paper gaskets - no goop) and removed the flywheel.  i was going to try it in place, but that's just too rough.  using a cut off wheel on an air die grinder i shortened the lump at the leading edge (lh when at the top, engine spins ccw from the lh side) by around 6mm, probably a bit more once it was done.

the lump is 40mm long, and theoretically (if my theory was correct) relates to 26 degrees of advance (6 at idle, 32 at revs), so thats 1.54mm per degree.  i chose 4 degrees so it'd be a clear change, which gave the 6mm.  i also took the chance to punch marks into the flywheel every 3 degrees so later when i fit the dp boxes i have i can see exactly what they do.

so when i buttoned it all up and got it running we put the timing light on it again and wala - idle as was, full advance retarded by around 5 degrees.  so it has about 27 degrees advance now instead of 32.  might have felt a bit smoother on the way home at lower rpm, but i didn't really notice any change.  of course, this engine had around 80,000km on it when i bought it and altho i did bearings, etc, in the rebuild i left the original rings in it because i have some fbf hi comps to fit at some point.  so it's potentially a little poor in the cylinder pressure dept.

but that's not really the point.  the experiment was as to whether or not this would work, and it does.  marvin had done this at one point after i told him about it and he said it worked, but then someone else tried it and said it didn't, so i wasn't sure.

i have a 600 flywheel i can fit to go back if need be.  the 2 phase alt 600 flywheels are a stamped steel piece, not machined like the 750/900 flywheel.  they weigh about 650 grams, compared to nearly 2,000 grams for the 750/900 part, and are functionally and dimensionally the same part.  light flywheel anyone?

i also fitted a dp electronic tacho (one with a grey face) - put it where the speedo was and mounted a little cateye bike computer on top of the dash.  doesn't seem to be too accurate (who'd a thunk it?), but i'm surprised by how low i may have been revving it anyway.

so i had a fun day.