[Ducati] Singles - Dellorto SSI carburetor tuning help?

[Kerry and Brad Black]

never seen a SSI, but i've played with lots of vhb and phf/m carbs, so here's my input.  ignore as required.

float level - usually the dellorto floats will sit parallel with the carb base when the carb is sitting on the inlet (air filter) face, engine end at top.  if the floats are round there's usually a line cast/moulded into them that is your parallel.  this may not apply, but it's a convention of sorts among quite a few manufacturers.

idle mixture - the idle speed scrwe and mixture screw are very closely related.  the higher the idle, the richer you need the mixture screw to keep them running.  when i'm tuning some thign i often wind the speed screws in quite a lot to get them running when colder, and you need to wind the mixtures screws out a lot.  as they warm up, the idle goes up, so you wind down the sped screws and the mixture screws in.  i've seen quite a few bikes over the years that wouldn't idle due to the speed screw being too far in (slide too high), negating the idle circuit somewhat.  so, to set idle speed and mixture, wind the speed screw out to get your lowest stable idle and adjust idle mixture.  if the idle speed goes up (often does) wind out the speed screw again to get your lowest stable idle and then adjust idle mixture again.  repeat this until you find the desired idle speed at the lowest speed screw setting possible.  you might be surprised how much times you repeat this process.

the other thing with idle holding up - particularly for the early electronic ignition bikes which yours isn't i believe - is that the ignition timing can stay at an advanced setting, holding the idle up.  for instance, a darmah, mhr or ss with std electronic ignition  - the speed at which the ignition control units advance the timing (one or two steps) is very dependant on pick up gap.  if the gap is too small it may advance at say 1200 rpm, not 1800.  so it will hold that first advance amount - they go from (from memory) maybe 6 degrees @ idle to 16 @ 1800 and 26 @ 2500 (something like that) when returning from higher rpm to idle.  meaning it'll idle at 16 degrees advance until you wind the speed screws right out and it suddenly loses 10 degrees of advance, going back to 6 degrees and idling at 800 rpm before it stalls.  and nothing you do will fix it, apart from resetting the pick up gaps.

as yours is mechanical, maybe the advance system is not returning to it's idle position - rotating plate sticking - and that's why it holds up, then suddenly drops.  or does it pick up a few degrees advance say from 900 to 1200 rpm?  i'd look there.  maybe try fixing the plate and see if that stops it.  or load the springs to make sure it doesn't start advancing until 1800 or so rpm and is returning fully once below that again.

other things.

pilot jets - if it stumbles as you take off, go up (or down even) in pilot.  if not, leave it.

mains - if it goes off with the last little bit of throttle opening, they're too big.  usually when i'm tuning mains i'll wind it full open, then back off just a touch.  if it doesn't go harder with the little roll off, i go up in mains (steps of two or three sizes so you can feel a difference) until it does.  that's my too big point, so i then i work back from there.

not sure how any of this applies to the SSI, but i hope it may be useful.  the main thing is you need to sort the ignition or vacuum leak issues, if they exist, before thinking about the carb tuning.

brad