Join Date: Jul 2006
Location: Galena, Illinois
As you pointed out you changed the compression ration, so jetting may have to be found through experimentation. Some will argue that you don't touch the extra emission screws, but I found I needed to get my Nuova Super to idle properly. I also use the factory mercury manometer ….. don't see how you can make these things work without it …….. but others will disagree.
But here is a little of my research when I did my first emission control carbs about 2 years ago.
Instruction from “Emission Control Weber Carburetor Tuning Manual” page 54-57:
• Set idle mixture.
o Establish an idle at prescribed value for vehicle (e.g. 900 RPM)
o Working on one “idle mixture adjustment screw” at a time, obtain the highest RPM, most stable running as possible
o After all idle mixture screw adjustments complete reset idle RPM to prescribed value.
• Re-check vacuum balance with vacuum gauge.
The following is provided by others and has not been verified:
• Opinion #1: Emission style needle tipped air correction screws normally will adjust somewhere around 6 turns off the seat.
• Opinion #2: 6 would be unusual, 3-4 is normal.
• Opinion #3: if you start with the needles over the barrels closed, all adjustment with the fine rear needles, you can end up with them 6 turns off the seat. Yes, I know that's not the way you are "supposed to do it", but many prefer to forget the perfectly clean idle with different cams or in racing, and begin with the forward needles closed, using the fine rear needles only.
• Addition comment by #3: If your forward needles are seated, then yes, 5-6 turns off the seat is Ok. The CORRECT adjustment with these is to close the rear needles, and get a rough adjustment with those over the barrels first. This is usually much like single adjustment classic (old) Webers, 3/4 turn off the seat. Then locking those down, get a fine adjustment with the rear fine needles, often in the four turn off the seat.
That aside, most users don't bother and leave those coarse needles closed, doing all adjustment with the fine needles.