Interesting Weber jetting

[traindriver]

Reading Eljaro's tuning work has given me some motivation. I purchased a PLX R300 logger. I'm logging AFR, RPM and TPS. I made a base line log on my 3 mile test run and found one particularly interesting item. It always bothered me that if you set the idle mixture to 'best lean idle' like you would with a 4bbl, you get horrific popping through the exhaust during letoff and slight throttle openings. By opening the mixture screws about 1/2 turn as Eljaro did, it goes away. My logging session shows why. With the idle AFR set correctly for BLI at 13.5 it would lean to 18-20 just off idle (major poppping) (TPS <=4%). As the throttle opened more (TPS>5%) the AFR dropped back to 13-14. To figure out why...I removed an idle mixture screw and the transition hole plug. Shining a flashlight in the airhorn revealed both transition holes above the throttle plate...not good. Then I closed the idle speed screw unitl the lower hole was covered. This is the correct idle position according to the Weber tuning manual, the TPS now read 5%. Of course with this setting the engine would idle at 2000 rpm. So to combat this, you have to open the mixture screw to make it rich at idle, then as the throttle begins to open but has not yet uncovered the lowest progression hole to engine vacuum the mixture will lean out to 13-15 as the throttle opens.

All for now, back to tuning.

Chuck