Rennlist Technical Specialist Rennlist Lifetime Member
Join Date: May 2001
Location: Portland Oregon
Those kinds of hesitations are caused by an incorrect fuel mixture.
SC's equipped with lambda controllers (oxysensors) run notoriously lean at part throttle and I'd guess that given your displacement increase, thats exacerbated the inherent problem.
In my experience, these cars run much better in open-loop (oxysensor disconnected) and the CO set manually at the adjusting screw to 2.0-2.5%.
Without question, a properly setup and jetted set of PMO's will make that SC REALLY go, but you may have troubles passing emissions testing (if that applies). We've had good luck with things like that, but your fuel mileage will suffer a bit.
After throwing parts at mine it ended up being the fuel accumulator. Fuel Filter some help, Fuel Frequency Valve more help, and then the Fuel Accumulator finished the job. My 911sc screams! There is a combo on e-bay now, but, I'd buy new. Start with the Fuel Accumulator if you want to save cash.
The board's gonna think I'm nuts but this falls into the 'it really happened to me!' category. My '86 3.2 Carrera began to exhibit a throttle pulsation at speeds above, oh, 20mph or so. At higher speeds it became worse and was actually accompanied by a noticeable pulsation of the throttle pedal.
End result was that the left inner CV boot had split and after sufficient rotational speed was reached it created enough centrifugal force to fling a portion of the CV boot up against the throttle rod that runs just above the CV joint. One pulse for each impact of the 'flap' that was sticking out.
Weird but true. A goog hint was that the pulsation increased with the rotational speed of the axle.
Neat huh? Probably not your issue but I just had to share this... 8-)
As mentioned, the frequent cause of surging is the fuel mixture setting.
Another very common source of surging, which is inherent, is the close-loop Lambda
system as the O2 sensor corrects for AFR (air fuel ratio) values. A value of 50 to
100 RPMs variation (surging) in idle is common.
I recall a John Walker post on the Pelican board that dealt with this subject. The mixture adjustment screw that we all know and love is for the most part an idle mixture adjustment. The WUR has a factory adjustment screw that adjusts the springs inside for throttle up pressures. I think John felt there was a strong possiblity that DIY'ers could really muck up the pressures playing with it but he felt it was the needed adjustment for a clients car that surged.