looking for any info on uncontroled idle surge. base info stock dcnf's 135main 195 air cor. e36 tube 65 idle base time set at 3btdc I set base adjustments 2.5 turns out on idle mix, air bleeds closed. the car has a clean idle at 950rpm. I get some small tip in pops through intake as I open the throttle and then the rpm climbs to 1600-2000 rpm all on it's own. the linkage has all been zeroed and the sync is good. thanks mart
Do you know if you have stock 308 carbs or the replacement DCNF40-12 carbs? Makes a difference for the rough mixture guess -- 4 turns for the stock and 2 turns on the replacements -- so 2.5T would be way leanish on a stock set IME (and are you confirming that you subsequently tweaked each cylinder to lean cut-out + a fixed amount back and those weren't far away from the 2.5 turn presets?). When you say "sync is good" what is the average airflow value at your 950 RPM idle in Kg/hr in each barrel from the Syncrometer? PS 3 deg BTDC is a very conservative idle timing for a single point/event system even on a late US carbed 308 -- are you targeting this value this intentionally or do you have the stock R1/R2 system and just mistyped BTDC for ATDC?
I believe the the dcnf's are stock. my wide band shows 14.5 base afr @ 2.5 turns out. the car needs to smog and has not in 8 yrs. The hc's are high on the 4 gas so I'm setting it up on the lean side. the ignition system has been played with prior to my tune. the stock distributors are still there with pionts however someone installed a pair of msd boxes. I asumed the timing was set to the advanced side. the fender tag shows 3 deg base +/- 2 deg.(again smog) is the factory timming on the retard @ idle?
If you've got it instrumented, I can't argue with that so let me ask it the other way -- how much do you have to turn each screw "in" before each cylinder cuts out at idle, and can you detect each cylinder is going from running to non-running? Since your symptom is surging, you might have some cylinders not actually contributing consistently at idle. Yes -- the fender tag says "3 +/- 2 deg ATDC" which is for when running at idle on the R2 points (which are 10 degrees crankshaft retarted from the R1 points). As soon as you open the throttle on a stock '78-'79 US 308, the microswitch opens and the timing changes to 7 deg BTDC at 1000 RPM idle running on the R1 points. If you have an MSD modification, you most likely have a "single event" firing system equivalent to just having the R1 points (otherwise you'd need 4 MSD boxes) and should be using the 7 deg BTDC at 1000 RPM spec for the R1 points -- but confirm your configuration and understand your advance settings (if electronic) before picking your idle target. And what about -- When you say "sync is good" what is the average airflow value at your 950 RPM idle in Kg/hr in each barrel from the Syncrometer?
I see where you coming from. I have not leaned out each cylinder to the point of a lean mis-fire to check if 1 have one not playing. I will scope it later this morn. is the base time set on the stock dist. btdc or atdc? I assumed btdc (advanced 3 deg +/- 2deg)? PS also noted the exhaust has much more pressure out on driver side than pass. thanks mart
You shouldn't -- the label directly says "ATDC" -- "A" = after, not advanced (and the marks on the flywheel are at 7 deg BTDC and 3 deg ATDC) Image Unavailable, Please Login Also see this site for the 308 WSM -- Section "O" covers the '78-'79 US 308 ignition: http://ferrari.stevejenkins.com/books/308GT4_workshop.pdf This is only significant if you have a symmetrical exhaust system that keeps the two exhaust flows separate. The stock muffler does not keep them separate and is asymmetrical inside so the flows aren't equal at the tips (if you have a stock muffler).
At 9 Kg/hr, you definitely are not running on all cylinders at idle -- should be more like: 3.0~3.5 Kg/hr at 7 deg BTDC and 1000 RPM or 3.5~4.0 Kg/hr at 3 deg ATDC and 1000 RPM In addition to the "turn in the mixture screw to detect miss" technique, you can also just pull off each plug wire one at a time at idle when you think you've got it all working -- if they are all contributing, you should be able to detect the same running change (RPM drop and audible noise) for each cylinder when the spark is removed.
