Still trying to get this car to transition from idle without spitting through carbs. 1. How much authority does the idle mixture screw have versus idle jet? B. Does the stock airfilter being on or off effect mixture? Thanks in advance, Tom
From my experience: 1. Idle screw settings work at idle alone, i.e. the throttle plates are about closed. Once the throttle starts to open, the transition holes tale over which also are metered by the idle jet. Therefore, the idle jet is more a player in transition than the set screw. 2. On the dy.no a/F meter, with the same carb jetting, I have found that the air filter does make a difference - usually (99% of the time) making the mixture a wee bit (.5 A/F) or so richer with the filter on - however, this has not always been 100% true for reasons I am not sure of yet. If everything else is set just right, and it still spits WITH THE AIR CLEANER ON, would raise the idle jet just a hair depending on what you have now (e.g. 50 to 53 or 55). hope this helps rt
i agree with the timing setting for spiting thru the carbs. I thought the idle circuit was always 'on'. that circuit still receives fuel pump pressure from the wells, even if the throttle is open. with the throttle open, the progression and idle circuit holes are still exposed to the venturi. there is less vacuum as compared to when the throttle plates are closed for these circuits, but fuel pump pressure will still deliver a mixture no matter what position the plates are in. correct me if i am wrong. i bet if you opened up the mixture screws way up, you will have a rich mxture across the board no matter what RPM.... just my thoughts from reading up on these webers......
Mixture screw only impacts idle. Beyond this the progression circuit kicks in. IME, spitting through the carbs under progression (say 1300 rpm) is due to: - lean mixture: idle jet may be too small - throttle connections being uneven: have someone depress the throttle once the car is warmed up and hold it at 1300 rpm, measure the venturi flows again with the SK. BTW, before #2, to get the throttle shafts about right, cut a playing card into strips, put a strip under each of the two throttle stops (makes the movements easier to see). As you move the accel cable, the pieces of card should move at the same time, indicating the carbs are advancing at the same time. Philip
Thanks for everyone's help: Jim- do you mean cam timing? It sure runs well off idle. I got a set of motorcycle carb sticks which allow me to watch vacuum for 4 carbs at the same time, both at idle and above and they're close enough. Mixture screws seem to all respond, out about 4 turns. I got this car with a bad motor so I don't know how it worked before it blew up. It starts and runs fine except for a lot of spitting back thru the carbs at idle and transition. It also is slow to return between shifts. Sometimes FI does look appealing.
Next on my list to do----55 could I be battling distributor trouble? Originals supposedly rebuilt at great expense by the local deities.
i have a thread here with the carb jetting and specs on all three of my carb 308's. 55/135/200 is about standard. 60's have always been too big for me here at sea level with the high humidity, and 125's are always a bit too lean. the dino has the crane ignition system and it seems to tolerate more advance and a richer condition for full power than the points set up. i am going to hotter plugs and leaner air correctors ( 190 to 220) to help balance out the A/F ratio. Russ turner has some super cool threads here with great details, as does Birdman. ill see if i can post a link to the thread i started on carbs. Michael edit http://70.85.40.84/~ferrari/forum/showthread.php?t=63519
Yes, I mean cam timing. I was having the same problem and Rifledriver pointed it out. I was trying to synch my carbs one day, and I was getting a fuel spray out of the front bank of carbs when I would blip the throttle. How long have you had the car for? I thought my car ran well off of idle too. Then when the problem was corrected the engine had a significant increase in power.
I got this thing running about a month ago. I've done about 300 miles. Both banks seem to respond (or not) the same. But I appreciate the advice and I'll check cam timing.
Carreaper, I've had your post on my desk for a week. Tomorrow I'm going to put it on an exhaust gas machine and see what it's really doing.
please let us know what you find, if i may be of any more help, please pm me. i learn as i go along with this also!
When I was getting spitting out of the carbs after a rebuild, some suggested I look into and I found it was the result of a vacuum leak causing overly lean mixture. Good luck.
i have a 1980 308 gtsi...it has a horrible haltech computer that has been nothing but a constant problem...too much gas, stalling, high idle, etc. ferrari suggested we ditch the computer and add carbs. basically turning the 1980 car into a 1979. anyone ever heard of doing that before???
I work on one such conversion for a friend- we know nothing of the prior history- It seems to work fine. By the way, the predominant problem with mine was bad carb base gaskets. It ran like it had an air leak and it did, it's major fault being slow to idle down. My nonscientific research says spitting at/off idle is lean. Backfiring on decel is rich. I haven't worked on carbs in years, bought this thing broken. It's been a learning experience.
