Am having issues dialing in my carbs. My elevation is about 300ft above sea level, so no complications there. Engine is a pre cat '77 with air pump and hardware removed, otherwise stock (so early cams?) Exhaust is European style headers with no ports. Aftermarket low restriction muffler (Nouvolari) Ignition is single distributor 123+ set at 10 degrees advance at idle, 39 total. New Denso 5307 iridium plugs, new Pertronix wires. Fuel pump is a new Delphi DFP FE0539 rated 5-7psi max with stock fuel bypass circuit. Valve clearances are at spec, as is cam timing. Air bypass screws are both closed on two carbs, one screw slightly open on the other two. Carbs ultrasonically cleaned, new gaskets and pump diaphragms, new floats set at 48mm. Carbs are nicely synched and flow about 4.5Kg/h at hot idle (using an STE synchrometer) Emulsion tubes are 36. Current Idle jet is 57 with idle screws about 1.5 turns out, Main is 140 with 205 AC. Have tried 60 and 65 idles, but car seems happier with the 57's. It really didn't like the 65s. On the main circuit have tried 135/220, 140/190, and 140/220 (which it really didn't like). FWIW my permutations are based on the what I have on hand. Situation (description is with car full hot) Car idles nicely around 900rpm. On very gentle acceleration from a level stop revs are clean. Seems happy to cruise at any speed (only tested up to 70-75mph) at constant throttle. But below about 3500 rpm, with anything other than a slow application of throttle there is significant stumbling. Heavier acceleration above 3500 gives an initial stumble then light 'popping' that appears to be through the carbs, though could be exhaust, I'm not certain. That same popping was there with 140 main/190 AC, btw. Data point: when rebuilding the carbs, two had a plastic pump cams, two had metal, so I replaced the two plastic with new metal ones (only ones available) as they matched the two metal ones I had. Is it possible the pump cams are an issue here? I've done several 'from scratch' set up of DCOE and Dellorto carbs on custom Lotus twin cam builds and not had this much trouble. So any advice or observations would be welcome.
Carb model number would be a real help. 308 used 4 different carbs. I will say in advance I strongly believe carbs belong in a museum exhibit but most of all US version 308 carbs belong anywhere but under the hood of a car.
Assuming all carbs are functioning the same, I think you need to get at least a CO reading at idle and throughout the transition range. can you get a bung welded to at least one of your muffler IN pipes? Maybe a stick in the exhaust pipe sniffer? Do some carbs seem to misfire more than others? When I tune my carbs, especially synching, I use a bank of 8-manometers, monitoring each manifold's pressure (vacuum). Misfires are easy to identify. My 78 GTS had metal tubing purging charcoal canister fumes, one to each of the intakes through a 1 mm factory drilled hole. I removed the two vacuum manifolds (light press fit) from the intake manifolds, exposing a tubing diameter hole that can be tapped for brass or SS hose barb rusulting in near perfect monitering points for synching purposes. The hose barbs can be capped or collected together to provide a dandy vacuum signal for measuring engine load during on road testing with an LM2 or similar O2 sensor based data collector. I found a virgin Heathkit gas tester that I put together and plan to use for reading my gas (CO) sampling tubes. Sorry you don't have the sampling tubes, they are important for setting idle mixtures. You want carbs to all act the same. That requires synching first and then getting all the idle mixtures as uniform as possible. Now you can start playing with jets, AC's and emulsion tubes. Years ago I was able to put together a uniform set of earlier carbs, 45/46/47/48 IIRC. Actually, I think there were two 47's, but by changing linkage arms I got what I needed. In addition to linkage arms, other things that cause part number permutations are jetting in general, chokes, and transition hole numbers, sizes, and patterns. I think you can make any of them work, but best to have a matched set.
Bill, I did make a good effort to get all the carbs matched for parts and back to factory specs, even comparing emulsion tube hole patterns. As I received the car, the idle jets were various sizes which had been soldered on the ends and re-drilled, had undersized 125 main jets, and there were two different sizes of floats, along with the aforementioned different pump cams. The carbs do have the 45/46/47/48 suffixes, but I believe those numbers are only related to external features like the linkage arms you mention. One thing I forgot to note during the rebuild was the pump jet size, which would be really helpful, as apparently there there were at least 3 sizes used on the 308's. I do need to look into some sort of CO or similar tester.
Having set up several of these I know the 140 main and 190-200 air corrector on the main circuit works well. 5-7psi fuel pressure is too much, should be 3-4psi. Ignition timing is spot on right? Sounds like it could be Significant stumbling below 3500rpm is the idle circuit and pump jet not playing nice. It's one or the other, or both. Make sure the pump jets are squirting fuel! It's very common they are clogged even after having been rebuilt. The pump jet is pretty much entirely responsible for proving fuel under quick throttle movements. If that's ok then you technically should have a wideband O2 sensor or you're just guessing, but 57 or 60 idle jets should get you there. I always have 3 turns out as the baseline place where I start with the idle screws, then typically land at 3.5-4.5 turns out with a 57 and 3 or so turns with 60s. The idle jets also feed the progression holes through the idle circuit, which transitions to main around 3-4k rpm.
Possible suffix numbers are what I refer to as 30 series for early 308 and GT4. 40 series for later GT4 with the mild cams 50 series for early 308 with mild cams 70 series for late 308. They have a variety of jet set ups, progression hole sizes, float levels, and pump discharge nozzles so yes, it is very important to know which you are dealing with and if the engine has the correct cams. Been tuning them since they were new cars so I know them a little. I will not work on fuel injection without a 4 gas analyzer but I have never found it much of a benefit on carbs. More of a crutch. If 4 turns in the idle screws is not in the ballpark you have a slow running circuit issue. Often caused by progression hole issues. The idle jets do a great deal more than just idle. They cover most fuel delivery below about 30% throttle opening. Seriously reconsider your commitment to carbs. They really do belong in museums.
Seriously reconsider your commitment to carbs. They really do belong in museums. On the shelf next to ignition points.
Points are already gone. One of the first things I remove on old cars. Carbs don't bother me as much as usually I get them sorted in short order. But I have been researching DCNF throttle bodies and ECU options....
My 78 GTS had the 72/73/74/75 OE carb set as OE. The progression holes are difined in the Owner's Manual as 1.50, 1.15, 1.50, 1.60 and 1.80 mm ID's. The 45/46/47/48 are shown a used on 75 GT4 USA. This is what I use now. My notes from Owner's Manuals show progression holes were 0.80 1.40 1.30 1.60 and 0.80 mm ID's. I think it might be useful to remember later carbs had to deal with emissions more and more. Do you know what your car originally had? My Weber book has original jet settings. Not sure which one is for you.
Come on in! The water's fine!!! I'm sure you know all this but just incase. If you make the change, don't be afraid to go big. Carbs need enough vacuum signal to draw in fuel, ITBs do not so there should typically a 10-15% power gain with large ITBs, none if you chose the same size ITBs as the carb size. Idling though is probably more of a challege with ITBs and bigger makes it more of a challenge because air imbalances cause mixture imbalances. So ITBs get kind of a bad rap on the street. Mine idles fine with 54mm ITBs, but I read MAP and correct mixture on every cylinder so getting a good sync is way less important.
Those numbers do not fit together at all. At 10 deg BTDC idle timing, you should be at about 3 Kg/hr/barrel, and having a hard time keeping it at, or below, 1000 RPM warm idle. You must have some cylinders not contributing at idle so you need to identify those first IMO. For example, turn the mixture screw fully in to see if the warm idle changes, or not (it should). Or unplug the spark plug wire to see if the warm idle changes, or not (it should - but you need to be careful to not get zapped).
Good input, gives me something to pursue. Those are the most recent observations, and the idle seemed very smooth to me. If it makes any difference, my car has the earlier ('76> '77) cams which I verified when setting the valve clearance. Unfortunately I won't be able to get to the car for a week or so, but then I can do a fundamental check of everything again and report back.
I think there was a thread a couple months ago with similar issues and the answer ended up being set the mixture screws to something reasonable (3 turns?) THEN sync the carbs and it sorted everything out. Something about the high airflow means the throttle plates are open and that lets it will fuel from a blead hole that is meant to add fuel off idle? Something like that? Maybe search high mass flow at idle, I think that was the thread
This thread https://www.ferrarichat.com/forum/threads/carbs-pulling-10kg-hr-at-idle-should-be-4kg-hr.692643/#post-149644867
I have had more nightmare experiences with Fuel Injection than ever with Weber carbs. Best thing I did for my Alfa was to ditch the SPICA for dual Webers. A Fuel Injected car with an intake plenum will never sound like the roar of carbs. Had a 1977 308, new in 1977 and it ran fine with the retarded points, pumps etc as delivered from the factory. In terms of popping, perhaps I'm not the only one to have witnessed break-down of the carbon recovery tank, whereby the carbon chips get caught in the carb. My 1978 GT4 had such a problem. This caused a lean low speed mixture and much popping. After much flushing and back flushing and blowing to remove the carbon chips, the carbs have been fantastic performers for the last 15 years. Image Unavailable, Please Login
I have one car with no pollution equipment at all. Even dumps crankcase bypass to the ground. The dual Weber throttle response is like no other car legal for the road; instantaneous and firm. The sound fantastic too! Image Unavailable, Please Login
I have extensive experience and factory training on Spica. I still have all the factory manuals from 79 to 82 when they wisely gave up on it and went to Bosch electronic. The 2 liter Alfa never ran so well. Spica was unmitigated garbage. I was speaking of quality fuel injection. Not the worst known to mankind. But the worst carbed engine (US version 308) or the worst fuel injected engine (1750 and 2 liter Alfa) I'd still have to go with the Spica fuel injection. What they did to a DCNF to make them pass smog was a very bad joke perpetrated on Ferrari owners. So I guess dual Webers on a Fiat surpass any Ferrari with Bosch Motronic in throttle response? Got it. Every North American version 308 dumped its charcoal into the carbs. Factory paid us a lot of warranty money to fix that. You are supposed to put an inline fuel filter on that hose to prevent it. Carbs are for people either stuck in the past or those who do not understand fuel injection.
The throttle response is mostly having the throttle plate(s) close to the intake valves. ITBs are the same in this regard, so just as snapy but with a bit more hp. Carbs do have a nice burble at idle all there own, but ITBs can have a nice inlet sound too, different but still nice.
The quad twin barrel Webbers are an endearing feature of early Ferrari 3#8 s . Well worth sorting .Once sorted ( I don’t profess to be an expert so shall refrain from technical advise ) by someone who knows how to tune them they bring an extra dimension to the driving experience. I don’t buy into any electrotwackery mods like swapping out the points for a “ box “ awaiting to brain fart . Think easy as wheel barrow or garden2T bush whacker , then think Lamborghini Miura …..Who’s gonna retro fit fuel injection, bin the points . Yes I tend to agree the throttle response is addictive .I run a Pork 987.2 with port injection the 2.9 variant flat 6 version , with hydraulic PAS . There’s something missing in the modern pork technologically tour d force , when it comes to throttle response that only Webbers can give . Enzo was very late to the “ tI “ and “E” injection party during the 70 s for a reason . Arm bent up his back by emissions reg he fell into line . This doesn’t mean it’s better .It’s just different.
You are an expert in many things!! I learn from all your posts. Mine ran fine. Yes, you know that the rotating mass of the 1756cc engine is less. Also, a cable attached to the butterflys won't have the inherent delays in this complex system. If you can't feel it, hear it, smell it, then, yes, it is not for you. Otherwise embrace this: Image Unavailable, Please Login Yes!! Stuck in the past and loving it!! You know I'm only referring to vintage cars and vintage induction systems. I don't own a contemporary Ferrari and maybe never will. My next Ferrari will most likely have carbs.
First I have read about an inline fuel filter being required on the carbon canister. I thought there is a one way valve in that line but don’t recall a filter. Anyone got a picture of the placement? Sent from my iPhone using FerrariChat
My 1978 308 GTS also had a failure which caused the suction of the carbon from the carbon canister into the carbs when the airbox flap went shut while driving on the highway. Hollywood Sport Cars removed the carbon granules at no charge. I used stainless steel wire to keep the airbox flap open at all times and removed the fuse for the electric pump connected to carbon canister. I use my garage building exhaust fan to remove any gas fumes.
(raising my hand slowly) Where is this carbon filter located? I have a US 77 GTB with no emissions. But the bypass system looks more like a GTS. (table 14) Image Unavailable, Please Login
Wrong system. That's the crankcase emission control system -- the "filter" to prevent the charcoal pieces entering the airbox is added into the fuel evaporation control system. This really isn't a problem on 1977 (and before) US 308 as there is no path for the charcoal bits to get into the airbox. It's more a problem on the 1978-79 US 308 (with the system that pumps the fuel vapors out of the airbox into the charcoal canister at engine shutoff) that do have a path for the charcoal bits to get into the airbox (and therefore they can get ingested into the carbs when something goes wrong like Richard described).
