Hi all Very conscious that there are hundred of posts on carb jetting on the forum, so am hoping I haven’t missed this (apologies in advance if I have) but… Does anyone know what the standard specs are for a 1979 Euro 308 GTB? I am contemplating a replacement carb. These are the specs for the replacement which I know are not right for a 308, but just not entirely sure what I should be speccing, and am wanting to start from a base line of standard, before tweaking with alternative jets. Thank you. Image Unavailable, Please Login
The "standard" carb parts for your model are shown in Section 3 of the 155/78 OM for your model (give a shout back if you need a pdf copy). However, if you are thinking of buying 40DCNF-12 carbs to replace yours, you might be disappointed as: 1. They have 4-bolt top covers, not 5-bolt (like stock 308 carbs) 2. The air horns are not a separate piece (so you need to add washers to retain the airbox), and do not have the best inlet shape for working in a 308 airbox. 3. The choke lever mechanism works backwards compared to the stock 308 carbs (so it can't be used/connected). Not saying that they can't work (I bought and used a set of 40DCNF-12 carbs near the end of my 308 ownership), but, after seeing and learning the differences, I would've opted to have my original carbs rebuilt as it was about the same cost (mostly for reason #2).
I would check one of your current carburetors first and match accordingly. Euro spec cars, from what I've seen, vary from early to late. Some use F24 and some use F36 emulsion tubes, and those really alter the personality of the carburetor quite a bit. From my experience, all 308s I have set up on a dyno or just with a wideband O2 sensor run optimally with F36 emulsion tubes, 140 main jets, 190 or 200 air correctors, 60 or 65 idle. Make certain the engine is getting full ignition timing of 38-39 degrees, which with the Marelli distributors is realized by 6000rpm.
In the OM! Thank you and another rookie mistake on my part! Thanks for the advice on the replacements - for the price, I was thinking keep the original carbs, but just replace for new for ongoing use and be certain everything is as new, rather than worrying about the skill of the person doing the refurb. As per usual, it seems it’s not so straightforward. On matching to the current carbs, that was my original thinking, but since they aren’t behaving, I wanted to make sure I was staring in the right place. Really appreciate the advice on the best set up, though. Thanks again for all the help.
"Make certain the engine is getting full ignition timing of 38-39 degrees, which with the Marelli distributors is realized by 6000rpm." A bit of a HiJack and I apologize for that but found that statement very interesting. I've done a LOT of dyno and track work with US v8 engines and we typically found that 34-36 degrees was max advance for most power on those engines. I was under the impression that these 3x8 motors needed considerably less advance with their head/combustion chamber design compared to typical US v8 head design so I'm surprised to see that amount of advance.
It's the hemi combustion chamber in these 2v engines, they are inefficient thus demand lots of timing. Alfa 4 cylinder engines of the same era call for about same timing. The 4v pent roof chamber of course needs less.
Aha, I didn't realize the combustion chamber shape was a hemi on the 2Vs. I'm VERY familiar with Hemi Mopars/their advance requirements - some liked 40 degrees!! So the advance for the 2vs now makes perfect sense. Thanks!!
I didn't do it, but would follow this next time... Set your ignition first and make it perfect. Then tune the carbs. When I was setting up my 1975 308 GT4, the biggest single improvement I saw was sending my ignition distributors to Patrick Ottis and having him tune them on his Sun machine. He installed new weights and springs that I gave him from Superformance in the UK. Then set them to spec. The difference was huge. Just my 2 cents.
2V's only have 8.8:1 CR. My 3.4L is happy at 34 degrees BTDC on the dyno, but it's running good pistons giving 10.5:1 CR. I think this timing is consistent with my Lotus twincam, a very similar engine running 10.3:1 CR. Bill
One other thing to look at when matching Webers is to note the number, sizes and locations of the progression holes. Most everything else can be changed out except for the 4-bolt, -12 variant. DCNF40/72/73/74/75 carbs differ only in linkage. Changes in series like DCNF40/35/36/37/38 or DCNF40/45/46/47/48 generally denote changes in progression holes and/or jetting or more. Bill
Hi pedders, I think I got the nomenclature not quite right. I believe it should be: 40 DCNF nn, not DCNF40/nn where 40 is the basic bore in mm. DCNF40/72/73/74/75 should read 40 DCNF 72,73,74,75 for the 4 OEM carbs on a 1978 US spec 308 GTS. I believe these carbs are fully tunable but mine were abused by a PO. I found 3 nos examples of the 40 DCNF 45,46,47,48 family of carbs from one source and a nos duplicate numbered one from another source. A change out of a lever arm and maybe the fuel inlet banjo and it was good to go and correct in all respects, but with a wrong number stamping on the base. Bill
