Best jetting for webers for 78 308 GTB

I have read that the stock jetting for 308 1978 GTB were set too lean in order to pass emissions. Does anyone have good experience as what is the best setting if the car does not have cats and I am more interested in the best performance for the car. I live in a state there are no emission testing. The car is essentially stock with electronic ignition. I read that the idle jets can be increased as well as the main jets. What about increases in venturi (choke) size? Does 33 or 34 mm make a difference? Thanks Ben

 

search for mine and Birdmans threads.

in a nutshell : 55 idle,135main, F36 emulsion tube, 200 air corrector, you can go to a 195 air corrector if the car has a flat spot up in the top end with the 200.


i have found the 220 air corrector to be too lean.

make sure the sync is perfect also.

Michael

 

Try calling Pierce Manifold at 408-842-6667 and ask for Mike the owner, He is a weber carb expert, and he is the biggest Weber carb distributor in North America. He has a huge warehouse full of weber carbs and part and hehas helped me alot, he also has a 79 308 GTB wich he played around with his jettings and preety sure he would know exactly what you need for your application.

 

Ben,
For a '78, I agree with Michael's recommendations, especially with no cats.

Birdman

 

in a nutshell : 55 idle,135main, F36 emulsion tube, 200 air corrector, you can go to a 195 air corrector if the car has a flat spot up in the top end with the 200.

60 idle helped mine smooth out a flat spot between the transition.

 

Style points or discussion only, but instead of raising the transition overlap reaching up from below with a 60 idle, another option would be to reach down with a 140 main to bring the main circuit in a bit sooner keeping the 55 idles; This saves on cruise and low throttle opening richness caused by the 60.

 

Thanks for all of the recommendations. I'm sure it will save a lot of time and effort in trial and experimentation. Ben

 

I agree with the above statements. With the cams that year car has, and cats, the largest main you will want is a 135 with 200a/c.

 

Not to mention the expense of buying lots of different jets and parts to experiment with. Just putting the carbs back in my '78. Good time to check the jetting and upgrade as appropriate. Thanks guys.

 

Interesting. Could you expand on this approach? Does the 55/140 setup work better than 60/135 in performance or gas mileage or does it depend on each indivual car?? What symptom are present with low throttle richness...problems in idle? How would 140 mains affect off idle preformance?

 

Just my modest experience here. A brief review
I look at jetting as a balance between idle, transition and main circuit. Idle mixture is controlled by the idle jet when the throttles are closed. Transition is controlled also by the idle jet operating through transition holes at low throttle openings. The main jet does not take over until higher throttle openings (variable, but think mid 3K). THE secret to Weber jetting is management of the transition range, where most of normal driving as done to avoid holes or 'flat spots' in throttle progression.

The main circuit comes in is detrmined by the velocity of the air (venturi size and throttle opening) and the ease at which the fuel is sucked into the stream (main jet size). As the throttle opens, the idea is to have enough richness provided by the transition circuit until enough air velocity is reached to bring in the more efficient main circuit. Depending on the driving you do, usually most of driving is done on this transition circuit, depending on how low or high the main circuit comes in. In reality Weber transitions like rich mixtures as they cover up a number of ills and give a smoother ride through this transition circuit, and many folks cheat a little on the rich idle side as it provides a high coverage of the transition circuit into the main cuicuit flow range. A big misconception I think for new folks is to think that you are tooling around town or cruising to Aunt Blabby's on the main jet. Think again, Buttercup, you are probably still on the idle jet controlled transition circuit -- and can be as high as a 60+ mph cruise!

My preference is to set up the the main circuit to come in a bit earlier (smallest venturi possible, size up on the main) while keeping the idle/transition circuit as lean as I can (but in reality that still translates to A/F of 13+ or so). When you put a mobile A/F on most Ferrari Weber jettings, especially older cars, you'll find you're tooling around town and cruising at A/F 11 or 12 if you're lucky at factory settings - no wonder they had a reputation for fouling plugs!! So for me, I'll lean out (relative term here) the idle transition as you will rarely be under load here, and when you might be that couple of seconds is well covered by the way over-rich accellerator pump from the infamous -12 pump cam we all have. So when you are at high throttle settings, you are typically on the main circuit, and power, not economy is desired - so a bit fat A/F with a deliberate sized up mains can at deliver A/F 13 or high twelves AND - HERE'S THE SECRET - bring in the main circuit earlier (larger main, remember - easier to flow) allowing a slightly leaner transition circuit for cruise and covering any lean transition flat spots. Thusly you have better gas mileage, less propensity to foul plugs better throttle response, and a top end more suited to power operations.

Now some folks will say their car runs well with 60 idles - richness in tuning covers up a lot of ills because it covers flat spots so well and gives that smooth transition. However, I'll bet you a coke they are cruisng and driving around town with A/F's in the 11s with all that entails.

How do it know? I've had good luck woth the Innovate technology LM-1 I got from Pelican Parts as mobile A/F which I can download and analyze on the fly as well in real world driving. (NO affiliation, yada, yada). I think it is the best $300 any Weber tuner can make to tweak those babies to the gnat's ass. There also others available I'd imagine.

Summary: I like to jet as lean as possible on the low side for low power settings and cruise, and using a larger main bring in the main circuit earlier to cover transition flat spots and provide a more optimal power jetting (A/F 12.8 - 13.5). Just my experience and preference with good results for 3 or 4 years now.

best
rt

 

great write up. i have always found 60s just too darn big for street and normal in traffic driving . i have electronic ignition on the black gtb, i would not want to try to keep a points car lit with 60s. the only 308 i had that ran with 60 idle jets had the float level set a bit too low when i got it. setting the float level to the higher spec( there are two specs floating around) worked great and the car preferred 55's.

there is an art to this, so at least we have a good baseline for someone to fine tune from. perhaps other folks can use 60s in their cars.

 

Oh crap! - got carried away in previous post and forgot to answer original questions. If you read my philosophy of life as expressed through carburation below, these answers will make sense (I hope):

1. Yes, although it varies car to car, I think a 55/140 setup is intrinsically better than a 60/135 if they feel the same while driving without transition flat spot

2.Ferraris can tolerate a LOT of low throttle richness without driving symptoms. You may however first notice a blue exhaust smoke, the smell of unburnt fuel and fouling of plugs.

3. 140 mains in themselves have nothing to do with off idle performance if that is defined as low partial throttle openings - you're usually on the transition circuit. They will effect the rpm/throttle opening point where the main circuit comes in a bit higher up as we said above. Again, I think what you may be after is the balance between transition and main, and that is what we talked about in the rabbiting previous post..

As far as venturi size, 32 is what you want in a 308 unless going for high rpm performance with other mods; see
http://www.ferrarichat.com/forum/showthread.php?t=147926 (start at beginnng)

See, Webers are fun! Good luck!

 

On a dual 44 idf equiped 2 liter FIAT motor I had just the issues described with 55/60 idle jets. Turns out you can special order 57s-they hit the spot-no flat spots in transition and the people driving behind me didn't have to gag on the exhaust fumes. PB

 

Thanks Russ
That was most enlightening and a great explaination of an ideal way to approach jetting of the Weber Carb. I am assuming that this is best way for the average Joe and not a setup for the track. I might give it a try. Thanks again.

 

Russ how about a 50/145 set up?Would this be ok?

 

Russ,
Outstanding job at explaining a tricky concept in simple terms. When you say "smallest venturi"....have you swapped out the venturis in your carbs?

Birdman

 

Depends of course, every car is different - but it very well could be fine. When people ask what a 308 should be at I always want to blurt out "140/52", but every car will need a little tweak usually. It depends how rich the 145 is - you do not want the car on main circuit much below 13 A/F if you can help it. If you take it to the dyno and punch it and the 145 does well from 4K to 6K rpm, ( defined as above 12.5 A/F and above to about 5.5K rpm, you can fine tune above 5.5K with the air corrector jet as you know), then I would - BIG RULE HERE - start rich and start bringing back the idle lean one step at a time until you sensed a flat spot or poor slow speed running, then up a 1/2 size or previous, whichever feels good (just like you do the A/C jet) If you are 50 idle or above the chances of it being to lean on idle/transition are pretty low, but would then re-dyno with A/F or use a A/F meter and verify everthing is no wuckin furries.

You good folks will notice slight personality changes as your car has different jets - that is the cool part, and very important. Just listen to what it is telling you.

 

Yes - have run 34s and 36s back and forth numerous times trying different combinations. Takes about an hour to change all eight.

 

Thanks Russ I might try that combo if I can get a hold of some jets

 

One more question Russ if I go with a 45 idle,145 main what A/C jet should I use?Thanks again

 

Where can you special order 57 idle jets?
Thanks

 

Pierce Manifolds
www.piercemanifolds.com

 

Between Pat Bradens Weber book, and Russ's findings with the LM-1, its amazing how close the two are. I gotta get me one of those things.

 

Had my 308 (carb) out today and it purred like a kitten however I noticed as the ambient temp went up to 90+ and the water and oil temp increased about 5-8 degrees (F) the car started to run a little irregular. It acted like it was too rich. Now is this my imagination or is there a techincal reason i.e. linkage is effected by the heat or whatever? Don't want to change the adjustments or jets if this to be expected.