Rejetting carbs

Hans, they mucked something up a lot worse than jets Im afraid. These cars run a bit rich with as big as 140's, but nobody has complained about backfiring and super poor mileage from that. In theory the 125's were too lean, so if anything, the 135's shouldnt have hurt fuel economy, if fact its more probable they could help it do a bit better as the mixture would be closer to optimum. Im running 135's and I can pull off over 20 MPG if I nurse it around staying off the carbs, and my car has never backfired. But loose or broken exhaust or exhaust leaks can cause backfires.

As far as changing the jets, take off the airbox cover and look down between the throats. There is an opening in the top of the carb and the jets are screwed down right inside the opening. You will see two large brass screws, those are the jet assemblies. They are actually a stack of three parts, the main jet at the bottom, then the emulsion tube, and on top the air corrector. Go unscrew one and look it over carefully for numbers and report back your findings. You may need a small grabber or tweezers to get it out after its unscrewed. Also the numbers are tiny and hard to read, but they are there. Emulsion should be a F24 or F36, AC should be around 200 +/-. You could probably swap all eight jets in less than 15 minutes if you had them in front of you.

 

This is all very confusing, isn't it?

I recently discovered that my car is running very lean at full throttle as you can see here http://www.pless.com.au/308mixtures.txt (this was done with a portable a/f ratio reader with a lambda sensor)

Imagine my surprise when i pull things apart and find 190 air correctors and 130 mains with F36 emulsion tubes. It SHOULD have been almost perfect with that.

also i'm running 55 idles...i'm going to chage those to 50s

 

It's amazing to find what is actually in there.
With F36 (vice F24) ET the car will run leaner.

 

i thought i had 220 a/c

my mechanic bought a wrecked engine (from fire)...this car was setup up to race, apparently...had 135 mins, 200 a/c and 50 idles.

i just put in the 135s, and was disappointed that in fact, the 200s are bigger than what i already had. i thought i'd got lucky!

i'll try the 50s tomorrow too....just for the heck of it.

friend had 140 mains 170 correctors...i thought that was absurd, but maybe it isn't

 

140 mains with 170 a/c should make it WAY too rich and it should backfire on decel, flames out the back, etc. I experimented with 140 mains and 190 a/c and that was too rich, but the 200 a/c was right on. I imagine 190 a/c with 135 mains would be okay too for top end, but the main would not kick in as early as with 140 and 200a/c which is what I prefer personally.

 

well, i've got a/f ratio of 15.1 at full throttle @ 7000rpm with 130/190, is 140/170 going to go from that to WAY too rich? i know it should, but who'd have expected it to be as lean as it is with the jetting i've got?

Anyway, time will tell. going bigger jets and smaller correctors and will play until it's right. first step is just stepping up to 135 mains and seeing what that does.

OTOH to 55 idles are a bit rich.

 

Hans, here is a link to an adjustment procedure for the Weber carb.

http://www.webercarburetors.com/ppw/html/tech/carburetor_set_up_and_lean_best_.htm

It is not a difficult procedure. As for exchanging the main jets, it is also not a difficult procedure, but I don't know the level of your expertise with wrenches either. I would go back to the shop and have them give you your old jets back. You can order any jets from the above website but it's best to take your's apart and inventory first.

Here's a brief primer:

1 Remove the air cleaner top. Base, and rubber gaskets held on by nuts around the carb bases.
2 Locate the machine screws on the top of the carb either 5 or 6 can't remember.
3 Remove the fuel inlet to the carb at the barb fitting.
4 Remove the machine screws holding the top of the carb to the body.
5 Gently tap on the top of the carb to loosen the gasket, then carefully start lifting the top.
6 As you lift the top, look at the side, and see that the gasket is coming off with the top, and also the floats are coming up smoothly.
7 Remove the top, and set it aside. Look in the well of the flaot chamber at the bottom, you will see the main jets.
8 Use a large screwdriver and remove the main jets from the well of the float chamber.
9 Read the number on the side of the jet.
10 Replace with the original, and carefully guide the gasket, float, and top back down on the body.
11 Cross tighten all the machine screws once the top is flush with the body flange.
12 Attach the fuel supply, Check for leaks.

That's it, if your car was running right before, and the only thing they changed in the circuit was the main jet, you should be good to go. As for the 'ideal' size, there are some cars that work best with 125s, some that work best with 130 and even some that work best with 140 depending on the air corrector and the emulsion tubes. It's a matter of trail and error somewhat, so don't be certain of one size fits all type theory.

Remove your plugs after an hour or so of spirited driving. They should be a nice chocolate color, not too dark, and not white. Maybe between chocolate and tan is pretty perfect.

 

You must be thinking about the wrong carburettor. The main jets and air correctors are sitting on top of the 40 DCNF carby. all you need to is remove the air cleaner box to access. couldn't be easier, really.

 

DOH! Of course, these are the DCNF. My apologies. I was thinking of the DCEV. The main jets are at the bottom of the emulsion tube stack in the DCOE. Much easier to service. Pair of brass jet stacks right near the center, just loosen and the whole stack comes out in your hand.

Very sorry, ignore all that stuff about taking the carb top off.

 

haha...sorry Doc....I'd never seen you make a mistake before

 

stepping up to 135 mains would be a good start I think. I would be interested in hearing what our WOT a/f ratio is with those installed. keep us informed.

 

And unless your going to yank out the idle jets, you dont even need to remove the airbox. Just take off the top cover and voila, they are right there ready to jump out at you like a snake. Item numbers 11, 16, and 24. You gain access through that small opening in the carb top between the barrels.
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actually, you have to remove the trumpets too. that takes more time than anything else...and it's a bit terrifying too, for the obvious reasons with all those washers and nuts!!!


more amazement today. i found i had one 130 main and 7 x 125 mains. it's all making more sense.

so now the car has 50 idles, 135 mains and still the 190 correctors

i'll go for a drive later and see how it feels.

 

Ahhh, I have those pesky -12's, they have a crude airhorn built into the top cover and getting the jets out is easy. But I was sure you could get the jets out of the others too. You sure you cant slip a screw driver down in there and sneak em out?? My other carbs are boxed up or I would go take a look.

 

it doesn't look like it to me...but then, i've been wrong often!

 

There's a chart of the Factory configuration by year, as far as a baseline goes.....

Not sure if I can find it, anyone else have that?

 

What kind of cofiguration?

 

When checking jets do not go just by the numbers that are stamped on them (especially idle jets). Get a Weber jet gauge and actually check them. A lot of mechanics will drill out jets ("weber drills") when they are tuning your engine, and then fail to file off the numbers.

I pulled the jets on my 308 Gt4. (Has aftermarket exhaust, and the car keeps up with a euro QV)
The idles said 55 but in fact were drilled to 60. These are probably little too fat. I will replace them with 57 (Judging from Fchat correspondence, 55 may be a bit too lean).
The mains are 130 (maybe should be 135 to help top end?)
I changed the AC from 220 to 200, and that helped a lot on the top end. (maybe I should change to 190 if I can find them). On the top end it is better to err on the rich side than the lean side.

The car's current charactersitics are
1) a little stinky at idle, but smooth
2) excellent mid range reponse in all respects.
3) Probably still a little anemic on top end. But maybe I expecting a little too much from those still relatively mild early 308 GT4 cams (at least compared to early Porsche 911S cams that I am used to)

Just my 2 cents.
Martin

 

140 mains with 200 a/c really work well with my '76. I have not tried 135 mains with 190 a/c, but I imagine that would work well too. I have tried 135 mains with 200 a/c and that was still a bit lean. Many on this board have great luck with the 200 and 140 setup. Try both!

 

that is absolutely amazing. i had 125/190/F36 and was FAR too lean! obviously the difference is the emulsion tube...and i'm amazed that it made that much difference.

 

so now the car has 50 idles, 135 mains and still the 190 correctors

i'll go for a drive later and see how it feels.[/QUOTE]

Aircon
How did this work out?

 

I second the wow. Not that fuel economy is a real big deal with owning a Ferrari, but I grew up with the belief that a good performing engine could have both good performance AND good fuel economy. All the cars up to about 1970 were like that. All my Dads big 440 Chryslers would easily reach to near 20 mpg on the road, until the 1973 car came along which was both strangled performance wise and guzzled fuel. The US emission ideas are a joke. They make cars burn more fuel to feed the catalytic convertors to get better emissions, when all we really needed was just the unleaded fuel by itself. I have read several times of early european 308's achieving fuel mileage above 25 mpg. Thats how they should run IMO.

What size idles are you running??