More Carbs: Mixture Screws

You should try the .60 idle jets. I had all kinds of popping and spitting from being lean throughout the lower RPM range. Having the mixture screws wide open did not help. I switched from .55 to .60 and my problems went away.

 

What is the difference between the two different types of screws? I believe my screws look like the one you have pictured.

 

Hey Geno,

I've got the same exact symtoms. I tried the 60s and IIRC, it did not help and might have been slightly worse. All of my mixture screws are out between 4-1/2 to 5-1/2. If you end up trying the 60s, please let me know what kind of response you get. Also, take a look at this thread...

http://ferrarichat.com/forum/showthread.php?t=264511

 

Funny,

My mixture screws, which I am pretty sure are Euro ones, are yet again different:

(the carb body you've sent me is still in customs by the way. Been there for two days now. Sigh)

Hans
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Sledge

My experiences are similar to yours except that I had installed 60 idle jets so that I could screw in the idle screws further as is the general consensus around here. I did that and it idled great and ran great (I thought) until I went to the dyno and the broadband A/F meter. My A/F mixture was WAY too rich (10-11) or so on the idle circuit (1000-3500 rpm or so). I pulled the 60's and put in 55's and the A/F ratio creeped back up to the high 11s and low 12s which was much better but still richer than ideal (12.9 -13.2) Power was increased at these lower rpms with the leaner A/F ratio and smaller idle jets. Of course I had to screw out the idle screws further than "optimal" or "the book" which is to be no more than 3 turns out but the dyno and the A/F meter do not lie. I am expecting my plugs to be less sooted now. With the larger idles they were pretty black (from the larger idle jets) even though my main jets were way too lean. My idle jet sizes are not representative of what would be needed in the typical 3 liter motor (P6 cams and hi- comp pistons) but the point here is that these engines are typically run far too rich with the bigger idle jets so the screws are not turned out so far. Virtually everyone I know has black sooty plugs and I think it is from the idle jets just being too big. Roc of Rock's Auto in Toronto said to forget about the number of turns on the screws- too many different types of screws- get the engine to run right. After reflecting on this a bit and the dyno experience I think it is almost impossible to have the engine be too lean on the idle circuit and still have the engine be able to idle at all. So, try making your idle jets smaller rather than larger (but not so small that the engine will not idle no matter how many turns the screws are out and see what happens. It is just in the nature of having this much carburetion.
Not true with the main jets and high rpms.

Don

 

Guys, don't you just SMELL the rich mixture?

When I was running it on 56 idles (drilled out 45's, hence the rather odd number), the car just stank. Nobody wanted to drive behind me, and I was doing 10 mpg.

50's made things a lot better, except for that one coughing barrel on the messed-up card.

Geno, the thing is STILL at customs! They're really trying to figure out how much money to charge me for bringing this back into Europe... sigh...

(Good weather on Monday, and I was planning on fixing the carb tomorrow. Guess not...)

 

I had similar problem with my carbs. After updating my ignition and resetting the timing it ran WAY too rich. Now, after synching and resetting the screws it idles smooth and runs and pulls strong all the way up to redline. My screws are about 1 3/4 to 2 turns open. No backfiring or popping and the smell is gone.

 

What size idle jets are you using?

 

And what emulsion tubes?

I've got the F24's...

 

Now I remember...this is the same results that I got. It didn't help the stumble and the mpg dropped significantly. So, I started to think that maybe I was going in the wrong direction. Maybe I was too rich. The stumble was not there when the motor was cold (needs rich mixture when cold), but appeared after the motor warmed up.

Correct me if I'm wrong, but the mixture screws are supposed to be adjusted to get max rpm at idle, right? For me it is still at around 5 turns out.

 

I am running what the car came with, which I think were the 55's

 

It is important to remember that the idle mixture screws only adjust the mixture at the idle orifice on the carb throat, not at the progression holes. That means the idle mixture screws only adjust the mixture literally "at idle." The minute you step even slightly on the gas and open the butterfly valves, you are now getting most of your fuel from the progression holes.

This is important because adjusting the idle mixtures is NOT a proper way to correct any kind of performance issues. Opening the idle mixtures to their maximum to correct a hesitation on acceleration is not going to work. (Or if it helps, it's the wrong way to correct it). If the thing is too lean, you put in a richer idle jet to make the progression holes richer, then close the idle mixtures to lean out the idle circuit to the right amount for idling.

I find that modern fuels with ethanol, plus my Ansa exhaust and "Russ" airbox mod require me to run .60s. The car runs like crap with .55s. I confirmed it by borrowing a friend's A/F meter and measuring my A/F with the .60s and it was right where it should be. You don't necessarily need a dyno. You just needs sniffer up the tailpipe to give you Air/fuel ratio at various RPMs. There are A/F meters for $300.

MANY experts have told me that the modern fuels are so different from the ones being made when the 308 was made that you simply cannot rely on the factory jetting as a guide. Additionally, don't forget that factory jetting was designed to make the car run as cleanly as possible for emissions. Thirty years later with half the emissions gear gone and ethanol fuels, the recommended jetting the owners manual is at best a starting point.

My car did have one curious quirk that took a while to figure out. It would run great when cold, and great when hot, but it had a hiccup on off-idle acceleration when it was "in the middle." In cold weather, the car could never get warm, so the car was always halfway between being warm and being cold. Under those circumstances, the hiccup never went away. The solution was to put a properly operating thermostat on the car which would allow the car to reach operating temperature in cold weather. It was a miraculous fix that happened after I spent god knows how much time trying to figure out which carb was causing the issue! (On the plus side, all the carbs got rebuilt as a result, which is never a bad thing). Not the solution I expected, but believe it or not, colder is NOT better for these engines. If they aren't reaching operating temp, they are not going to run correctly.

In case there is still anyone here who has not seen my carb synch tutorial, here it is:

http://www.birdmanferrari.com/service/sync/carb_synch_tutorial.htm

Birdman

 

Birdman, I've been tweaking my carbs and at present the off idle hesitation is almost gone but I'd like to have it all gone. I was wondering what your airflow readings were at idle and the next level rpm up (1500?) when your carbs are properly synched. Do you recall? Mine read about 4.5 on the meter at 800 or so rpm. Could it be my carbs are letting in too much air still?

 

Can you explain how backing the needle out more (increasing the opening) would inhibit the fuel flow out the lower hole?

 

I don't see how the mixture screw affects the upper progression hole. I may be misunderstanding things here but doesn't the upper hole only contribute fuel when the butterfly valve opens and there is enough vacuum to "suck" the fuel out of it?

 

Birdmans's comments regarding operating temperature made me think. Cars in the 70's typically ran at 180 F. Modern cars operate at 195 F. I believe it was an automotive engineer that told me this is due to changes in the fuel as well as changes in combustion chamber design (less hot spots). This seems counter-intuitive to those of us who have spent much effort trying to make our cars run cooler; however, I have found that allowing newer cars to run cooler does hurt performance. Like Birdman I have noticed that my '79 GTB does run much smother at higher temperatures.

What about the effect of spark advance on this transition zone. I have notices that a little more advance helps.

It could be that these things are only intended to operate at idle or wide open throttle.

I am no expert, I just though these may be a few different ideas.

 

Well, in its current condition mine does indeed work best at wide open throttle and high rpms!

Geno, the carb body arrived today. Thanks again!!!!

Waiting for my shipment from the UK to arrive, carrying some idle jets and a flow meter, and I'll be off to the garage rebuilding and tuning carbs!!