Carb Tuning Question

I don't have a lot of knowledge of subtleties of tuning carbs, but I did have a vapor lock problem in my 308GT4 about 13yrs ago. I talked with GT Car Parts who supplied a higher output Fiat (I believe) fuel pump that solved these problems. Could some of your problems be fuel pump related? Just a thought.

 

Blaine, if you study the design of the webers on your car I think you'll find that the transition off idle is also dependent upon the progression holes (both size and location) as any jetting setting, in fact, more so. In other words, the location and size of the progression holes (typically you'll see one or two very close to the edge of the closed throttle plate) dramatically affect the rate of fuel flow as the plate comes off it's (closed) idle setting in the transition from idle circuit to main.

Unfortunately, the progression holes are drilled, and difficult to change. They can be drilled out larger if done carefully, but then the location may not be ideal either.

There are different variations of the weber downdrafts on your 308, the main difference between the variations being the location and size of the progression holes. But, obviously, trying different sets will be expensive and labor intensive to say the least!

Not much help, I know.

 

Blaine,

I've got the same exact symptom and have thought too if it was too rich. I'm going to do some reading in my Weber book and I'll let you know if I find anything out.

Did you adjust your idle jet screws when you changed your idle jets? Maybe leaning them out (screwing them in) might alleviate some of the hesitation. Although, my understanding was was to adjust the screws for maximum idle rpm. I'm pretty sure that the fuel that flows through the idle jets then goes past the idle screws.

Henry

 

hope this helps..


I don't own a weber'd Ferrari, but have had lots of Lotus and other cars with DCOE and Delorto.

From my experience.. with a good engine, fresh carbs and fuel..

Ideally, start by taking the car to a dyno shop and do a map of the AFR to see what you have to start with.

Set the fuel pressure, float heights, and timing, then adjust your idle speed and idle AFR (13/13.5:1) and go from there.

The ideal AFR across the board under full power is IMO 12.5:1. You will probably find that with the Webers its not easy to hit that mark, unless you are lucky or have LOTS of jets to play with.

Here is a breakdown of the jet functions as I know it:

IDLE jet - adjusts idle and effects progression mixture strengths

F# jet - adjusts the transition mixture curve as the idle circuit goes from low throttle openings to where enough airflow passes the aux venturi to activate the main circuit. This transition zone is where a lot of cruise problems can develop.

MAIN jet - more or less uniformly effects the mixture strength of the full RPM range the main circuit is active.

AIR corrector - effects the upper rpm range mixture strength curve more so than the mains, but will also adjust the entire rpm mixture strength. So for example, if you have to richen the top end by going to smaller AC, you should also consider a slightly leaner selection of main. I use a 3-AC sizes = 1-main size ratio.

EMULSION tube - changes the shape of the mixture distribution curve of the main and air combination. This jet is the most difficult to source and select, and the one which I consider most important when fine tuning a modified car.

PUMP jet - effects the flow rate of fuel during the accel enrichment phase. The pump spring effects intensity/duration

PUMP BLEED - effects the response of the pump jet during light throttle application, and the overall quantity of fuel delivered to the pump jet.

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If the engine sometimes spits back thru the carbs on accel that indicates lean. If it stumbles and feels all muddy then thats rich. If this happens during light accel I would suspect the idle, progression or accel circuit is off... if it happens at higher rpm or hard accel I would suspect the accel circuit (if the mains are OK)

if you got an improvement going from 55 to 60 idles, I suspect you where either lean on the idles, or compensating for a pump jet/ bleed deficiency)

Ultimately, if you have a lean issue and use the idles to compensate for a pump deficiency you may solve the problem but will likely get worse fuel economy as a result.

I would look at the idle/ pump/ spring and bleed combination.. but make very small changes (and one change at a time) to test each combination.

 

I occasionally get "spitting" back through the carbs as well as a hesitation off idle and a stumble during light acceleration. Here's what I'm running right now:

idle jet: 55
main jet: 135
A/C: 200
ET: F36
accel pump jet: 45 (I think)

 

this may be helpful..

http://www.ferrarichat.com/forum/showthread.php?t=55165

 

I think this will be kind of hard to do. I've noticed that the slightest movement of the throttle will still squirt some fuel from the accel pump. I'm still not sure if my problem is too lean or too rich. I really need to get it on a dyno and see what my a/f ratio is.

Attached is the OEM jetting according to my Haynes Weber Carburettors book:

H - main jet
I - idle jet
J - air idle jet or hole
K - emulsion tube
L - air corrector jet
M - starter jet
N - accelerator pump jet
O - accelerator pump back bleed
P - needle valve
Image Unavailable, Please Login

 

I'm now wondering if I may have been going in the wrong direction (or maybe took too big of a step when richening up the mixture). My first step was to go to 135 from 125 on the main jets. I too have tried 60s on the idle jets but am back to 55s for now. I was thnking of going back to OEM settings and taking "baby steps".

Does anyone know exactly where each of the jets/circuits control? My assumption is that idle mixture screws only affect idle and initial transition into acceleration. The idle jets then take over and affect mostly cruising times. Then the main jet takes over for harder accel and the air corrector mainly affects the top end. The accel pump/jet only adds fuel during accel (the harder the accel, the more fuel is added) The accel pump will add additional fuel during the process of depressing the throttle and a few seconds after. Is this correct?

What I'm still not real sure of is when these jets and circuits operate and transiton. Are these dependant only on the RPM, or the throttle position, or both?

I'm starting to think that I I'm too rich during accel and getting the stumble. Like Blaine, it feels good when the car is cold and for the first 10 minutes, but once it is up to temp I get the stumble. Also, my top end is dead. The power really drops off above 5500-6000 (I'm thinking too lean here).

Looking at the OEM jet settings that I scanned above, it seems like the major differences between the different 308s are the "air idle jet or hole" and the "accelerator pump back bleed". Are either of these changeable?

 

I'm now wondering if I may have been going in the wrong direction (or maybe took too big of a step when richening up the mixture). My first step was to go to 135 from 125 on the main jets. I too have tried 60s on the idle jets but am back to 55s for now. I was thnking of going back to OEM settings and taking "baby steps".

GOOD PLAN
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Does anyone know exactly where each of the jets/circuits control? My assumption is that idle mixture screws only affect idle and OVERALL IDLE/TRANSITION CIRCUIT MIXTURE STRENGTH. The idle AND TRANSITION CIRCUIT affect mostly LOW THROTTLE cruising times. Then the main jet takes over ONCE THERE IS ENOUGH AIRFLOW PASSING THE AUX VENTURI TO ACTIVATE THE MAIN CIRCUIT (USUALLY FAST CRUISE WITH MORE THAN LIGHT THROTTLE APPLICATION) the air corrector mainly affects the MAIN CIRCUIT WITH A BIAS TO TOP END MIXTURE STRENGTH. The accel pump/jet only adds fuel during THROTTLE APPLICATION (the harder the APPLICATION DOESNT PUMP FUEL HARDER) THE PUMP JET WILL ALSO BLEED SOME FUEL AT HIGH RPM. The accel pump will add additional fuel during the process of depressing the throttle and ONLY FOR A FRACTION OF seconds after.

JUST TO GIVE SOME IDEA OF SCALE.. A SMALL PUMP JET W/LIGHT SPRING = LOW FLOW/LONG DURATION FUEL SPRAY.. BIG PUMP JET W/HEAVY SPRING=HIGH FLOW/SHORT DURATION SPRAY. DIFF ENGINES LIKE DIFFERENT MODES. THE BLEED WILL DIVERT SOME OF THAT CHARGE BACK TO THE FLOAT CHAMBER. THERE IS A BALANCE TO BE FOUND AND ITS EASY TO COMPLETELY MESS THE METERING. MY GUESS IS THAT OE IS CLOSE AND THAT CAREFUL SELECTION OF IDLE JET, PUMP JET, AND BLEED WILL DO THE TRICK (PROVIDING THE REST OF THE ENGINE IS IN ORDER).

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What I'm still not real sure of is when these jets and circuits operate and transiton. Are these dependant only on the RPM, or the throttle position, or both? BOTH

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I'm starting to think that I I'm too rich during accel and getting the stumble. Like Blaine, it feels good when the car is cold and for the first 10 minutes, but once it is up to temp I get the stumble. WHAT IS THE COLD START/ WARM UP CIRCUIT DOING? Also, my top end is dead. The power really drops off above 5500-6000 (I'm thinking too lean here). CHECK YOUR MAX IGN TIMING ADVANCE.. AN ENGINE OFF TIMING OR OTHER IGN WEAKNESS WILL SHOW SOME OF THE ISSUES YOU HAVE.

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Looking at the OEM jet settings that I scanned above, it seems like the major differences between the different 308s are the "air idle jet or hole" and the "accelerator pump back bleed". Are either of these changeable? NOT SURE.. IM A DCOE/DHLA GUY

IDLE 'AIR' JET WORKS A BIT LIKE THE MAIN CIRCUITS AIR CORRECTOR BUT ON THE IDLE/TRANSITION.

GO BACK TO OE SETTINGS.. CHECK THE REST OF YOUR ENGINE AND START/ WARM UP SYSTEM AND PLAY WITH THE IDLE/IDLE AIR COMBINATIONS AND THE PUMP/PUMP BLEED. CHANGE ONE JET AT A TIME AND TEST AND MAKE SMALL CHANGES.

YOUR TOP END, IF NOT TIMING, TRY THE AIR CORRECTORS FIRST BUT REMEMBER 3 AIR JET SIZES IS ABOUT EQUAL TO ONE MAIN JET SIZE. SO, IF YOU YOU NEED TO ALTER THE TOP END RANGE MORE THAN 3 SIZES YOU NEED TO CONSIDER BALANCING THE MIXTURE DISTRIBUTION WITH THE MAINS.

 

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The main circuit ACs do NOT have any effect on idle.. its the air corrector portion (or F#) in the idle jet they are referring to. You can play with the idle jet and the F# ("idle air" as I call it) to get the idle and transition sorted.. The pump bleed is the area I would explore in your case.

Fuel composition can have an effect of mixture strength, especially if the alcohol content gets above 10%. Also, other non-energy contributing additives in fuel will reduce the mixture strength..

 

OK, what is the proper procedure to tune these carbs? Do we start at getting a good idle, then transition into part throttle/cruise, acceleration, WOT (idle screws, idle jets, accl pump jets, main jets, air corrector jets)? Or, do you work from the reverse?

 

Blaine

The air idle jet is actually a fixed bush and cannot be changed easily in the DCNF. You can measure what you have and make it larger (this would lean the mixture, especially between 2K to 3K under light throttle) by drilling out. To make it smaller you would need to take the carb to a machine shop to remove the existing bush and replace with a smaller inside diameter one.

I believe the US and Australian version 308 had 1.70 and 1.60mm bush ID. I have a euro version 308 carb no.57/58/59/60 and the bush size is 1.50mm therefore making the mixture richer in the 2K to 3K under light throttle.

I also had the light throttle lean condition but this is eliminated now and the way I did this was to completely screw in the air bypass screws. Only one screw needed to be very slightly opened to balance the carbs nicely.

55 idles
1.50mm idle air bush

 

Blaine,

I had an off-idle stumble just like you had. And it was warm up temp related just like yours not noticeable when cold but pronounced when at operation temp. I tried many different sizes of jets, idle and mains, all to no avail. I have a dry sump engine with P6 cams and high compression pistons and thought that the off idle stumble was endemic to the modifications made to the engine. Turns out that they are not. David Fastradio suggested that I take a look at my synchronizaton again. He was right, my carbs were not synched properly despite my following the Birdman procedures you mention. The problem with that procedure is that it does not discuss the concept of play in the linkage. I think that you need to focus on geting some free play in the linkage as my guess is that it is too tight. For years my linkage was too tight meaining that the there was no free play in the linkage at all. There must be free play to allow the throttle return springs on the carbs to to bring each bank back to idle without being affected by the other bank. If the linkage is so tight so that each bank is effectively tied to the other when you let off the throttle you will have problems. The idea is that both banks should pull together when tension is put on the throttle cable but that when that tension is released the banks should return to idle speed stop screws independently of each other. They cannot do this if the linkage is so tight that the return to idle action of one bank affects the other bank. They must return independently of each other or you will have synchonization problems. I think that with the stock engine the off idle stumble caused by poor synchronization is masked by the general robustness of the engine and the cams at low rpms. In my engine with P6 cams there is not much low end and I always thought this was the cause of my stumble. Turns out that the problem was synchronization all along.

Hope this helps.