Carb adustments 101

My car doesn't use Webers but lots of Renault engined guys do. The below is from Tim Engle on basic tuning; I thought it might help. He's responding to a guy having trouble with the idle circuit.

Ken

The idle air correctors and the idle jets work together to produce an
air-fuel emulsion that is then fed into the carb throat. The mixture not
only must be in the proper ratio, it must be available in sufficient volume

to feed the engine up to the rpm at which the main circuit takes over. The

jets and air bleeds can be in balance to produce the correct emulsion ratio,

yet be too small to feed the engine up to the transition point.

So there can be two typical problems in the idle circuit jetting:
a) Too lean or rich. The jet / corrector balance is wrong so the air-fuel

mixture of the emulsion isn't correct. But even if A-F ratio is correct,
there can still be a problem with...

b) Insufficient capacity. Even though they may be balanced, both the
idle jet and the idle air corrector are too small to flow a sufficient
volume of proper-ratio emulsion to feed the engine until the mains take
over.


It's not enough for a kid's teeter-totter to be balanced. It must also be
large enough to be appropriate for the kids playing upon it. And it's not
correct to make the teeter-totter larger by simply adding to one end only.
The idle jets and the idle air correctors must be changed in balance.

The suggestion to try using a smaller idle air corrector would be
appropriate if the idle jet size was already large enough to feed the
engine, but that the emulsion mixture was too lean. Going to a smaller
idle air corrector would admit less air to the emulsion mixture, making it
richer.


The usual set-up drill is to start by selecting an idle jet that's in the
ball park. Use the appropriate rules of thumb or an example that works in
a similar capacity/ power engine. The get the idle air corrector in
balance with the jet so the emulsion air-fuel ratio is correct. Capacity
isn't much of an issue at idle speed, so just about any idle jet will work
if it's properly balanced with an appropriate idle air corrector. To check

that...



1) Make sure the ignition system is properly tuned-up and not contributing
to the perceived problem.

2) With the engine fully warmed up, set the idle speed as low as is
consistent with a smooth, even tick-over.

3) Open the throttle slowly while listening for any off-idle hesitation.
Open the throttle too quickly and the action of the accelerator pump will
mask the results, so move it on the slow side. If the engine hesitates,
the mixture is too lean. Adjust the idle mixture screws for the best
result, but if the idle jets and air correctors are not correct, the
mixture screws won't fix the result. The screws only "trim" the mixture
(like... between jet sizes), they don't set it. The jets do that.

If the idle mixture screws' best setting cannot trim-out the off-idle
hesitation, then the idle air corrector is too lean (too large/ admitting
too much air) for the idle jet that is installed (again... capacity not
being an issue at idle). Install a size or two larger idle air corrector
and try again.

The idea is to start with a too-lean idle air corrector and go richer until
the off-idle hesitation "just" goes away. No richer on the air corrector.

Right now it's probably worth mentioning that...
Richer:
Idle Jet = larger = admits MORE fuel
Air Corrector = smaller = admits LESS air
Leaner:
Idle Jet = smaller = admits LESS fuel
Air Corrector = larger = admits MORE air


4) Then drive-test the car. Under full-throttle acceleration, does the
engine stumble at 3200-4000 rpm? If so, that indicates the idle circuit
is running out of capacity before the main circuit takes over. To address
that, increase the size of the idle jet a step or two and try again.
Proceed in small steps and repeat as necessary. Go just large enough with
the idle jet to eliminate the stumble and no larger.


5) That makes the teeter-totter larger by adding to one end. That's okay
when you're blindly feeling your way toward a solution. But after the
stumble goes away, it's now necessary to go back and re-address idle air
correctors. Change the air corrector back to a leaner until the off-idle
hesitation just returns. Then go back richer until it "just" goes away
again.

6) If it was necessary to make a large change to the idle jet in step 4
above in order to eliminate the stumble; and at the time, that was done
without a balancing change to the air corrector, then the emulsion mixture
could be really off even though the increased circuit capacity seemed to
resolve the 3200+ rpm stumble. If the idle jet change that eliminated the
stumble requires more than one or two size change in the air corrector to
get the off-idle hesitation to "just" disappear again, then the system is
probably still not in perfect balance. Repeat it all again.

7) With the second-try idle air corrector tweaked for the new larger idle
jet, road test the car once more. If the 3200+ rpm stumble is still gone,

then go leaner on the idle jet until it just returns. Then back up to the
next larger idle jet that just eliminates it again. If the idle jet
changes more than a step or two after the idle air corrector was tweaked, a

perfectionist would repeat once again.

Go back and forth like that until changing one (idle jet or air corrector)
produces no more than a corresponding 1 or 2 step in the other to achieve
the correct result at their ends of the range (adjust air corrector at
off-idle / adjust idle jet at the transition speed... 3200+ rpm stumble).
Try to achieve a combination in which neither the idle jet nor the air
corrector is any richer than required to "just" achieve it's target goal.


Done? No.
That just establishes the best base settings. Sort of the perfect-world
laboratory setting. But we don't drive in a lab, carbs are not
computer-perfect with oxygen sensor feedback loops and they are sensitive to

the changing atmospheric pressure. Not to mention, you don't "drive" the
car at idle.

The engine may not hesitate off-idle when you operate the throttle slowly,
but does it feel strong? If you have to nurse it off the line when the
clutch is released, performance may be improved noticeably by making the
idle mixture a tad bit richer. Going one or two sizes richer in the air
corrector may make the engine easier to drive off the line and more pleasant

in traffic.

Similarly, as the barometer changes, you may find the 3200+ rpm stumble
returns. So going one or two steps richer on the idle jet may give the
engine a little more margin for accommodating daily changes.

But don't go overboard with going richer. Overall, doing so doesn't add a

lot of benefit, but it does consume more fuel. There's a 3-way balance
between smoothness, economy and power.

The procedure above establishes a baseline setting that solves the
smoothness requirements of off-idle hesitation and the 3200+ rpm stumble
without treading deeper into the economy range than is required. Going
richer yet may make the engine feel stronger/ flexible getting going, but
beyond that there's not a lot of benefit to "more is better". The real
"Power" end of the spectrum is controlled by the main circuit, so don't try

to address those issues with the idle circuit.

Also, for anything other than slowly opening the throttles, the
accelerator pump's affect cannot be ignored. "Off-idle" performance and
balancing the idle jet for the correct "emulsion" air-fuel ratio (which is
not the same as the engine's overall air-fuel ratio) is the idle air
corrector's rhelm. Beyond that, throttle transistion issues are more
impacted by the accelerator pump. If the engine still has issues there,
deal with the pump settings directly. Don't get duped into messing with
the idle circuit when the accelerator pump is the issue.

Follow the steps above to find the lean but balanced base-line combination
for the idle jet and air corrector, write it down and keep it. Venture
away from that with richer settings by only a few steps, and always
remember to keep the system in balance.

Spark plug condition and color don't always relate exclusively back to carb
jetting. The carbs can be working great and the plugs can still be black.
That may just indicate the plug's heat range is too cold. Try going to a
hotter plug. Black can also mean the engine is burning some oil, and
nothing you do with the carbs or plugs will fix that.

 

You may not like what you find. My 1972 car, when set to factory spec runs like total crap. This is because for emissions, they took a lot of liberaties with timing, mixture, etc. For best running, you likely do not want factory spec, you want it tuned for best running! In addition, modern coils and ignitions for upgraded cars may like different specs than original factory.

Ken

 

I have been also having the headache to balance up my buddies 246 at the moment. I have been told of 3 ways to do it, all are different form the other. I am going to try and attempt the job tonight. I have already stripped the carbs and removed the linkage bar so have no base settings, I am not looking forward to the job at all! There must be a definite way to do them somewhere!
Al Cowan

 

You can adjust the idle bypass screws and idle mixture screws all you want and the car still can run poorly, whats most important and takes skill is the linkages are set up perfectly and all carbs are opening exactly the same, this take some extra skill on the 12s, float height is critical as well and other minor items such as butterflys in relation to the progression hole. You can easily have one carb running off the progression cicuit bypassing the idle circuit if any of these items are off and run around in circles getting nowhere, BTDT as I went through it tuning my BB carbs over a period of time.

 

Birdman, from the 308 section, has a carb sync tutorial on his site (www.birdmanferrari.com) that is noteworthy for a thread like this.

 

now, after reading that you do not have the original carbs, well, this makes sense. the aftermarket weber 40DCNFs are way off in the progression hole department for the 308. On my webers, i like to get the idle down as far as it will go without stalling, then i can really detect any response by turning on the idle mix screws. In your case,with the aftermarket carbs, i would at least check one carb (pull it off the manifold, turn it upside down) and see where the progression holes are in relation to the throttle plate when fully closed. Compare that to a picture of how the 308 weber 40dcnf should look with progressin holes to see if your carbs are at least in the ballpark for the hole positioning and the number of holes.

 

I think what it boils down to is what ever the car likes setting-wise, go with it. Whatever it takes, be it plate position, mixture screws, etc. as long as you're achieving proper idle speed and mixture strength, then leave it. I run my car with the idle-bypass screws closed as my readings with my flowmeter show that the throats next to each other, have always been equal. So I adjust my idle speed solely with the throttle stops. Unless I tap into the manifold and hook up a manometer to measure intake vacuum, and get my exhaust analyzer up-and-running, I'll have to continue to use my Gunson Colourtunes and STE meter to tune the carbs. Three turns out (give or take a 1/4 turn more or less) with my U.S. spec idle mixture screws have worked for me (see results of mixture strength with my Colourtune picture below). I have no idea how many turns out the throttle stops are, as I have only turned them out to set the idle speed. And it doesn't take much to bump the speed up or down!

I recently fooled around with my carbs to solve one problem and caused another. I've changed settings and swapped jets/emulsion tubes to more of Euro spec. It drives much better now and I'm going to the dyno tomorrow and will see what she does.

Before any carb tuning is done - no matter what your procedure - ignition timing must be spot on and functioning properly. If you notice further, most people who have been posting here with running problems have been experiencing ignition issues...

FYI: There are five progression holes of sizes: 0.80, 1.40, 1.30, 1.60, 0.80. Note position/orientation in another one of my photos below (note that this is viewed from the bottom of the carb - upside down basically).
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Paul,
I spent a ton of time on my car battling the same idle problem before I accepted the theory of an air leak. Turns out the base gaskets were bad. Now I think the throttle shafts are leaking, but in the interim it ran pretty well.
Gently spray carb cleaner around the carbs and look for a change in idle.
Cheers,
Tom

 

great thread.

for stock 308 i use 55/125/F36/200.

with the open air cleaners and a better exhaust i use 55/135/F36/190. still working on the air corrector numbers.

 

Paul
I am one that believes that the air bypass screws play a bigger role than just balancing the mated throats in each carb. It seems to me that the throttle plates s/b exactly in the same position in relation to each other when the car is running off idle. So the air bypass can used to help equalize the air flow without using the throttle stop screws. (Now minor throttle stop screw adjustments...1/2 turn preferably less may be necessary). I mentioned this approach a while back but it was kinda ignored..so I'll bring it up again as it may help someone out. I tried to use the conventional method but it would not work on my car. I think if your car is low mileage that approach is more effective but most of the 308's are seasoned veterans and are not up to new car standards i.e. vacuum leaks (old gaskets and especially throttle shaft bearings without seals), unequal cylinder compression, carb dirt, spark plug wires, etc.) I talked to Mike at Pierice Manifold and he told me to do the following: Set all plates completely closed then open 1/2 turn. Close all air bypass then back out 1 turn, air/fuel out 3 1/2 to 4 turns. When you think about this it makes sense. The plates are essentially the same position and the air bypass can both raise and lower air flow. Now there are two different air/fuel mixture screws...the long skinny ones is more exacting and needs more turns out whereas the short stubby one is more abrupt and effect the mixture with less turns so you need adjust them a little differently. Adjustments s/b in small increments 1/8 turns and a breif wait for the motor the acclimate to the change. Without making this post longer I'll just mention that the idle can be adjusted with the air bypass and the air/fuel screws. The throttle linkage s/b disconnected. The passenger side throttle plates are a little harder to get closed and the plate must be held shut as the stop screws are adjusted to assure that the plates are closed (the small contact spring will affect the final position of the plate). (There are also differences in the types of 40 DCNF carbs and some DCNF's have stop screws on all four carbs and some have adjustments between the carbs on the passenger side on each bank.) Any slight changes to throttle stop screws are then done mostly on the driver side since this affects all plates and both banks s/b turned exactly the same once the air flow has been equallized . Well I've said enough but maybe this approach will provide some food for thought.

 

Paul, that is a VERY rich set up you have there. 60 idle jets have give myself and the other 308 guys here fits. we dont use them, even on cars with free flow exhaust and open airboxes. the 135s and the F36 i agree with. the 190 air corrector is something i want to try myself.

i need to make sure my ignition system is 100% before i touch the carbs in any way. seems i need to do plugs, wires and extenders. i would respectfully recommend the 55 idle jets and get the car running right, and then swap in the 60s and see if you need them.

best.

 

Some general remarks on 40 DCNF's and air balance.

There are lots of variations of these carbs. Ferrari used several variations over the years for 308's. They differed in jetting, float levels, and progression holes, both number wise and diameter wise. This is apparently in response to changing tuning requirements, fuel, emissions, etc. The commonly available 40 DCNF 12's (4-bolt covers vs 5-bolt Ferrari carbs) have built in air horns and I believe 3-progression holes. Because there are only 3-progression holes, the transition to the mains might take a little more tuning. They should work fine when tuned properly, but I don't know how well they mate with the standard air box.

Air bypass or balance screws: Some Webers, like the early DCOE's, don't have provision to balance air flow within throats of a carb. If flows weren't equal you either lived with this issue, twisted the throttle shaft slightly, or drilled a bleed hole in the low flowing throttle plate. Later DCOE's added an air balance circuit to remedy this problem. Early Dellorto DHLA's didn't have this feature either, but the later one's did. DCNF's always had this feature as far as I know. Porperly used, it will equalize flows between throats within a carb at idle. Start with both screws seated. Measure the air flows and open the screw on the low flowing throat to achieve balance with the high flowing one. Done. One can't turn down the flow on the high flow throat. Flow diverted through the balance circuit is not seen by the venturi and axuiliary venturi, so the fuel supply system doesn't know about this additional air. This is why you don't want to muck around with them more then you have to to balance flows at idle. The screws are not meant to balance flows between carbs, that's the job of the linkage adjustment. Don't use these screws for any other purpose or you will going off into uncharted and unintended use territory and will likely cause yourself a lot of grief, like jets, emulsion tubes and air correctors that are different than everyone elses. Finally, these air balance tubes really only work at idle and maybe a little past that point. Once you are well into the progression phase, they are no longer effective as their flow is so small relative to the air flow past the throttle plates.

Measuring air flow: The best available mass flow meter commonly available for this application is probably the SK Synchrometer. The BK, it's larger cousine, is probably too insensitive at the low end of the range were you will be setting your balance. I suspect the SK in also marginal in this region, but it and the BK are still far superior to the unisynch and tube to the ear method. The SK has its limitations. It will not detect air seeping in from air leaks like at the base of the carb or past the throttle shaft bearings. It only sees air coming through it. Air flow in a Weber with one throat per cylinder is more of a pulse than a steady state air flow, so its not particularly easy to move the gage from one throat to another and get good comparisons, especially if the idle speed is not rock steady. Off idle synchronization is more difficult to achieve but very important. It is here that many driveablility problems occur. Synchronization at idle almost always changes as soon as the throttles leave their stops. This is because the loading on the throttle linkage changes drastically. The throttle linkage in 308's is quite sloppy and there are a lot of parts to it. The only practical way to synch up the carbs here is to artifically hold the throttles open using the throttle cable or the fast idle mechanism in some fashion. This will load the bank-to-bank linkage rods in compression and the left to right side carb linkages in torsion. This state will remain as long as the throttles are off their stop. Rather than fight it, learn to tune to it. Synchronization off idle will have a great affect on driveablility, especially around town. The smooth application of torque will make launching the car from a stop a lot easier and will require much less care, throttle opening, and clutch slippage.

The best way is a balance a multicarb setup is with a multibank manometer system, measuring manifold vacuum, where one gage is dedicated to each cylinder continuously. A little time spent with a system like this will increase your knowledge as to what is happening in real time and will uncloud a lot of misconceptions and explain why things are happening the way they are. A system such as this will shine a spotlight on what happens when the throttles lift from their stops and distorte due to spring loading. A system such as this will also account for air leakage past the throttle shafts and past base gaskets and allow one to minimize their effects.

Bill