Synchronizing 40 DCNF Carbs

Pics?

 

Yes, you are correct on the throttle stop screws. They are only on the left side carbs. If you do a search on "308 carb synch tutorial" you'll find , well, a 308 carb synch tutorial. Good tips.

 

I've found that the early 308 carb technical materials (like in the 308GT4 WSM) list the 50mm float height number (which was probably the "official" recommended Weber number), but the later carbed 308 OMs (like 150/78) list the 48mm float height in the carb "setting data" table.

Just guessing, but I believe raising the float height 2mm to the 48mm value was F's (and probably Webers') response to try to reduce the stumbling under hard cornering (i.e., the 50mm value works OK on applications where the float chambers are mounted north-south relative to the chassis, but on 308s, with the float chambers going east-west, raising the level in the fuel bowl helps reduce the stumbling). You can check your OM, but I'd bet the farm that your '79 started life at the 48mm height.

 

I have always set mine to 48mm and they work well (40 DCNF and 44 DCNF).

On your "which screws do what" question, Jonathan (Birdman) has written an excelelnt tutorial with pictures. Search and ye shall find. As a quick guide:
- disconnect the linkages (f-r)
- start with the idle mixture screws all 3 turns out
- the air flow on each barrel should be sync'd to its mate. Close both screws all the way in. Adjust the venturi with the higher flow rate by opening the screw to bleed off some flow. Repeat across all four carbs. You should now have matched pairs, but four different flow rates across the 4 carbs.
- carbs on a common bank should now be matched using the adjustment screws (10 mm IIRC) that connect the throttle linkages (ie., the 1-2 carb to the 3 -4 carb etc). Repeat on the other bank. You should now have the front bank reading consistently and hte rear bank reading consistently but the two banks are unlikely to be matched
- now adjust the throttle stops (the two on the driver's side carbs you are referencing) to sync the front and rear banks.
- 6, connect the throttle linkages and check to ensure the linkages pull the front and rear banks evenly. Two tests. (a) make sure the throttle mechanisms are leaving the stops at the same time. Some of us use a thin piece of plastic or paper between the throttle lever and the rest to make sure they are sync'd; (b) have an assistant operate the loud pedal (or if single handed, adjust the throttle cable linkage on the front cam cover to , have the car idle at 1200 or 1300 rpm. Check flow and adjust throttle linkages to make sure the flow is even and the cable is pulling on all stops t the same time (under (a) above).
Philip

 

Steve, as you know, raising float height to 48 mm will raise the fuel level in the bowls and therefore in the wells thereby enrichening the mixture. On the rationale, I don't know, maybe it was due to the different progression holes in the earlier 308s' carbs. The prior transverse engines (like the 246) had longitudinally mounted carbs (N -S floats). The front mounted 12 cyl had E-W bowl carbs. Assume the Boxer had N-S bowls so Ferrari had a fair amount of experience of different carb settings before the 308.

 

You're very welcome, but if you can't feel this -- I'm slapping you around right now ...

1. You can download a copy of your OM for free as a .pdf file from www.owners.ferrari.com (the Owners Site) once registered there,

and

2. You can access copies of the SPC for your exact model/year/version at the FerrariUK spare parts site (which you access thru the Owners Site "spare parts" links).

 

Yes. You are trying to equalize the airflow through the carbs at idle. Small differences in throttle plate position and bypass screws make big differences in airflow.

Yes.

No. I do not believe the float level affects the idle. Different circuit. Only the main and progression circuits.

Yes.

It can. As you get a better burn (mixture) the cylinder can draw more flow and you have to iterate. usually the adjustments to the flow are limited at this point.

You are trying to make sure the throttle plates leave their stops at the same time, rather than one bank lagging the other which occurs if the linkages are not adjusted well. A small piece of paper (or a piece of playing card can be better, stiffer so it holds its shape) inserted between each stop and the screws enables you to see visually if the throttles are being actuated consistently.

HTH
Philip

 

Hi guys. I haven't posted in a while but I'd like to put my two cents in here.

1. When balancing both throats within in a carb, start with both bypass screws closed. Open the bypass screw on the low flowing carb to increase air flow to this carb. The air bypass screw adds air to the carb throat (it goes around the butterfly valve). You can only increase air flow with the bypass screws, not reduce it, if all bypass valves are closed to start with. To balance air flows, you must match all air flows to the highest flowing throat.

2. My research shows that the only carbs that specify a 50 mm float height are the ones I currently have fitted to my car. They are Weber carb numbers 40 DCOE 45/46/47/48 and were originally fitted to the early US spec GT-4's. A couple of years ago, I came to the conclusion that I needed new carbs for my 78 GTS, while they were available. I replaced my 40 DCOE 75/76/77/78 carbs with a 40 DCOE 45/46/47/48 set, which were the only complete new set I could find. To be truthful, I think it was more like 2x45/47/48 carbs, where I had to change the linkage to one carb to end up with a complete matched set. New carbs are wonderful. When I get the chance, I plan to try the 48 mm float height because I do have a stumble when cornering.

3. I see no reason why float height wouldn't affect idle mixture. All circuits, to the best of my knowledge, draw fuel from the float bowl. If the fluid level is lower, the resulting mixtures will be leaner, for all circuits.

4. I've investigated many methods of synchronizing multiple carbs. IMHO, a bank of 8-mercury manometers (no friction effects), measuring manifold vacuum, on all throats simultaneously, is by FAR the best method. All other methods are second class, by a lot. I measure manifold vacuum by removing the vapor evacuation rails from the intake manifolds, drilling, tapping, and installing brass barbs in the vacant holes, and running flexible hoses to a pair of 4-column motorcycle, mercury, manometers. When synchronizing is complete, I replace the vacuum lines with soft plumping that improves scavanging of fuel vapor from the carbon canister. I have a matched pair of BK Synchrometers that I used originally for balancing purposes. I would put the control flow meter in the high flowing throat and adjust all other throats to match it. Well, the truth of the matter is that adjustment of the flow in one throat, changes the flow in all throats. This is not widely appreciated if you are only looking a one or two throats at a time. Also, while its nice to know the air flow rate (in Kg/Hr), I believe what's really important is are all the throats flowing equally? Looking at all flow rates simultaneously, is the only way to go! If you don't like dealing with MERCURY manometers, there is a Carbtuner II available that replaces the mercury columns with metal slugs that float in a tapered bore. I haven't tried these, but they could work well if one pays attention to their verticle orientation. With mercury, a column height is a column height. One doesn't have to be fussy about the columns being perfectly veritcle.

My method of synchronization is a win win scenario, where the only drawback is replacement of the original vapor scavanging lines with soft plumbing. I use high temperature silicone tubing for this application.

 

OK, but keep pushing to get registered at the Owners Site.

They don't show a '79 OM in the available OM list so use the 150/78 for the '78 US 308 (I believe the '78 and '79 US 308 are virtually identical). You won't find an SPC in English (at least my '78 SPC was also Italiano only). You've now got about everything that's available documentation-wise (OM, SPC, WSM) -- Haynes does make a Weber manual that has a chapter on DCNF that can be a good read to blend with the carb info given in the OM and WSM.

 

Bill, I do not think this is correct. Here's why. The air for the idle circuit is introduced by the GAM air bleed. The air for the main circuits is controlled by the emulsion tube and the air correction jet. Fuel height in the bowl determines the oxygenation of the fuel in the wells and therefore the main circuit. It does not determine oxygenation for the idle circuit

Agree did the same. Not much help for me though. The cams give me 2 or 3 inches of vacuum at idle and the pulses cause a fair amount of movement. 15 to 20 inches of vacuum is a lot more precise.

 

Chuck
It is a feeling of accomplishment that goes with the tired back muscles, watering eyes and rough hands from fuel and being burned by hot water pipes. All worth it in the end. It is not difficult, just takes patience.
Good luck. Take your time.
Philip

 

No, haven't met Mr. Novak.

 

Phil,

Thanks for your comments.

Quote:
Originally Posted by bill308
3. I see no reason why float height wouldn't affect idle mixture. All circuits, to the best of my knowledge, draw fuel from the float bowl. If the fluid level is lower, the resulting mixtures will be leaner, for all circuits.


Bill, I do not think this is correct. Here's why. The air for the idle circuit is introduced by the GAM air bleed. The air for the main circuits is controlled by the emulsion tube and the air correction jet. Fuel height in the bowl determines the oxygenation of the fuel in the wells and therefore the main circuit. It does not determine oxygenation for the idle circuit

My response:

The idle circuit is really no different than any of the other circuits. Fuel is drawn from the bowl and lifted (sucked) up a channel to the idle jet. The idle jet restricts gas flow (volume). Immediately down stream is the idle jet air bleed which meters the air to be mixed with the liquid gas to create an emulsified air/gas mixture. The volume of this emulsified mixture is controlled by the mixture needle valve when the butterfly valves are on their stops and mixes with air that slips past the butterfly valves. How does the gas level affect the mixture? Well, a higher level means that for a given vacuum, less vacuum is required to lift the liquid more available to force the liquid through the idle jet. This results in slightly more liquid gas being mixed with air from the idle air bleed so the resulting emulsified mixure is richer.

Quote:

4. I've investigated many methods of synchronizing multiple carbs. IMHO, a bank of 8-mercury manometers (no friction effects), measuring manifold vacuum, on all throats simultaneously, is by FAR the best method. My method of synchronization is a win win scenario, where the only drawback is replacement of the original vapor scavanging lines with soft plumbing. I use high temperature silicone tubing for this application.


Agree did the same. Not much help for me though. The cams give me 2 or 3 inches of vacuum at idle and the pulses cause a fair amount of movement. 15 to 20 inches of vacuum is a lot more precise.

My response:

Wow, 2-3 inches (mercury?) of vacuum is not much, but it makes no difference to the manometer system I recommend. One would just equalize the vacuum for all throats at the 2-3 inch range. No increase in sensitivity is necessary.

The pulses are a little different matter, but easily controlled. The key is to privide an inline restriction to dampen the vacuum signal. The greater the restriction, the more the damping. The restriction is typically placed between the vacuum source and the mercury column. The manometer system I use actually came with restrictors for this purpose. In use, my system has some pulsing, but it is acceptable. For more damping, I would fabricate restrictors with smaller holes. The important thing is to ensure all signal lines use the same internal diameter restrictor. When all mercury columns are next to each other, the eye has little difficulty in comparing the response of one signal with the other seven.

Well, I tried to upload before and after photos of the synchronization but failed. I'll try again later.

 

Hey Bill
Good thoughts. I've used restrictors in the lines. Still hard when vacuum is 2 - 3 inches -- the P6s produce so little vacuum that they are a challenge to sync.
Philip

 

Just to be clear, when you say "timing to 7 degrees" do you mean that both distriibutors are set to fire at ~7 deg BTDC when the crankshaft is at 1000 RPM for their respective bank?

Also, have you confirmed (by maybe pulling off the spark plug leads one at a time) that all 8 cylinders are actually contributing something when you've got it down at the 1000 RPM idle? If it isn't an advance mechanism problem, perhaps once it gets to a slightly higher RPM some (dead) cylinders come to life -- just a thought...

 

Can't comment on dwell, don't use points. Dwell not total advance, right?

Your commentary suggests it is more timing than fuel. That said, in my experience, when the motor wants to "hang" at 1500 - 2000 rpm on the overrun, the revs fall until the motor hits preferred A/F mixture when it tends to stabilize. A slow return to "normal" idle, seems, in my experience, to correlate with a rich mixture. (A "sticking" advance mechanism could produce the same result). So:
- Run the car. Put the timing light on it. Confirm what you are seeing.
- Turn the mixture screws in 1/2 turn at a time. Does the motor gain RPM or lose RPM (give it 10 - 20 seconds to respond each time). If it gains, turn mixture screws in again, 1/2 turn until it slows, then back out 1/2 turn. If it slows, take the screw out 1/2 turn. If it gains again, another 1/2 turn until it slows. Repeat on all. When done, your mixtures will be right. If you still have the problem it is a timing issue.

HTH
Philip
P.S., sometimes an electronic tach is useful as it can be hard to detect rises and falls in engine RPM. Also, do all this after car has been run 5 - 10 miles to get it good and warm. Not just idled in the driveway.

 

Philip.... You're correct about the standoff with P6s (or any high overlap/ multi-carb setup) making carb synching difficult, even with valved manometer lines (cuz there's no way to know THEY are even). The only satisfactory solution I ever found is a totally PITA multi-step process.... retime cams for minimal standoff... balance airflows (making precise note of turns to establish baseline).... reset the cams.... go from there.

An obvious comment on the fast idle posts. Make sure the carb linkages, cable, and pedal assy are lubed and not sticky. Will your idle drop to where it should be if you manually pull the linkage to it's stop?