help setting up carbs after rebuild

Hi everyone,


I've just completed the rebuild of my '76 GT4 engine, it had done over 220000 km since new! I was expecting massive wear, but apart from the cylinder sleeves and small end bearings, everything was in great shape. Clearly, these are very robust engines if looked after well!


So, the engine went in two weeks ago with the help of our daughter and, guess what: started up on the first crank!
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Since then, I have been fighting getting the carbs tuned. They were sonicated and rebuilt and do not leak, no extra play, and stock nozzles.


I have done this before several times without too much trouble on our ex-Fiat Dino, but something is really off here. I have two color tune plugs and added O2 sensor ports on each side of the pipes going into the muffler.


To idle at all, the engine needs at least 2 turns of the idle set screws, at which point it is pulling 12 Kg/hr of air mass. The idle mixture screw has no effect and I conclude that it is running off of the progression holes.


To bring the throttle plates back to the point at which the first progression hole is blocked, I need to open the air bypass screws by two turns. Now, it's idling ok with 4.5 Kg/hr, but it looks like the mixture is very lean: lambda > 1.2 most of the time and the colortune plugs are pale bluish. If I close the air bypass, then I see the cylinder no longer firing. Again, the idle mixture screws have no or little effect (ie no way to make it go rich). The throttle plates are practically closed, so I'm sure that the progression holes are not uncovered. Interestingly, I can somewhat control the richness of the mixture with the air bypass screw, but I think that this may be a fluke from the cylinder not firing all the time and accumulating some fuel to give the impression that, when it does fire, it is rich.


I should add that the engine has the Wössner 10.5:1 compression pistons with 1st oversize bore sleeves, and that I tried going to the 0.5 idle jets as suggested by Birdman. This may have helped a little, but it's not huge. I've gotten to the point where I don't get any sneezing from the carbs, and it idles ok at 950-1000 rpm.


So, any suggestions or support are sorely needed! I have been spending too much time behind and under the car instead of behind the wheel…

Thanks!!

 

Forgot to mention that camshafts are stock and ignition is set to 10° BTDC using a converted Lumenition and Bosch unit!
(See Adrian's great setup here: https://www.ferrarichat.com/forum/threads/bosch-ignition-for-s159-double-distributor-setups.453748/)

 

4.5 Kg/hr/barrel at 10 deg BTDC for 1000 RPM idle is (still) too high. The throttle plates should be nearly closed, but the airflow should be closer to something like 3 Kg/hr/barrel.

One thing you might try (although it is a little dangerous), is, at your present warm idle (using a well-insulated glove), unplug the spark plug extenders one at a time (so the spark jumps to the metal spark plug tube and not the spark plug end) and note if the engine running changes, or not. Can you you an indication (amount of RPM drop and audible sound) if all cylinders are contributing equally at idle (or if some are not contributing or are contributing more weakly than others).

PS Update your Profile! (and please include year and version)

 

I can see that the front bank is contributing more than the rear one by disconnecting each timing sensor separately but, for the individual cylinders, it's not so clear to me. I did get a few sparks through the glove but will give it another try tonight I still don't see how to get a lower air volume without having the air bypass screws open.

Thank you for the tip: I updated the profile!

 

It's certainly goodness that you've confirmed that both banks are contributing something (and would be more goodness if you can confirm that every cylinder is contributing something). However, the unequalness between banks is a little concerning -- I assume that you've measured that both banks are at 10 deg BTDC at warm idle (i.e., cylinder #1 and cylinder #5 have the same idle ignition timing). Otherwise, something in the cams could cause a bank-to-bank difference.

I don't really follow your point about the air bypass screws needing to be open. The Synchrometer measures the air flowing thru both the throttle plate and the air bypass screw (so it's always the total airflow entering the cylinder regardless of the path). At least one air bypass screw on each carb should be fully closed (the other is only opened, if necessary, to equalize the airflow of the two barrels). I.e., if you close both air bypass screw more, you should be able to open the throttle plate more and be at the same total airflow.

 

I checked each cylinder by pulling the cable way out away from the plug (yikes those sparks are strong!): Cylinders 1 and 4 contribute more than the rest (rpm drop from 1050 to 950). For the rest, it doesn't make much difference. Now, both banks are contributing about the same and engine continues to turn for a bit once the timing plug is disconnected.

For the advance, I have checked both banks (they now run on a single distributor with two pickups). Timing and advance are identical for both.

Thank you for the info on the bypass screw: I didn't realise that it was connected to the plenum and was actually wondering where the air came from. The issue is that if I close the air bypass screws all the way, then I need to open the throttle plates so much that the progression holes are exposed.

I was wondering if the fact that the engine still was fresh from the rebuild would have anything to do with it: does it need to run for a few miles? At present, it's done about 30 Km.

 

Plenty of carb 308 out there running well without have to massively open the air bypass screws (i.e., with at least one air bypass screw fully closed on each carb as it should be). Approximately, how many turns open do you have the idle mixture screws? IME, they should be roughly about 4 turns open.

No, that shouldn't make any difference.

 

The idle mixture screws are of the early, stubby type and I opened them out four full turns. However, it doesn't seem to make any difference (on AFR, colortune plug, or idle rpm) wether they are screwed in or out.

I completely agree that the webers should not be running using the air bypass screws, that is why after two weeks of fiddling, reading weber manuals and forums, I'm reaching out for help

Is it possible that fuel is making its way down the air bypass passage? With the throttle plates covering the progression holes, I don't see where it's getting the fuel...

Thank you for your time and thoughts!

 

Have you double checked your floats, needle valves, and fuel pressure? I wonder if the float bowls are possibly overfilling and causing you to need to add more air and making it not respond to the mixture screws.


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The fuel for idling just dribbles out of the orifice ("28 calibrated bushes" beneath the throttle plate) that the mixture screw's pointed end restricts -- see Fig 12.2 on page 161 of the Haynes Weber Manual. I think you could remove the mixture screw and confirm the orifice is not blocked with a fine straight wire. The same figure shows that there isn't really a way for liquid fuel to pass thru the air bypass screw path.

Caleb raises a good point that liquid fuel should not be dribbling out of the center Auxiliary Venturis (nor Acc. Pump Nozzles) at idle.

 

Hi, great info, thanks! I did check the floats and set them precisely to 48mm as per the OM. I also checked that the idle mixture orifice is not blocked by squirting solvent through it.

The pump is a Pierburg pump which seems to be working fine. I did not measure the pressure but would think that the hose returning the excess fuel to the right tank would prevent too much pressure buildup. Could this be a problem? I do hear the fuel circulating, but maybe I should check that the return hose is not partially blocked. What pressure should I be looking for at the entrance to the carbs? Wouldn't this make the mixture go rich as I close off the air?

I found Fig 13.2 in my Hayne's "Weber, Zenith, Stromberg, etc carb." that shows the idling and progression phase. I have been studying it for some time, which is what led me to try to make it work with the throttle plates covering the 1st progression hole. In the Weber Tuning Manual, they discuss the issue of when the carb is undersized for the air intake of the engine at idle. To fix this, they say to try opening the air bypass screws or, as a last resort, drill a small hole in the throttle plate. Since I haven't read of anyone needing to do this on their 308, I'm thinking that this is not the right solution.

 

Sounds like your floats are spot on. Fuel pressure should be somewhere between 3-5 psi depending on where you read it. I believe Weber themselves recommend 3.5. I know the original Corona pump has an internal regulator in it. I’m not sure if the Pierburg does, but I know a lot of people here use them. As long as your not at too high of a pressure to push open the float needle valves you should be ok. Just check to make sure no fuel is dribbling down the carb throats at idle. If it is, that extra fuel could be why you need to have so much air flow needed to keep it running. It seems like you are on the right track, just something small for sure that is keeping the tune from being perfect.


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Can I see this by opening the throttle and looking down the throat without removing the carburettor? Also, would the fuel be dribbling all the time that the pump is on?

 

I'm not familiar with that one, but here attached is a pdf of Chapter 13 from the Haynes Weber Carburetors Owners Workshop Manual covering Weber DCNF. (Whoever made this pdf file for me added some good reference photos at the end -- I just scanned the individual pages.)

 

Good for her! I hope my daughter helps to work on my Ferrari some day.

 

Thanks!! I also have the Haynes Weber Carburetor Manual from 1995 that has the same diagram. The pictures at the end are exactly like what I have, so I think that the set-up is stock with regards to the progression holes.

Before going to work this morning, I checked to see if fuel was dribbling down when the pump was on. Appart from the accelerator pump, I did not see anything dribbling down the idle or progression holes. I will check again tonight with the boroscope because it's not so easy to see past the choke and venturi tubes. Too bad, I was hoping that it would be this!

I also disconnected the return hose from the last carb to the gas tank and blew through it to see that it was not clogged.

Still stumped...

 

So, after getting to work smelling like fuel, I spent my lunch trawling through the forum posts. Amazing what you can still find after two weeks by alternating search terms!

For others and future reference, here is a particularly great thread on the progression holes and their adjustments:

https://www.ferrarichat.com/forum/threads/dcnf-progression-tuning.423336/

At present (may change again in a few minutes), I am leaning towards air leaking somewhere: If the condition was as in C below, then the manual says that it should be running rich, whereas it is way too lean. So, I need to open the throttle valves to get more fuel in. What this doesn't explain is why it works if I open the air bypass screws. Perhaps because it reduces air pulled through the progression holes or calibrated bushes at idle? But then the idle mixture screw should do something...

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So, while waiting for a better idea, I'm planning to pull one of the carbs off again, double-check that it is flat (I don't really remember doing this diligently), and re-install it with a light touch of non-hardening sealant resin. I'll also check that no fuel dribbles down when it is connected to the fuel pump as suggested by cbmc.

 

Ok, so the good news is that I can remove all four carbs, linkages, etc in 30' despite those pesky 13mm nuts. That should help bump me out of my rookie status.

The mixed result is that the carbs decked flat, except two that had a minor ding that came right out with 500 grit. I also tried tuning again with the air bypass screws closed, and it definitively idles very rich pulling a lot of air (10 Kg/h at 850 rpm, the farthest closed I could get the throttle plates). So, definitively case C above. No way am I drilling anything, so I will try again using the air bypass screws but being more careful to see if there is a sweet spot where the idle mixture screw can be used to adjust the AFR.

 

When you had your carbs rebuilt did they replace the throttle shaft bearings? If not, they may be leaking air.

The old beatings were not sealed so on rebuild, one would pack them with grease. Eventually the grease would get compromised and a leak could develop.

A sealed bearing is now available which solves the problem.



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I recently rebuilt my carbs, during disassembly I recorded all the screw settings and jet sizes. My air bypass screws where either 0 or 1 turn out with one outlier 1.5 turns out. Idle mixture screws were from 1.5 to 4.25 turns out with no apparent correlation to air bypass screw settings. A mixed bag to be sure. Then again they hadn't been rebuilt during my 20+ years of ownership and by my observation the PO's "mechanic" was a hack. He even had the intake manifolds reversed with the brake vacuum nipple on the cyl. 8 position, he just extended the hose over to the other side.

There are lots of places for air leakage to occur. One leak or several leaks can really throw things out of "normal" settings ranges in an attempt to compensate for the extra air, that is probably why my settings were so wild.

 

Hmmm, I didn't go as far as removing the spindles from the carbs: I took everything else apart and sonicated the pieces separately and then put them back together with new gaskets. The spindles rotated nicely and had no play, so I didn't want to risk damaging the throttle plates.

You both raise a good point about leaks. I guess that the throttle shaft is now next on the list of suspects...

I've also been wondering if the increase in compression ratio from 8.8:1 to 10.5:1 could have something to do with it?

In the Weber tuning manual, they discuss the effects on increased CR on the timing, but do not say anything about Webers! I calculate that the difference in aspirated air is minimal, but there is a difference in scavenged exhaust gasses. The manual states that this means that the mixture can be slightly leaner, especially at idle. They also mention that this leads to adjustments in the jetting, but do not go further. Does anyone have any experience with jetting on a slightly higher compression engine?

 

Regarding the throttle shaft leakage, the leak would be between the ball bearing balls. That is why they are packed in grease.

My engine was rebuilt with a 10.5 compression ratio and I did not notice any major difference in the number of turns of the mixture screws. Mine are out from 3 1/2 to 4 turns. Flow rate is 3.0 at 950 RPM at 10 degrees of timing. Idle jet is 57.

Another potential leak is having a missing ball used to plug the drill holes.




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Thank you, that is really useful information! So, it is really down to something wrong in the setup. I need to backtrack and start over checking ignition and camshaft timing.

 

If the carb bodies with throttle spindles in place were cleaned in an ultrasonic cleaner would that have not removed the grease from the bearings?


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Yes, I would think so, too.

So, today I re-checked the timing (spot on for both 1 and 5 at 10° BTDC). The flywheel is so clean that I can even read PM 1-4 and PM 5-8 Each bank is getting the same number of sparks, so all cylinders are getting sparks.

I checked the one camshaft that you can see through the oil fill, whose timing is also spot-on. I remember taking the time to get all of the camshafts exactly right, so hopefully they did not move.

To be sure, the ones stamped with an "A" (amissione) are on the intake, and those stamped "S" (scarico) on the exhaust, right?

So, tonight the carbs came off again (second time in 24h). I am quite good at it now. I checked and packed the throttle shaft bearings with grease (they were indeed dry). Please tell me that white lithium grease is ok and there's not an NLA Weber special grease made from baby blue whale blubber.

Can anyone think of what else I should check on the carbs while I have them off? I am wary of taking them off and on...