Carb balance question

IMO if there is no obvious sign of leak I would increase idle of that carb.

 

Ken, the most effective method I found to tune my Webers by ear is to turn one idle screw in until that cylinder quits firing. Then back out that idle screw until it starts firing again, and continue back out almost a full turn. Repeat for each cylinder. If there is no noticeable change in idling as you turn an idle screw all way in, you may have found a dead cylinder, hopefully just a fouled plug. Also you may want to read up on leaky throttle shaft bearings.

Good luck with it, Jeff

 

All of those airflow values are much too high for a 1000 RPM warm idle (and indicates that some cylinders are not actually contributing at idle). All cylinders should be in the 3.25~4 Kg/h range depending on the ignition timing that you are using -- more advance (like 7 deg BTDC R1 only) = less airflow needed; less advance (like 3 deg ATDC stock '78 US R1+R2 = more airflow needed).

 

My advice on carb tuning is to be sure everything else is perfect.

Carbs are adjusted last for good reason. Most carb problems are not carb problems at all.

 

I go through this several times a year, explaining to a client about running problems. If I do a complete, thorough tune up and drive the car until running degrades and it needs attention it will not be the carbs, yet they are the one part people fuss with most when there is a problem. To a great degree your friend is correct.

 

As Brian said, you need to make sure that the ignition system is healthy and (even if the carb airflow balance at idle isn't yet correct) confirm that the ignition timing is reasonable at 1000 RPM warm idle for whichever ignition set-up that you have (stock R1/R2, R1 only, aftermarket R1 only equivalent). Then you can use Jeff's suggestion to use the mixture screw on each barrel to confirm/deny that you can close it completely to cause the cylinder to miss, then open the mixture screw slowly until the cylinder starts to function, then open it another fixed amount (although I'd try something closer to +3/8 to +1/2 of a turn as the extra fixed rotation addition). Another method to confirm/deny if a cylinder is contributing at idle is to temporarily unplug the spark plug wire to see if the engine RPM/sound changes (and it should if the cylinder was working) -- but you need to be very careful doing this to avoid getting shocked.

However, since your air flows are already well above where they should be (and unequal), if/when each cylinder is contributing at idle = your idle RPM will be much too high. Birdman has written a good tutorial on how to make the various throttle plate and air bypass screw adjustments -- it's here somewhere...

 

You may want to read the Birdman guide on tuning the 308 webers. I think it covered all you would need to know and how to do it. I did mine after the 1st year I owned it right after I replaced the points and plugs. I would also suggest you go to a hotter plug to prevent fouling during the process. I sold the car 13 years later and never touched the carbs again.

 

I am not sure of the exact sound (through the carb/intake or from the exhaust). You may have a small leak on the header, perhaps where the sniffer tubes enter or around one of the plugs if the sniffers have been removed. Sometimes you can see it by running the car in the dark. Just another thought for the mix.

 

You may want to consider re-jetting as well.

I have just recently re-jetted my 246 by changing the idle jets one size larger and dropping two sizes on the air corrector jets.

Rolling road tests before and after showed positive results.

Snapping and popping out of the carb is an indication of lean condition, make sure all the idle jets are clear

 

+1 on rejetting. There are threads on the correct jets, air correctors and emulsion tubes. The US cars were intentionally tuned to run lean to meet emissions.

 

Hi Dang.

I think you're going in the right direction now. let me see if I can help.

Confirm timing at idle and 5k. Ensure advance is smooth over its range.

Balance using the throttle cable method (off the throttle stops). Set the balance speed in the 1200-1500 rpm range, where it's most important for driveability.

Ease the throttle cable (still of the idle stops-back off if necessary) to give the desired idle rpm, say 1000 rpm. Check and fine tune balance again. When acceptable, nip up the throttle stops to just maintain the linkage geometry.

How are you setting mixture?

Bill

 

Assuming you get rid of all vac air leaks, any timing issues etc and intermittent plug firing and assuming all compressions are good and that all the carb bowls are full to the correct level.
Them you may want to think about debris in the idle jets ie take the screw out counting the number of turns, blow it out and replace the adjustment screw. Repeat on all 8.
Ideally you'd want to clean all the carbs through but you might get lucky.
A piece of debris the size of a grain of salt will cause the issue you describe.
Does the issue stop when you raise the RPM, if it does and assuming you've done all your other checks then idle jet screw is a strong contender.
Regards Bell

 

Diagnosis on the Internet is a very slippery slope but if carbs do give problems here's a quick goto.
Please bear in mind that I'm writing this on a film set between takes and it's off the top of my head.
Carb issues.
First find the carb that's giving issues or just check them all systematically
1. Popping and spitting on idle= poor adjustment of jet screw or blockage or partial blockage
2. Popping and spitting or loss of power in the mid range = blockage or partial blockage of the mid range jets.
3. Popping and spitting or loss of power in the high range=blockage or partial blockage of the main (large) jets.
4. Hesitatation or audible popping at low RPM when throttle is suddenly pressed for acceleration = accelerator pump bladders split or accelerator jets blocked.
Note all accelerator pumps are easy to check by removing the airbox
cover and looking down each choke. As the accelerator is depressed slowly and fully with the fuel pumps on you should be able to see a clear uninterrupted 1mm jet of fuel go straight down the choke.
If the stream is interrupted or broken ie spluttering then your carb accelerator pumps will need servicing.
Please note that you should only depress the throttle no more than 6 times with the engine off before a start is required to clear the unburnt fuel, as this test is done with the engine off.
5. Carb bowls overfilling can be checked quickly by looking down each choke with the engine off and pumps on.
Any fuel entering the chokes in this condition indicates a problem with the float or carb float fuel stop valve.
6. Carb bowls underfilling is harder to check since it involves removal of the top of each of the carbs and then visually check the fuel level.
However with some carbs you can see the fuel level around the immulsion tubes or main jets.
Ok like I say this is a very rough down and dirty quick guide, I hope I haven't left anything out.
It's a big subject but this basic approach with get you started.
Just remember a Ferrari engine is just a single carbed lawn mower X8!!
The trick is to get all the carbs feeding all of the cylinders at the same rate and with the right combustion ie not too rich or worse not too lean.
7. Carb adjustment screws are only responsible for engine mixture at IDLE, they do nothing to the mixture once the throttle is operated.
Throttle open mixtures are controlled by the main jets and mixtures can only be changed by physically changing out the jets.
So twiddling your idle mixture screw does just that, it only adjusts the Co at idle for better low speed running and emissions.
Hope this helps, all the best Bell

 

I found this cleaning an old computer....
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Recall that is for "1977 fuel" however....any thoughts, Bell Bloke (or others) on a change to reconcile to modern blends, with ethanol??

 

Hi BigTex.

Some data points I came across.

A/F Mixture gasoline E10 (10% ethanol)

Stoichiometric 14.7 14.1
Safe full power 12.5 12.0

You need about 4% more E10 for equivalent performance.

If the engine was tuned lean for gasoline, one could make a case for a larger jet size.

Example idle jet, 0.55 increased to 0.57, yields a 7.4% increase in flow area. Probably not a bad approximation for expected mixture results.

Bill

 

You know I run a simple test to see how my cars are running and it works like a dream.
In the absence of a rolling road I just take the car in question on a run down a 10 mile flat piece of road.
1. For the mid range jet test.
First I drive at a constant 40 mph light throttle, at the end of the road there is a pull in. I then pull straight in engine off and pull all the plugs, be careful as they obviously will be hot. Look at them all, photograph them with your phone and note their colour and position. If they look rich or lean then this is the jet you change.
2. For the large main jet test.
I do a higher speed run the other way at say 70-80mph. Then pull straight in again and repeat the process.
Next I do high speed run with lots of hard acceleration, I then pull in again, remove the plugs and note the colour.
Again, if they look rich or lean then these are the jets you change.
These tests will show you how the engine is running in a real world scenario under load better than any colour tune system or similar.
Looking at plug colour is the best way to see how your engine is running and you can make tuning adjustmensts based on that.
Don't forget though this is done only by swapping out the main jets. The idle screw adjustment only adjusts the mixture at adle, for some reason many folks mistake this as an overall mixture adjustment.

My 1981 Aston Martin V8 Vantage was a little bit of a dissapointment when I first bought it, it seemed to feel a bit flat all the way through the rev range.
On checking the plugs I found that the engine was running lean, Bloody hell!!
Why?
Because some penny pinching ape had swapped out all the jets for smaller ones in an effort to save fuel! Unbelievable.
Anyway fortunately being a 'production' engine I got all the correct jets from Aston Martin (yes they were bigger) and the car took off like a scolded cat! Plug colour was restored along with the performance, as for fuel economy? err best not talk about that, but it was par for that car shall we say.
Regards Bell.

 

I wrote a few articles on carb tuning in my Dino blog. The principles and procedures for a 308 are nearly identical so the information contained can be of help to 308 carb owners

Dino 246 Restoration Blog: Carb tuning #1: Weber DCNF carbs and their adjustments explained

Dino 246 Restoration Blog: Carb Tuning #2: Tools needed, pre-tuning prep, & general pre-requisites

Dino 246 Restoration Blog: Carb Tuning #3: Setting idle, synching carbs, & air correction

Dino 246 Restoration Blog: Carb Tuning #4: Setting the idle mixture & understanding the low speed circuit

Dino 246 Restoration Blog: Carb tuning #5: The High Speed Circuit (HSC) Explained

Dino 246 Restoration Blog: Carb Tuning #6: Modern Fuels & Jet Selection When Tuning

 

Excellent post Nuvolari!!