Carb tuning question

That's the accelerator pump. Carburetor - Wikipedia, the free encyclopedia
It shouldn't be temperature dependent.
Does it run poorly below 3000 both cold and hot?

 

I'm Thinking you have plugged holes or air leaks in there somewhere..usually the popping and spitiing is an indication of too lean a mixture..Bidmans tutorial is excellent and if you follow his procedure it is actually quite easy to get the carbs to work correctly..cleaning and changing gaskets and O-rings is the first step and unless you have done that there is no chance that you can properly tune the carbs..

 

Unfortunately, the "nominal" number of turns open for the mixture screws varies on Weber DCNF due to differences in the shape/angle of the mixture screw ends and the thread pitch -- some are 2 turns and some are 4 turns. The usual strategy is to start rich, turn in to lean it out until the cylinder under adjustment starts to miss at warm idle, then turn back out a fixed rotation amount to re-richen (like 1/4~1/2 turn).

 

Idle jets are too small. Please take the time to read the carb tuning posts in the blog at Dino 246?Home and a detailed explanation of why your jets are small are there

I will add that the needle screws are for adjusting mixture at idle only. If you have a running problem at anything over idle the problem is with the jets. Lots of mechanics start turning screws to try and correct running issues and that is the litmus test to show that they have no clue of what they are doing or know how the carb operates.

 

Sounds like it is too lean (that is how they US cars are out of the box) and an air leak. Rejet your carbs and you will feel the difference in how the car runs. There is plenty of information on rejetting else where and I would offer my recommendation but it would result in yet another carb tuning thread/discussion.

Please note, it is not just the jets but the air correctors, emulsion tubes, and jets.

You may want to check that your sniffer tubes have no leaks and if there are no sniffer tubes that the ports are all well plugged. An easy way to test is to run the car in the dark, when it pops you may see the flame. That is how I found my problem, the PO had removed the sniffers and plugged all but one.

 

If I have understood his first post correctly,there's no fuel coming out of the accelerator jets, when the engine is warm. This has absolutely nothing to do with jets or adjustment.
Accelerator pump seems to work o.k., since fuel is coming when cold. But stops, when warm. Seems, that there's not enough fuel in the float chambers after running for a while. Since it's on all carbs I would suspect the fuel pump first. Then needle valves and the small filters on each inlet.

Best Regards from Germany
Martin

 

I like Martin's thoughts there.
Did run well recently?
If the solution remains elusive, I'd think about rebuilding the carbs. And installing sealed shaft bearings. When the fuel supply is correct and the carbs are known clean and adjusted, you'll be driving happy!

 

I think he is consistent. Idle jets control from idle to midrange (3000 or so) but the idle mixture screws don't affect the mixture significantly once off idle. If you look at a schematic of a Weber 40 DCNF it will make sense.

 

Main jets, air correctors, and emulsion tubes control the fuel flow past 3000rpm. Because your problem is below this speed the place to look first are the idle jets. As a general rule I have found 50 idle jets to be too small. I had a serious popping problem that was cured by going to 55 idle jets.

 

Modern fuel blends have pretty much negated the "original selection" of the Factory.

Read up on some current tunings and get the jets and emulsion tubes sized properly...

Or, go find some leaded, no ethanol gas...

It's the ethanol, that is doing heavy damage to any rubber parts of the fuel system.

 

Sounds like you are lean on your idle jets. Also if you have the square tip idle mixture screws 4 turns out will "probably" be better. The lean backfire causes fuel to be pushed out of the bowls. This is why you will see fuel dribbling. I recommend getting a pin gauge to measure the jets. I had some jets that were bigger/ smaller than stamped.


I rebuilt and synced my carbs with 55 low speed jets, 135 main 200 AC F36 emulsion. 48mm float. 3.5psi fuel pressure. Best lean idle + 1/4.

60 jet was good, but a little rich. 55's are good but I can hear a little lean pop on light accel when cruising.

 

I've just serviced my carbs after rebuilding my '78 308. There is what seems to be a temperature controlled idle control mounted on the coolant distributor between the carbs. The bimetallic coil appears to rotate a flat on a circular cam.

I haven't been able to find any information or set-up instructions for this.

Does anyone have any info on this ?
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It's called a "fast idle device" in the owner's manual. In the 20+ years that I have owned my car I have found it to be useless. It may be that my thermo spring is not up to spec but it never seems to rotate to the desired amount or there is drag preventing it from doing so. BTW, I found early on that the choke is useless as well.

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Most carburetor problems are in the distributors.

Is your car otherwise in a perfect state of tune? If not or unknown do that first. There is a reason that when a tune up is performed in order
Adjust valves
Adjust cam timing
Service distributors and time them.
Make all other ignition components work perfectly.
Then and only then do you touch the carbs.


I will never bring in a car for carburetor work. It gets a complete tune up or nothing. I hate wasting time.

 

Hydrocarbons are so high with 50's there was no way to pass smog. It took 55-60.

 

There is additional information about it on pages O4 & O5 of the 308GT4 WSM, but most people disable it (as yours is shown with the adjustment screw fully backed out), and use the acc. pedal to squirt extra fuel and add some extra air to keep the RPMs up at cold start-up (though not to that kind of crazy 3100 RPM level ).

 

Thanks for some great information !

I have the car 11 years. It seems the fast idle never worked.

I certainly couldn't live with a 3100 rpm idle !