Carb Synch

Been trying to synch my carbs with little luck. Upper and lower linkage rods disconnected, idle adstment screws unscrewd. Idle mixture screws 4 turns out the the engine idles at 1300 rpm with front carbs flowing at 1 3/4kg/h rear carbs at 4.0 kg/h.

If I close the rear carb idle mixture screws the flow reduces near 2.0 kg/h and the car idles smoothly at 1100 rpms.


Any ideas? Visually I tried to determine if the throttle valves were closed and it was difficult to determine. I sprayed carb cleaner around joint of intake manifold and head to see if the cleaner was being sucked in by any leakage and there was none detected.

 

I would suggest you go to the birdmans web site and look at his carb sync tutorial. It is very informative and should help you . Once you do it you should never have another problem. I did mine before I saw his process and although I thought I was in sync I decided to follow his outline. It worked well and was not to difficult. You may want to make sure you do it outside. Good luck

 

Steve I have done Birdmans procedure with know luck. Thxs

 

What year and version 308GT4?

Also, when you do get it to idle at 1100 RPM, where is the ignition timing?

 

is this tuning exercise random service to unsatisfactory running, or, have the belts been off just prior?
Have the distributors been on a SUN machine recently for advance, dwell, and phasing???

 

1979 GT4. For some reason the front carbs are making a loud suction noise coming from the two triangular cutouts adjacent to the two start valves. I put my finger tips over the two cut outs and felt alot of suction. The carbs were recently rebuilt. igntion is set statically at 7 degrees btdc. The idle mixture scews are 4 turns from their seated position.

 

check the timing on the low flowing bank.

 

Never a good sign to have markedly different behavior between (supposedly) identical carbs, but I'm not sure where these "triangular cutouts" that you speak of are located. Can you indicate their locations in either of these photos (or provide your own photo)?
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Do you have all the air bypass screws fully closed to start with?

(Assuming that your Synchrometer is accurate), the air flow numbers that you report for the front bank (4 Kg/hr) at ~1000 RPM idle are a lot more "normal" than that rear bank number of less than 2 Kg/hr. One thing you might check (if yours is a US version) is that on the '78-'79 US version 308, there is a large vacuum fitting on the cyl #8 intake manifold that is used to run the airbox flap system. This is a "closed" system so no air should be entering there -- IME, the emission stuff on these cars gets mangled sometimes so make sure that that fitting is either completely blocked or hooked up to a closed system.

That's the right ballpark for a stock F 308 carb, but there are different mixture screws with different tip geometries. Also, the "replacement" 40DCNF-12 carb has these different mixture screws that are more in the 2 turns from seated ballpark (just in case yours have been replaced with 40DCNF-12).

 

I think he's refering to the air inlets for the "choke" (aka "starting circuit"). If he feels air rushing by there, that means the choke/starting valves are opened up, allowing extra air/fuel to go into those cylinders. This would throw off the tuning adjustment completely if that were the case (and the reason why his air flow rates are considerably lower than the rear bank of cylinders - more air is going through those "triangular cutouts" instead of going down the throat of the carb).

md88plt, you need to find out if your choke lever on the center console is set open, or if the cable is jammed, or if the little levers on the front carbs themselves are stuck open.

 

Thankyou guys for the input. Peter you were right about the cold start circuit being engaged. I had the cable engaged fully on the front two carbs when I installed them.

 

I don't know if you noticed this, but there is a thread in the 308/328 discussion about carb cars and using (or in this case, not using) them. As you see for yourself, this can cause problems and so it may be best to simply disconnect the choke cable...