1979 308 GTB inconsistent idle question

Below is what I wrote in a separate thread on a similar issue. Did the car suddenly started doing it or has it been like that during your ownership? You issue could be due to sticking ignition advance weights, vacuum leaks or the position of the progression holes.

"Regarding the creeping idle, are you sure the progression holes in the carbs are not exposed at idle (thus releasing more fuel into the engine than it should)?
On my carbs when the progression holes were not uncovered by the butterflies the idle was way too low and would slowly die. If I increased the idle with the throttle stops to open the butterflies the holes would instantly be uncovered and the idle would jump very high.
The fix was to close the butterflies and instead adjust idle with the air bypass valves. I have all my air bypass valves two turns open and now it idles rock steady at 1000 rpms."

 

I would suggest that if a stock, carb'd 308 will not idle at 900-1000 rpm (fully warm) with the idle bypass screws closed, then something is amiss. Opening the bypass air bleeds to manage the idle is a bodgey crutch. I would be looking at the thermostatic idle control in the center of the valley and giving it a good clean. Then ensure the carbs linkage is free and synchronization is spot on and that there is just a wee bit of slack in the cross bank linkage to allow all the butterflies to fully close at idle. Oh, and if you are running stock idle jets, you might want to move up a jet size before getting into the full carb setting process.

 

Are you running the R1 and R2 points? If so I would take a look at the micro switch to insure its adjusted properly. If you've got only the R1 points or some kind of electronic ignition that doesn't have the idle retard feature it can be difficult to get the 1K rpm idle. I have been fighting this problem for the better part of a year and have finally resolved it, however I have the carbs completely closed and the air bleeds completely closed. The idle speed screws are set so the carb stops just touch when the carbs are completely closed.

My 78 US has a Pertronix ignition which mandates the initial timing be advanced from 7 degrees ATC to 3 degrees before TDC in order to achieve 34 degrees at 5K. These numbers may vary a little due to inconsistencies in the mechanical advance. The increase in initial timing caused a 900 to 1000 rpm increase at idle with no other changes.

In my case I had a little more advance than I should have, about 39 degrees rather then 34 at 5K which caused the fast idle. The engine is not sensitive to total ignition advance as mine ran great at 39 degrees and may be al little better at 34 degrees but its hard to tell.

The things I did prior to discovering the timing advance include.

Built a smoke machine to check for vacuum leaks. Didn't find any.
Adjusted and synced the carbs to obtain the slowest idle regardless of balance.
Closed all air bleeds.
Put a restrictor in the carbon canister vacuum line.
Changed the throttle shaft bearings to sealed bearings. (probably a waste time and money as the original bearings have a tight fitting washer which seals it).
Reset the butterflies
Changed one throttle shaft as it was binding.

All to no avail. The only way I could get it to idle slow enough was with extra springs.

After adjusting the timing it now idles at 1K with AC off and 950 with AC on with no extra springs.

The way its set now the idle mixture screws are 3 1/2 to 4 turns out with 60 idle jets. 55 are stock.
The idle speed screws are set so the carb stops just touch when the carbs are completely closed.
Main jest are 130. Stock are 125.
Accelerator squirters are one size larger.
The carbs flow between between 3.5 and 4 KG/hr.
Camshaft separation angle is 111 degrees vs 109 stock. Which increased the vacuum at idle slightly and may have contributed to the increase in idle speed.
Camshaft advance is 0 degrees (straight up) as opposed to 2 degrees advanced stock. Don't know if this would affect the idle speed.

Cold start is as follows.
No choke.
Cold idle speed about 850 rpm on cold idle cam.
As it warms up while being driven it will rpms will gradually increase to 1100 to 1200.
When fully hot with original 195 degree thermostat it drops back to 1000 rpm with no AC.

It takes a while, about 10 min driving, for it to cycle off the cold idle cam.

Barry