308 Carbs idles fast when hot

Vacuum leaks. Could be either throttle bearings are dry or more likely carb base gaskets, among other causes. Try spraying WD 40 or carb cleaner lightly around the carb bases and listen for a change in idle speed.
Cheers,
TomB.

 

I would suggest that you put a timing light on it and see if the advance is returning at idle. Easy to do and will verify if the advance mech. is not sticking.

 

Ben, when you say it takes a long time to settle down, do you mean to 1000 rpm or to 2500 rpm?

In any case setting up the idle is a bit tricky, (certainly took me a long time even with a color-tune and a CO device), which is why there are so many threads on it. There is a good tutorial from Jwise or Birdman but you've probably read it.

Richard

 

Ben,

Is this high idle a new event?

Are you saying that during the warm up, idle speeds increases for a while and then abruptly returns to about 1000 rpm there after?

Carb 308's do have a fast idle device which will increase idle speed by moving the throttle linkage to a larger throttle opening, until it sees warm/hot water. Then, a cam is rotated by a bimetallic spring to release the throttle linkage for normal operation. This device is housed in an aluminum casting between the heads.

Bill

 

If you run around very much with it running lean you could burn a piston or a valve. The cars were lean to begin with and any air leaks will only exacerbate the problem. And if the ignition advance is sticking, well, you really want to solve this before it snowballs on you.

Be sure the ignition is functioning correctly before you start playing with the carbs. As stated earlier, you can make a quick check of advance and idle speed timing with a light, a dwell meter will tell you points condition, an ohm meter will tell you wires resistance.

But also make a quick check of the following:

Make sure the throttle cable is slack at the adjustment screw and not pulling the carbs open. The car should be running on the idle speed stops not hanging up on the cable.

There are vacuum lines on the manifolds under each carb, you want to make sure they arent leaking.

If you dont have a unisyn, etc., handy to check carb sync, you can make a very accurate check with a length of small hose. Simply place one end in the carb throat, the other end to your ear and listen to the hissing sound. Now move it from carb to carb. The sound level in all 8 throats should be equal. A quick listen from bank to bank and side to side will tell if your synced. Alternatively you can use a medical or mechanics stethoscope the same way after removing the amplifier. If you can tune a guitar by ear, this really shouldnt pose any difficulty. Just be careful around popping carbs you dont hurt your hearing, as the pops can be quite loud.

 

I'm not kidding, the first thing you should check is that the floor mat is not sliding forward and pushing against the gas pedal. I problem I discovered on my 78 308GTS. I ended up using some Velcro to hold the mat in place.

 

Hello from Germany,

I suspect, that on the affected carbs the idle mixture is too lean with the throttle plates too wide open.
My car did this, when I tried to bring up a low idle caused by idle mixture screws not enough turned out, by further opening of the throttle plates.
Firstly idle was o.k., but then it kept hanging at 2500 1/min.

Turn out all idle mixture screws 3 or 4 turns. Depending on whether you have the long point of short point screws.
Then adjust the idle rpms with the help of the throttle stopping screws.

Best Regards

Martin

 

If you carefully spray carb cleaner around the carb bases one at a time the idle will rise if you have a vacuum leak. Be careful not to let the cleaner get near the carb mouths or it will distort the diagnosis. This will also help locate leaks at the throttle bearings. You can accomplish the same thing with WD 40 as it will briefly seal small openings, but I find this harder to use than carb cleaner. Just keep in mind the carb cleaner is as flamable as gasoline.
Cheers,
TomB.

 

the problem of carbs spitting back, is caused by your ignition too retarded. my 308 gtb had this trait, even though it was "correctly" timed. a couple of degrees of advance will stop spitting back. but beware you will advance the whole curve. my car is now fitted with a computer program system which allows me to set the idle timing at 11 degrees up to the maximum advance. via a non linear curve. all the above is based on the assumption the everything else is ok with your engine.