Carb gasping

[pasvorto1]

I have a 78 L48. As far as I know, it is bone stock.

When it is cold (under 200 degrees), I really have to feather the carb when I leave from a stop. If not, it will gasp and die. Embarassing, to say the least. However, once it hits 200 degrees, it takes off nice and smooth.

The guy I bought it from didn't drive it much in the few years he had it and the guy he bought it from had it stored for many years. I am thinking that it is 'gummed' up.

What, if any, value is there to pouring a can carb cleaner in the gas or running some gumout through the carb? Or, heaven forbid, does it need to come off and get gone over by a pro? I've never 'rebuilt' a carb and don't feel real comfortable doing it.

Or, is it that these cars are just 'cold blooded'?

 

[titan81]

i'd assume you have a lot of build up in there.

 

[tomtom72]

IMHO it sounds like the choke is not set up correctly, the car runs fine after warm-up. I would think if the carb was contaminated from varnished fuel it would not run well even after warm-up. I would say with the history of little use you relate that the previous owners were aware of what happens to improperly treated fuel during long storage. I would think a shop manual would help in figuring out how to check & adjust the choke.

 

[78SilvAnniv]

I agree that it sounds like a choke problem.

My own is having that type of trouble right now and we are looking into perhaps replacing the choke mechanism or converting it to an electric choke. Either way, we need to fix this before winter cold weather sets in.
We recently had the carb rebuilt by a professional in Texas and were very please with his work.
Heidi

 

[MMM]

Check two things:
First the choke system including the choke pull-off. Choke operation can only be checked with the engine cold. Let the car sit overnight. Before you start it in the morning, remove the air cleaner and pull on the throttle linkage to open the primary throttle plates. You should see the choke plate SNAP closed. If this works, then replace the air cleaner and hook up any vacuum lines you may have disconnected to the the air cleaner off. Next, start the car and see how far the choke opens. This is the choke pull-off function. The choke plate fully (or nearly fully) closes for cold start up to get as much gas into the engine as possible for a quick start. But this setting is TOO rich for the engine to run correctly, even cold. So as soon as the engine starts and builds vacuum (almost instantly) the pull off opens the choke plate to a position that allows the engine to run properly. The settings for the initial choke and pull off settings are in the factory and even the Hanyes Service Manuals. If the choke pull off is not working then the engine will run too rich. However, in your case it sounds as if the engine is running too lean. So my guess is the choke is either not closing or the pull off is incorrectly adjusted to open the plate too far. The bi-metallic strip in the choke housing controls the rate the choke opens after the engine starts and begins to generate heat. '78's use an integral choke system vs. the divorced system used on pre-'75 models. Unless the heat cross over passage in the intake manifold is plugged, these systems are easy to fix. I base all of the above info on the assumption the original Q-Jet is still installed.
Mike
Mike

 

[MMM]

please excuse the typo's in the first message and I forgot to add the second item. Check your initial timing. I have seen a situation where retarded timing caused identical symptoms. If it checks ok, try advancing the timing by a couple of degrees and see if that helps.
Mike

 

[pasvorto1]

The replies are all great, folks. Looks like I've got some work to do.

 

[black_81_vette]

I would put bets on the choke thermostat is dead

 

[The73vetteman]

pasvorto1,

I have a '73 L48 with EXACTLY the same issue, although the carb is a Holley. I know my choke is working, and I have it adjusted to what seems to be the best possible position, but it still struggles to pull away when cold without a lot of clutch feathering.

I think on mine part of the problem is that the fast idle mechanism doesn't work - it seems when the previous owner mounted the Holley he couldn't get the fuel line in without removing the fast idle mechanism. I have also wondered if this is something to do with the Power Valve that I hear people talk about. I've had quite a bit of spitting back thru the carb when it's cold.

Anyone have any further suggestions?
John

 

[Montana]

You might pull your plugs to see if they are fouled. If you find just one or two with black soot on them then check your timing. Look up what it should be for your engine because they are not all exactly the same. Black soot is an indication of a "rich" mixture, if you find it on just one or two plugs it means the spark timing is off and its like your running without that cylinder which could make it hard to get off the line. Just a couple of cents........