1968 C2500 engine dying

[Grumpy old man]

It's a fuel starvation issue , that crimped fuel line would be the first thing I'd replace along with the fuel filters and checking the lines , It's not electrical if it keeps restarting .

[gmc396]

An Update.

I took off the choke plate. Went out yesterday and at the 2 mile mark engine would die. Got to my destination about 4 miles and idled fine.

Yesterday I got out my vacuum gauge and rest the idle adjustment screws. They are both now set about 1/2 turn out. I replaced vacuum hose to choke pull-off as it seemed a bit loose. This morning is warmer (high 50s) and no problems. So it appears to be temperature related or I solved problem.

I'm going to reinstall choke plate and give another report on the next cool morning.

Thank you for all the ideas.

[HO455]

Just for a bit of context on icing. I drove my 69 GTO with the heat risers blocked off for well over 150k miles and only encountered carburetor icing once. Driving between Pasco WA and Spokane WA in December.

The car had a 160 degree thermostat with two 1/8" holes drilled in it. The hood scoops were removed and I was running a open air cleaner. The outside temperature was well below zero and the engine would only come up to about 140 degrees.

Earlier that day it had snowed and as always for that area the wind was blowing. So the road was about 75% bare with hard packed icy snow spots from drifts. Driving 45 mph the rear wheels would spin when they hit the icy spots. When I lifted the throttle the engine would die from the icing in the carburetor. If I immediately pushed the throttle down the engine would catch and spin the tires until they hit dry pavement. Eventually I got to a place I could pull over and I shoved my coat in front of the radiator and that got the engine to run above 160 and the icing went away.

Long story but, the chances of encountering icing on one of our trucks at normal altitudes are pretty slim. You really have to stack the deck against yourself like I did.

[1967GMCC20]

Originally Posted by Grumpy old man

It's a fuel starvation issue , that crimped fuel line would be the first thing I'd replace along with the fuel filters and checking the lines , It's not electrical if it keeps restarting .

My 56... wagon would hit bumps in road and die periidically.. thought
It was fuel... wrong. Ignition switch was worn beyond life.

And yes it would restart after sitting. Solved with new wire and toggle..been running perfect for 4 years without a hitch. I'm not putting an ignition switch I n as long as I own it.

[Sheepdip]

A bit more on the context of "Icing". A cut and paste from an old Hemmings article.

The conditions required for carburetor icing to occur are moist air with a temperature as low as 13 degrees Fahrenheit and as warm as 55 degrees F. In most instances, once the ambient air temperature gets down in the low teens, the propensity for ice to form is greatly reduced so as to be almost nonexistent. At lower temperatures and humidity levels, there is just not enough water in the air for the problem to manifest itself. The confusing part for many is that a carburetor will suffer the most dramatic icing when the ambient temperature is between 45 and 50 degrees F--well above freezing.

If an engine at idle begins to slow down and run rich, it is likely due to ice forming on the throttle plate(s), effectively making them larger and choking off the air. Once the ice buildup gets large enough, the opening of the throttle plate created by the idle speed screw is negated, and the engine stalls. In very short order (30 seconds to a minute), the ice melts from the underhood heat, and the engine starts and runs fine, as if nothing happened. Usually by then, it's warm enough under the hood for the condition to not recur. The motorist drives off not giving it much consideration, chalking it up to an anomaly--that is until the weather conditions are right again. This event is known in the industry as "idle icing."

Another cut and paste article:

Carb ice can form over a wide range of outside air temperatures and relative humidities. While the word “icing”
typically brings to mind blustery winds and frigid conditions, carb ice can form when outside temperatures are
as high as 100 degrees Fahrenheit with 50 percent relative humidity. At the other end of the spectrum, the risk
doesn’t go away until the humidity falls below roughly 25
percent and/or the outside air temperature drops well
below freezing.
In other words, carb ice can form at pretty much any
time.

Here's a chart I also downloaded to share:

As you can see by the chart here Icing can occur as high as 100 degrees as long as all the humidity conditions are met, this is exactly why carbureted airplanes take off and land with carburetor heat on/applied. Yes I know it's an Aviation Chart but a carburetor is a carburetor and air/humidity is air/humidity.

[Richard2112]

" The confusing part for many is that a carburetor will suffer the most dramatic icing when the ambient temperature is between 45 and 50 degrees F--well above freezing."

That is odd but it seems like exactly what I've been experiencing since the weather has dropped into that range.

" In very short order (30 seconds to a minute), the ice melts from the underhood heat, and the engine starts and runs fine, as if nothing happened"

I've replaced both fuel filters and though that has helped in the acceleration department, when I pull to a stop, the engine dies. Thirty seconds later, starts and runs fine (for a little while).

Possibility of icing hadn't occurred to me. Interesting.

[Steeveedee]

I've been in any number of environments, from -60º in Idaho, to 118º in California, with all kinds of humidity range and never experienced icing. There is no special de-icing equipment on motor vehicles on the ground. I think that aviation is a special case.

[Richard2112]

Originally Posted by Steeveedee

I've been in any number of environments, from -60º in Idaho, to 118º in California, with all kinds of humidity range and never experienced icing. There is no special de-icing equipment on motor vehicles on the ground. I think that aviation is a special case.

Yeah, it's strange I've never had icing problems in the winters before either. Maybe I will get a new fuel pump. My truck will accelerate without problems but once I downshift to slow down and eventually push the clutch in, it just dies. It's like I drained the carb bowl.

[geezer#99]

Originally Posted by Steeveedee

I've been in any number of environments, from -60º in Idaho, to 118º in California, with all kinds of humidity range and never experienced icing. There is no special de-icing equipment on motor vehicles on the ground. I think that aviation is a special case.

I’ve seen it once while living in cold country. Temperature near zero F, driving in a ground blizzard. Pulled over to check rad heat. Had a cover in front of rad behind the grill. It had slipped out of place. Found the carb encased in ice. It started to melt as soon as I fixed the rad cover and idled for a bit.
I set up a heated spacer one time to get some extra heat under the carb for winter driving. Had a blocked off intake crossover. Plumbed the coolant thru it. Helped a lot with quick warm ups.
Used a plate like in this link.


https://jetpumpparts.com/carburetor-...er-heated-nos-

[Richard2112]

I have just resolved an identical problem. After changing both fuel filters, with little improvement, I drained the fuel from the tank. It was murky and didn't resemble gasoline at all.

Upon letting a sample sit in a glass jar, there eventually was a distinct interface between two substances, one being gasoline the other being water with particulate matter. Drained half a tank and went for a spin with fresh fuel. Things are looking much better. Now I can do a tune up.

I have a 32 gallon tank.

[Ironangel]

I'm thinking the timing is a bit retarded. What's the timing set at with the vacuum advance disconnected and port plugged? What's the idle rpm? You have a "Divorced Choke" not a hot air choke. My 396 liked 15 degrees initial and 750 rpm at idle. Thats with a warm motor and the curb idle kicked off...