C3 runs poorly under operating temp
Thread Starter
4th Gear
Joined: Mar 2022
Posts: 4
Likes: 0
C3 runs poorly under operating temp
Hi,
I am currently trying to understand why my 1974 corvette small block runs poorly when it gets to operating temperature. I have tried for some time now to find the problem on my own but have failed to do so. When starting to reach operating temp the car will start to run worse and worse, after about 20 minutes of normal driving the car will not run at all and will not fire up again until cold.
I think I ruled out the chances of it being related to electrical components/timing/egnition since the car starts up again when using starter fluid which from my knowledge tell me it has to so with fuel being lacking source.
Now more to the question I suppose, what could cause this lack of fuel? And if you have any other suggestions about what it could be I am very open to suggestions.
noted should be that the fuel filter has been replaced along side with egnition wiring, spark plugs, air filter. All the easy simple stuff. I have thought about vapor lock and read about that this could be an issue from heat soaked lines? Could it be that the carb is bad? (rochester quadrajet) the carb jets have all been cleaned out.
I am very thankful for all help that you can give and if you have had this issue please tell your story. Thanks!
I am currently trying to understand why my 1974 corvette small block runs poorly when it gets to operating temperature. I have tried for some time now to find the problem on my own but have failed to do so. When starting to reach operating temp the car will start to run worse and worse, after about 20 minutes of normal driving the car will not run at all and will not fire up again until cold.
I think I ruled out the chances of it being related to electrical components/timing/egnition since the car starts up again when using starter fluid which from my knowledge tell me it has to so with fuel being lacking source.
Now more to the question I suppose, what could cause this lack of fuel? And if you have any other suggestions about what it could be I am very open to suggestions.
noted should be that the fuel filter has been replaced along side with egnition wiring, spark plugs, air filter. All the easy simple stuff. I have thought about vapor lock and read about that this could be an issue from heat soaked lines? Could it be that the carb is bad? (rochester quadrajet) the carb jets have all been cleaned out.
I am very thankful for all help that you can give and if you have had this issue please tell your story. Thanks!
Melting Slicks
Joined: May 2021
Posts: 2,274
Likes: 1,321
From: Fleming Island, FL
I have a Honda powered pressure washer that sits for a good part of the year. In spite of using ethanol free gas in it won't run with less than half choke for the first 15 minutes of operation. Here's a trick for purging any garbage in the carburetor. Rev the engine up and then slam the choke close. Release the choke right before the engine is going to stall. Repeat this as many times as needed.
Le Mans Master
Joined: Jun 2019
Posts: 7,658
Likes: 2,470
From: Indiana
Well… here is a list until finally solved that may give you some ideas..same condition
https://www.corvetteforum.com/forums...e-on-road.html
Assume you have a return line and stock..post a few pics..start at fuel measurements.
https://www.corvetteforum.com/forums...e-on-road.html
Assume you have a return line and stock..post a few pics..start at fuel measurements.
Thread Starter
4th Gear
Joined: Mar 2022
Posts: 4
Likes: 0
Well… here is a list until finally solved that may give you some ideas..same condition
https://www.corvetteforum.com/forums...e-on-road.html
Assume you have a return line and stock..post a few pics..start at fuel measurements.
https://www.corvetteforum.com/forums...e-on-road.html
Assume you have a return line and stock..post a few pics..start at fuel measurements.
If anyone knows the measurements for fuel at crank for X amounts of seconds please post otherwise I will just look around or compare it before and after operating temp.
Melting Slicks
Joined: Nov 2000
Posts: 2,507
Likes: 114
From: St. Louis Mo
Bought my 74 in PA in 2013, Drove it home to St, Louis and I had the same problem as you, Would not run after driving for awhile, This happened several times then we finally changed the coil and it ran perfectly the next few hundred miles to home.
Thread Starter
4th Gear
Joined: Mar 2022
Posts: 4
Likes: 0
As i stated in the initial post first I wanted to rule out egnition but I guess i cannot fully, I read about egnition coils for a bit and apparently as the copper gets warmer the resistance in the copper increases a lot as well. Could it be that since the starter fluid is more easily burnt that less spark is needed for it to run?
Thanks for the input, anything that might help is very appreciated.
Le Mans Master
Joined: Jun 2019
Posts: 7,658
Likes: 2,470
From: Indiana
Thanks, yeah I looked through the posts made in that thread and it gave me some good ideas along side with what you said about measuring fuel. I will start there since it is the easiest and go for the more demanding projects after.
If anyone knows the measurements for fuel at crank for X amounts of seconds please post otherwise I will just look around or compare it before and after operating temp.
If anyone knows the measurements for fuel at crank for X amounts of seconds please post otherwise I will just look around or compare it before and after operating temp.
spec one pint in 30 seconds..old pump 30 seconds maybe half pint
new pump 2 pints in 30 seconds!
old pump cranking gets to 7 psi
new pump cranking 8.5 psi.
spec with flowtest 7-8.5 in 79 anyway..must pass both tests
i would see if accelerator pump shoots fuel in carb to see if out of gas not starting fluid.
like the video in post 11
Last edited by interpon; Mar 26, 2022 at 10:25 PM.
Race Director
Joined: Mar 2017
Posts: 11,953
Likes: 4,510
From: Virginia
First, can you take off the air cleaner assembly, and post some photos of the carb, dist, and engine, from all sides? You'd be surprised what members of this Forum will spot.
Do you have a true dual exhaust (no crossover), and can tell if your heat riser is opening? If not, exhaust leaving the right-side exhaust will be greatly reduced, forced through the intake manifold, and can cause overheating of the carb, vapor lock, and other fuel issues.
Do you have a true dual exhaust (no crossover), and can tell if your heat riser is opening? If not, exhaust leaving the right-side exhaust will be greatly reduced, forced through the intake manifold, and can cause overheating of the carb, vapor lock, and other fuel issues.
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Thread Starter
4th Gear
Joined: Mar 2022
Posts: 4
Likes: 0
First, can you take off the air cleaner assembly, and post some photos of the carb, dist, and engine, from all sides? You'd be surprised what members of this Forum will spot.
Do you have a true dual exhaust (no crossover), and can tell if your heat riser is opening? If not, exhaust leaving the right-side exhaust will be greatly reduced, forced through the intake manifold, and can cause overheating of the carb, vapor lock, and other fuel issues.
Do you have a true dual exhaust (no crossover), and can tell if your heat riser is opening? If not, exhaust leaving the right-side exhaust will be greatly reduced, forced through the intake manifold, and can cause overheating of the carb, vapor lock, and other fuel issues.
great respons and a lot I had to research and figure out. Unfortunately I do not know if i have the true dual exhaust although I know that the exhaust system is stock and it is a -74 which if I understand correctly means that it is the last generation with true dual exhaust?
Regarding the diverter valve I will have to see and have to get to the garage and look. I will update as soon a I do. I have one picture of the engine bay looking at it from the driver hand side. I will attach it and take more next time I am next to the car.
Race Director
Joined: Mar 2017
Posts: 11,953
Likes: 4,510
From: Virginia
Hi,
great respons and a lot I had to research and figure out. Unfortunately I do not know if i have the true dual exhaust although I know that the exhaust system is stock and it is a -74 which if I understand correctly means that it is the last generation with true dual exhaust?
Regarding the diverter valve I will have to see and have to get to the garage and look. I will update as soon a I do. I have one picture of the engine bay looking at it from the driver hand side. I will attach it and take more next time I am next to the car.
great respons and a lot I had to research and figure out. Unfortunately I do not know if i have the true dual exhaust although I know that the exhaust system is stock and it is a -74 which if I understand correctly means that it is the last generation with true dual exhaust?
Regarding the diverter valve I will have to see and have to get to the garage and look. I will update as soon a I do. I have one picture of the engine bay looking at it from the driver hand side. I will attach it and take more next time I am next to the car.
Was the engine warmed up when this image was taken? Your choke is wide open.
From this angle, it looks like you have a points distributor with a mechanical tach drive. I see no evidence of a vacuum advance canister, or a hose leading to it (that I can see anyway).
If your exhaust is original, then yes, you should have a dual exhaust. If the car is heated up, the heat riser valve should open, and you should have equal flow out both pipes (you can feel it with your hand).
Melting Slicks
Joined: Nov 2000
Posts: 2,507
Likes: 114
From: St. Louis Mo
hi, how would you if you remember describe how the car ran, would it get progressively worse as it got warmer? Also did this ever become a problem again later on or did it just go away after that?
As i stated in the initial post first I wanted to rule out egnition but I guess i cannot fully, I read about egnition coils for a bit and apparently as the copper gets warmer the resistance in the copper increases a lot as well. Could it be that since the starter fluid is more easily burnt that less spark is needed for it to run?
Thanks for the input, anything that might help is very appreciated.
As i stated in the initial post first I wanted to rule out egnition but I guess i cannot fully, I read about egnition coils for a bit and apparently as the copper gets warmer the resistance in the copper increases a lot as well. Could it be that since the starter fluid is more easily burnt that less spark is needed for it to run?
Thanks for the input, anything that might help is very appreciated.
Dr. Detroit
Joined: Mar 2012
Posts: 10,097
Likes: 4,027
From: New Braunfels Texas
When it quits....get out real quick and look down the carb throat.......whack the throttle and look to see if the squirter is squirting fuel....if not...then you have a fuel delivery problem. Check the 6" piece of rubber hose in the back off the sender first, it can collapse internally...then check the "S" hose off the fuel line to the pump up front.....or just replace both if they are old.
The 66' in the avatar had your exact same problem and it was the 45 year old hose at the sender at the rear.....it was collapsed and deteriorated internally.....$5 fix!!!!!
Jebby
The 66' in the avatar had your exact same problem and it was the 45 year old hose at the sender at the rear.....it was collapsed and deteriorated internally.....$5 fix!!!!!
Jebby
Burning Brakes
Joined: Dec 2005
Posts: 1,153
Likes: 341
See if driving with your gas cap off changes anything. A tank with a clogged vent will oil can and can't supply adequate fuel to the engine.
Valves. On small engines I worked on when the valves were far too tight, they wouldn't shut when the engine warmed up. Engine wouldn't run or re-start. You got solid or hydraulic lifters? Might
be time for a valve adjust.
Well, those are the things I know of when an engine will start but runs progressively worse or dies. I'll post more when I think of 'em.
EGR on this car?
Last edited by wadenelson; Mar 30, 2022 at 05:54 PM.
Safety Car
Joined: Jul 2005
Posts: 3,697
Likes: 833
From: WI
Um ....did I miss the part where you put a vacuum gauge on it? What's it running for manifold vacuum at cold vs warm/hot? And what's the needle doing? Maybe you have a bad carb or intake manifold gasket that doesn't seal when the engine warms up? Or another vacuum leak.
Have you gone around the top of the engine with a propane torch and hose (not lit) and see if the engine 'smooths' out as you wave the gas in various locations? (or you could squirt ether here and there ..if you have that on hand).
Have you gone around the top of the engine with a propane torch and hose (not lit) and see if the engine 'smooths' out as you wave the gas in various locations? (or you could squirt ether here and there ..if you have that on hand).