71 LS5 Fuel delivery problem
Thread Starter
Safety Car
Joined: Oct 2021
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From: Iron Range MN
71 LS5 Fuel delivery problem
Ive run into an issue where after driving about ½ hour to 45 minutes the car starts sputtering. It's stranded me a couple of times lately but after letting it sit for 10-15 minutes it fires right up.
When cruising the vehicle just start sputtering and if I step on the accelerator the carb gets an occasional fuel burst but not enough to keep it running.
It has the mechanical 3 port fuel pump so I don't think it's vapor lock. The carburetor is a Sean Murphy industries Quad jet flowing at 850 CFM. My mechanic also thinks it's not vapor lock but thinks the carb needs to be rebuilt.
What would cause the carburetor to act like this when running warm / hot ?
It is a 1971 LS5 with the 454 and 4 speed manual.
Thanks
When cruising the vehicle just start sputtering and if I step on the accelerator the carb gets an occasional fuel burst but not enough to keep it running.
It has the mechanical 3 port fuel pump so I don't think it's vapor lock. The carburetor is a Sean Murphy industries Quad jet flowing at 850 CFM. My mechanic also thinks it's not vapor lock but thinks the carb needs to be rebuilt.
What would cause the carburetor to act like this when running warm / hot ?
It is a 1971 LS5 with the 454 and 4 speed manual.
Thanks
Dr. Detroit
Joined: Mar 2012
Posts: 10,097
Likes: 4,027
From: New Braunfels Texas
What is your timing set at?
Next time it quits....hop out real quick and look down the carb.....give it a few pumps and see if you see fuel squirting in......if so, then it's ignition, if not, then it is vapor lock or a restricted fuel supply.
Jebby
Next time it quits....hop out real quick and look down the carb.....give it a few pumps and see if you see fuel squirting in......if so, then it's ignition, if not, then it is vapor lock or a restricted fuel supply.
Jebby
1967 Pedal Car Champion
Joined: Mar 2020
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From: US-PNW
Thread Starter
Safety Car
Joined: Oct 2021
Posts: 4,186
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From: Iron Range MN
Inserted key and car fired right up. Let it idle for about 2-4 minutes with no issue. Got back on interstate and made it 15 miles before it happened again.
Before the trip I was having the same issue so I replaced the fuel filter which was quite dirty so thought that would take care of it.
Thread Starter
Safety Car
Joined: Oct 2021
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From: Iron Range MN
Update on this.
So the mechanic where I had it towed to was able to recreate the problem but could not pinpoint it.
What he did say is that the Carburetor had a lot of build up of a Whitish substance that he thought was from ethanol. I have a Sean Murphy Quad jet.
I'm going to pull the carb over the Winter and have it rebuilt and see what happens in the spring.
So the mechanic where I had it towed to was able to recreate the problem but could not pinpoint it.
What he did say is that the Carburetor had a lot of build up of a Whitish substance that he thought was from ethanol. I have a Sean Murphy Quad jet.
I'm going to pull the carb over the Winter and have it rebuilt and see what happens in the spring.
Dr. Detroit
Joined: Mar 2012
Posts: 10,097
Likes: 4,027
From: New Braunfels Texas
I mention timing because retarded timing can elevate EGT's and make it eventually get hot under the carb......not to mention that the stock LS5 has an exhaust passage under the intake so having 800 degree heat under the carb is a recipe for boiling....this may or may not be happening but if your timing isn't at 36 degrees total....or close to.....then you are running basically a sick engine......
Jebby
Jebby
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Le Mans Master
Joined: Nov 2006
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From: I'd like to propose a toast... to internal combustion and wind in the face.
No problem.
I don't know a lot about Pertronix ignition but I do believe it requires a full 12 volts. If the ignition coil is oil filled (stock type) and receiving a constant 12 volts the coil will become so hot you won't be able to touch it. I know this for a fact as my '72 had a HEI installed in it and then later switched back to a points distributor but the wiring wasn't corrected which lead the coil to receiving a constant diet of 12 volts. One day at a car show the car simply shut off but later easily restarted and I was able to make it home but I did have to replace the engine wiring harness to correct the issue. Your Pertronix may require an epoxy filled coil to avoid this situation.
Also, does your car have a Q-Jet? I would check is the fuel filter.
I don't know a lot about Pertronix ignition but I do believe it requires a full 12 volts. If the ignition coil is oil filled (stock type) and receiving a constant 12 volts the coil will become so hot you won't be able to touch it. I know this for a fact as my '72 had a HEI installed in it and then later switched back to a points distributor but the wiring wasn't corrected which lead the coil to receiving a constant diet of 12 volts. One day at a car show the car simply shut off but later easily restarted and I was able to make it home but I did have to replace the engine wiring harness to correct the issue. Your Pertronix may require an epoxy filled coil to avoid this situation.
Also, does your car have a Q-Jet? I would check is the fuel filter.
Dr. Detroit
Joined: Mar 2012
Posts: 10,097
Likes: 4,027
From: New Braunfels Texas
At the tank sender in the back is a 6" piece of hose that can partially collapse inside.......I had this problem stalling while running down the road and tried all kinds of ****.......this is with the 66' L-79 in the Avatar.....I replaced that piece of hose on a whim and the problem disappeared.......it is worth a look.
Jebby
Jebby
Thread Starter
Safety Car
Joined: Oct 2021
Posts: 4,186
Likes: 68
From: Iron Range MN
I mention timing because retarded timing can elevate EGT's and make it eventually get hot under the carb......not to mention that the stock LS5 has an exhaust passage under the intake so having 800 degree heat under the carb is a recipe for boiling....this may or may not be happening but if your timing isn't at 36 degrees total....or close to.....then you are running basically a sick engine......
Jebby
Jebby
The info on the dyno sheets shows - BARO: 30.1 - DISP 489 - BORE: 4.28 and some other info that is greek to me.
Thread Starter
Safety Car
Joined: Oct 2021
Posts: 4,186
Likes: 68
From: Iron Range MN
No problem.
I don't know a lot about Pertronix ignition but I do believe it requires a full 12 volts. If the ignition coil is oil filled (stock type) and receiving a constant 12 volts the coil will become so hot you won't be able to touch it. I know this for a fact as my '72 had a HEI installed in it and then later switched back to a points distributor but the wiring wasn't corrected which lead the coil to receiving a constant diet of 12 volts. One day at a car show the car simply shut off but later easily restarted and I was able to make it home but I did have to replace the engine wiring harness to correct the issue. Your Pertronix may require an epoxy filled coil to avoid this situation.
Also, does your car have a Q-Jet? I would check is the fuel filter.
I don't know a lot about Pertronix ignition but I do believe it requires a full 12 volts. If the ignition coil is oil filled (stock type) and receiving a constant 12 volts the coil will become so hot you won't be able to touch it. I know this for a fact as my '72 had a HEI installed in it and then later switched back to a points distributor but the wiring wasn't corrected which lead the coil to receiving a constant diet of 12 volts. One day at a car show the car simply shut off but later easily restarted and I was able to make it home but I did have to replace the engine wiring harness to correct the issue. Your Pertronix may require an epoxy filled coil to avoid this situation.
Also, does your car have a Q-Jet? I would check is the fuel filter.
(With this particular issue)If I remember correctly, the Carburetor is a Sean Murphy Quad -jet 800 CFM.
Last edited by Stallzer; Nov 15, 2023 at 08:50 AM.
1967 Pedal Car Champion
Joined: Mar 2020
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Likes: 4,247
From: US-PNW
If the fuel filter was so clogged, the debris had to be coming from the tank. Look inside the filler neck with a flashlight and see how the bottom of the tank looks. You should be able to see through the fuel to a clean metal shiny bottom.
Le Mans Master
Joined: Nov 2006
Posts: 5,262
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From: I'd like to propose a toast... to internal combustion and wind in the face.
You may want to install an addition metal fuel filter between the gas tank outlet and the frame rail. My '72 would bog down when I throttled it after the carb filter becoming partially clogged. I installed the rear fuel filter and no more problems.