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Old 03-13-2019, 07:59 AM   #76
67 chevelle
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Re: Intermittent power loss (need help troubleshooting please)

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The Saga continues. We rebuilt the carb. It worked for a few days, then it started stalling again. No amount of tuning would fix it. So we bought a remanufactured carb and tuned that. Still stalling.

I started suspecting it wasn't the carb at this point.

I was going through every vacuum test I could think of when it did this on the exhaust restriction test: https://youtu.be/Z_I_cT5yRos

Is it possible this has been an exhaust restriction the whole time?
Most certainly , when a catalytic converter goes ,it does this but its full time , not intermittent , the diverter valve that runs gas to your air cleaner may be causing the issue ,
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Old 03-13-2019, 10:18 AM   #77
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Re: Intermittent power loss (need help troubleshooting please)

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Most certainly , when a catalytic converter goes ,it does this but its full time , not intermittent , the diverter valve that runs gas to your air cleaner may be causing the issue ,
gas to the air cleaner? Are you talking about the flapper valve in the snoot of the air cleaner? I just checked it. That valve is fairly stiff and defaults to the open position (fresh air, not exhaust air). It has a vacuum port on it, but it was disconnected during the test. I just tested it with a vacuum pump. It's definitely broken. Does not hold vacuum and does not close. I don't see how this could be causing the vacuum drop during cruise speed as it was disconnected during the test, but I definitely need to fix it.

Last edited by trevarthan; 03-13-2019 at 10:56 AM.
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Old 03-13-2019, 12:28 PM   #78
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Re: Intermittent power loss (need help troubleshooting please)

I wanted to remove the air cleaner as a possible source of air restriction, so I remade the video with the air cleaner off:

https://youtu.be/S_2tQ-CapG8

And then immediately after that video I made another video showing how it behaves when restarted immediately after a stall:

https://youtu.be/ZSGQz21wrsI

I also made these videos with the engine cold to prove to myself that I could make it stall cold or warm. I think this indicates it's probably not vapor lock too.

I checked the exhaust pipe under the truck. There is no catalytic converter I can see, just a muffler.
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Old 03-13-2019, 02:08 PM   #79
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Re: Intermittent power loss (need help troubleshooting please)

there is a valve in your exhaust with a metal hose that runs to the botom of the air cleaner horn , that is the valve I was referring to , most remove it
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Old 03-13-2019, 02:40 PM   #80
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Re: Intermittent power loss (need help troubleshooting please)

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there is a valve in your exhaust with a metal hose that runs to the botom of the air cleaner horn , that is the valve I was referring to , most remove it
The heat riser flap?





The spring broke, but it moves freely when I move the lever. I suppose it's possible it's blocking the exhaust, but it doesn't feel like it's become detached from the rod. I would have to the intake manifold off to know for sure.
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Old 03-14-2019, 09:36 AM   #81
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Re: Intermittent power loss (need help troubleshooting please)

You've ruled out ignition , You have compression , Have you checked the fuel lines all the way to the tank and the in tank pick up ? Your videos sure look like fuel starvation to me, collapsing fuel line ? . If you slide the seat all the way forward you should be able to pull the sender / pick up from the tank . LockDoc suggested it 2 months ago
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Old 03-14-2019, 12:18 PM   #82
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Re: Intermittent power loss (need help troubleshooting please)

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You've ruled out ignition , You have compression , Have you checked the fuel lines all the way to the tank and the in tank pick up ? Your videos sure look like fuel starvation to me, collapsing fuel line ? . If you slide the seat all the way forward you should be able to pull the sender / pick up from the tank . LockDoc suggested it 2 months ago
I will reinstall my soft fuel line with gauge and re-run the test to check for fuel starvation under sustained throttle. Good suggestion. Thank you.
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Old 03-14-2019, 02:52 PM   #83
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Re: Intermittent power loss (need help troubleshooting please)

FINALLY a test that makes sense! It is indeed fuel starved. I didn't catch it before because pressure starts high and slowly declines under sustained throttle. I made a 3 minute video with the fuel gauge attached showing the problem:

https://youtu.be/D8EWqHjmkqA

I guess the next step is to remove the fuel tank and check the sock. Thank you for the suggestion! This rocks. I feel like I finally understand what is going on now for the first time.
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Old 03-14-2019, 04:00 PM   #84
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Re: Intermittent power loss (need help troubleshooting please)

Don't thank me ,Thank LockDoc ... I just re read your thread and noticed it . You don't have to pull the tank to replace the pick up / sender just move the seat forward and lean over it . also check the hose under the truck . Old guys RULE !
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