Intermittent power loss (need help troubleshooting please)

[67 chevelle]

Originally Posted by trevarthan

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 ,

[trevarthan]

Originally Posted by 67 chevelle

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.

[trevarthan]

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.

[67 chevelle]

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

[trevarthan]

Originally Posted by 67 chevelle

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.

[Grumpy old man]

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

[trevarthan]

Originally Posted by Grumpy old man

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.

[trevarthan]

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.

[Grumpy old man]

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 !

[trevarthan]

UPDATE:

Removed sender. Filter sock was definitely falling apart, but I was dubious that it was enough to block flow. Ordered a new sender.

Went on a vacation out of the country for a week. My new sender had arrived when I got back. Installed it last Thursday, I think. Replaced the two soft lines between the frame rail lines and the tank at the same time.

It still exhibited the same behavior. I knew at this point it was either the hard line or the rubber hose between the pump and the hard line, so I reached down and traced my hand along the rubber hose immediately after a stall. Sure enough, there was a kink in the hose right where it met the hard line.

I had checked this hose previously and it felt and looked fine, so I think what was happening was that it had collapsed internally and as it ran pressure built up, causing it to kink.

Replaced that hose and haven't seen the problem since.

We rebuilt one carb then bought a remanufactured carb when the rebuilt carb didn't do the job. I ended up keeping the remanufactured carb because it came with a nipple on the float vent (my old carb didn't have a nipple, remember). The old carb ran way rich and the new carb ran way lean, according to the spark plugs. The old carb's metering rod arm was adjusted to proper specs and the new carb's metering rod arm was a few millimeters high. So we split the difference and lowered the new carb's metering rod about a millimeter and a half.

After a few days of running successfully I pulled the truck into the garage again and pulled the number 1 spark plug. It looks nearly perfect to me, so I guess we nailed it.

The truck gets much better mileage now.

Did I have to rebuild the carb to fix this intermittent problem? No. Absolutely not. If I had run the vacuum test and fuel pressure test for longer than a minute before I removed the carb I would have found the problem and been able to fix it.

Am I sorry I removed the carb and rebuilt it? Hell no. I loved learning how to do this and the vehicle gets much better mileage now. I'm also now confident that this motor is in pretty good health and will last my son a while longer.

Anyway, thanks to everyone who commented for all the help. Hopefully this saga will help someone else with the same problem identify their problem quickly and resolve it faster than I did.

[HO455]

Congratulations on getting to the bottom of the problem. Your tenacity is admirable.


And thank you for the follow up post on the solution.