Intermittent power loss (need help troubleshooting please)

[trevarthan]

Originally Posted by Grumpy old man

Automatic chokes need to be adjusted correctly , The choke ( top flap on carb) should be closed when engine is cold in the morning and open fully once engine has warmed up . AUTOMATIC DOESN'T MEAN FOREVER

You made me curious, so I filmed two videos showing the auto choke actuation.

The first is manual actuation (short video): https://youtu.be/eWjX0byOYRQ

The second is automatic actuation as the vehicle warms up from 40 degrees F (8 and a half minutes long - skip to the end if you want - turn your sound down - loud): https://youtu.be/94MVneCeIDE

Everything appears to be in proper working order to me, but I don't really know what I'm looking at beyond the whole air - fuel - spark triangle here.

[trevarthan]

Originally Posted by RichardJ

The carb may need rebuilt, but before I make that suggestion I need a better understanding of the existing vacuum line connections.

In my first post I ask about the "T" on the PCV vacuum lines. From the first picture above, I thought B-A was a single line that looped across the valve cover and back to the PCV "T". I now see that A and B both go across to the pass fender and to the Charcoal canister.
My question was and still is, what is the vacuum line on the driver side of the "T" and what is it connected to?

I'm happy to collect photos and video of this vehicle.

The vacuum line on the driver side of the "T" Looks like it is connected to a port in the intake manifold:

Originally Posted by RichardJ

The large casting port on the carb air horn usually would have a fitting that would connect a hose to the charcoal canister. Sometimes the casting is there, but not drilled out.

The open port directly below the fuel inlet is probably for the air cleaner Thermac door. Just checking.

There are two vacuum lines on the air horn. One goes to a pancake on the inlet:



The other goes to that port on the carb below the b-nut:



Both go to a sensor of some kind on the air cleaner assembly.

top of air cleaner, open and removed:



bottom of the air cleaner:



That sensor is labeled AC (auto choke?) and looks like it has a port for an electrical connector (inside the air cleaner), but it's never had anything plugged in there for as long as I've owned it. No clue what that is. Let me know if you know. I'm curious.

Originally Posted by RichardJ

I see a vacuum line connected to the distributor vacuum advance, but I can't see it on the other side, connecting to the carb? There is a steel line laying against the valve cover, but would still like to see where it is connected.

It is connected to the carb here:



I've tested the vacuum advance on the distributor with a rented hand held vacuum pump. Seems to be working (saw the lever move), but I don't know if it's within spec or not because I don't know the specs or where to find them.

EDIT: I just noticed you asked about that sloped port on the carb. It does appear to be drilled to me, according to this photo with the cleaner removed:



I took a video of the auto choke opening for the previous poster. At the end of the video I pan around the carb a bit for you. This link should take you right to that point in the video (turn your sound down - loud):
https://youtu.be/94MVneCeIDE?t=473

[Average Joe]

I dont mean to throw you a curve ball here, but I had a similar situation in my last truck. It would stumble randomly. In the beginning it appeared to be doing it under load. I replaced fuel filter and pump to no avail. I then rebuilt the carb. Still happened sporadically. IIRC I tried a known good coil. Still no luck. Eventually I traced the problem to a crappy splice in the main power wire coming from the battery, via the small distribution block on the passenger fender, to the fuse box. I soldered and shrink wrapped the faulty splice and was back in business.

Years later I had a similar problem in my Suburban. I traced it back to a loose bulkhead on the backside of the fusebox.

[trevarthan]

Originally Posted by Average Joe

I dont mean to throw you a curve ball here, but I had a similar situation in my last truck. It would stumble randomly. In the beginning it appeared to be doing it under load. I replaced fuel filter and pump to no avail. I then rebuilt the carb. Still happened sporadically. IIRC I tried a known good coil. Still no luck. Eventually I traced the problem to a crappy splice in the main power wire coming from the battery, via the small distribution block on the passenger fender, to the fuse box. I soldered and shrink wrapped the faulty splice and was back in business.

Years later I had a similar problem in my Suburban. I traced it back to a loose bulkhead on the backside of the fusebox.

Nah, it's fine. I've got 50/50 money on fuel or electric at this point.

How did you determine it was a bad splice? Jiggle the cables? I've been doing that and I haven't noticed any odd behavior. I may still not be jiggling the right one though.

[LockDoc]

-
Have you replaced the gas cap lately by any chance? If it runs for a while before becoming sluggish it could be creating a vacuum in the tank. I know someone earlier mentioned taking the cap off to see what happens but not sure if that was tried.

The sock filter on the end of the pickup tube in the tank would be at the top of my list of things to check, as would be a plugged fuel line from tank to pump. Here are pictures of a plugged sock filter and pieces of a filter that were plugging up a fuel line.

LockDoc

[trevarthan]

Originally Posted by LockDoc

-
Have you replaced the gas cap lately by any chance?

no. the cap may be original for all I know.

Originally Posted by LockDoc

If it runs for a while before becoming sluggish it could be creating a vacuum in the tank. I know someone earlier mentioned taking the cap off to see what happens but not sure if that was tried.

It has not been tried yet. Removing the cap after a stall doesn't seem to help with the hard restart issue and I certainly don't feel or hear any vacuum when removing the cap. I'd be shocked if the cap on it now is air or gas tight.

[RichardJ]

The valve inside the air cleaner operates with heat, to open the snorkel door for cooler air to enter, after the engine warms up.
The vacuum port for the Thermostatic Air Cleaner (Thermac) should be capped or plugged with the air cleaner off and the engine running. You are providing a vacuum leak the upsets idle mixture and idle speed.

You have three hoses on the charcoal canister, visible in your video. One hose goes out of sight under the battery.
Does the end of that hose have the nipple that is missing from the carb?

The choke rod coming out of the Bi-metal Choke Stove on the exhaust manifold is working in your video.
The choke has two functions.
When you step on the throttle of a cold engine the high-speed idle is set along with the choke. I.E. together.
Heat turns off the choke (opens the choke valve) , independent of the high-speed idle. By once again stepping on the throttle, the high-speed idle mechanism is released. The rod on the back side of the carb is connected to a high-speed idle cam.

Your high-speed idle mechanism is obviously not working properly.

Some carbs have a idle speed screw and a separate high-speed idle screw. Yours may not.

Fix the missing large nipple on the carb, cap the lower vacuum port, warm the engine up and adjust the idle speed down to a reasonable rpm. At that point see what you need to do to get a correct high-speed idle.

That missing nipple would certainly have allowed dirt and water to get into the float bowl so you may need to rebuild the carb. I am not certain if or how much that open hose would have upset the vacuum in the charcoal canister.

[trevarthan]

Replaced the ignition switch. No change. Still happening. My timing light arrived this week. Plan to have a look at that this weekend and have a look at the vacuum lines/ports as suggested. May replace the fuel pump too.

[VetteVet]

Back in the early days when catalytic converters were new they would sometimes plug partially and performance would drastically decrease. I know you don't have a converter, but have you checked your muffler and exhaust system for a plug? If the truck has sat for a while the critters might have built a nest in there and that might be your issue.

[trevarthan]

Originally Posted by VetteVet

Back in the early days when catalytic converters were new they would sometimes plug partially and performance would drastically decrease. I know you don't have a converter, but have you checked your muffler and exhaust system for a plug? If the truck has sat for a while the critters might have built a nest in there and that might be your issue.

Seems unlikely. It billows smoke when it starts up (stops when warm).

[trevarthan]

Originally Posted by RichardJ

When you step on the throttle of a cold engine the high-speed idle is set along with the choke. I.E. together.
Heat turns off the choke (opens the choke valve) , independent of the high-speed idle. By once again stepping on the throttle, the high-speed idle mechanism is released. The rod on the back side of the carb is connected to a high-speed idle cam.

Your high-speed idle mechanism is obviously not working properly.

Just to be clear, you say it's not working properly because when I manually actuated the throttle the choke plate didn't move?

[trevarthan]

Originally Posted by LockDoc

-
The sock filter on the end of the pickup tube in the tank would be at the top of my list of things to check, as would be a plugged fuel line from tank to pump. Here are pictures of a plugged sock filter and pieces of a filter that were plugging up a fuel line.

LockDoc

Do I have to pull the tank to check that?

[trevarthan]

Originally Posted by RichardJ

You have three hoses on the charcoal canister, visible in your video. One hose goes out of sight under the battery.
Does the end of that hose have the nipple that is missing from the carb?

No. That hose goes to the frame rail and converts to a metal tube. I assume that is the vent line for the gas tank.

[LockDoc]

Originally Posted by trevarthan

Do I have to pull the tank to check that?

No, You just have to remove the sending unit from the tank. You should be able to get to it by sliding the seat forward as far as it will go, disconnect the fuel line fitting, remove the sender lock ring (spray it with some penetrating oil before trying to remove it) and pull the sending unit out. If you disconnect the inlet line from the fuel pump you can then blow through the line from inside the cab with an air hose to clear the line of any internal junk. Make sure there are no filters between the tank and fuel pump.

LockDoc

[trevarthan]

Originally Posted by LockDoc

No, You just have to remove the sending unit from the tank. You should be able to get to it by sliding the seat forward as far as it will go, disconnect the fuel line fitting, remove the sender lock ring and pull the sending unit out. If you disconnect the inlet line from the fuel pump you can then blow through the line from inside the cab with an air hose to clear the line of any internal junk. Make sure there are no filters between the tank and fuel pump.

LockDoc

I'll do that. Thank you.

I also bought a fuel pump and a carb rebuild kit and cleaning kit from autozone today.

But first I'm going to check the timing and pull the spark plugs and photograph/replace-as-necessary. Stay tuned.

This would suck a lot less if it wasn't pouring rain today. All week it's dry, but I work. Of course it rains on the weekend. Sigh.

[trevarthan]

I'm excited because I finally found something that is definitely broken! Ha.

The heat riser valve spring is definitely broken:



Here's a quick video of me manually actuating it: https://youtu.be/IL0KQFsW2K0

I'm googling, but I'm having a hard time figuring out how important this is.

What affect will a broken spring have on engine operation, and what do I have to do to fix it?

Thanks!

[trevarthan]

Granted, I haven't set the idle yet, but a quick check with the timing light and the vacuum advance disabled indicates 14 degrees before TDC.

The plate under the hood says 4 degrees BTDC, but I see a lot of people on the forum here saying 12 to 14 is fine.

[LockDoc]

Originally Posted by trevarthan

I'm excited because I finally found something that is definitely broken! Ha.

The heat riser valve spring is definitely broken:



Here's a quick video of me manually actuating it: https://youtu.be/IL0KQFsW2K0

I'm googling, but I'm having a hard time figuring out how important this is.

What affect will a broken spring have on engine operation, and what do I have to do to fix it?

Thanks!

The exhaust should open it up when the engine is running. The spring closes it. I would try an auto parts store for the spring. Not sure if you can replace it without taking it apart or not.... maybe.

LockDoc

[trevarthan]

Spark plugs removed.

I don't really know what I'm looking for, but I notice:

1. the plugs are coated in oil, but they don't look quite like the "oil deposits" photo in my Haynes manual
2. the insulators are blackened, looks kinda like the carbon deposits photo in my Haynes manual - mixture too rich?
3. the arms are rusted - seems normal

from left to right: 1,2,3,4,5,6



another view, bottom to top: 1,2,3,4,5,6



#1 plug immediately after removal:



#2 plug immediately after removal:



#6 plug immediately after removal (definitely in the worst shape - blackened, makes sense given the dieseling issue):

[trevarthan]

We're removing the fuel pump now.

After sitting for 20 minutes in the driveway, the line from the fuel pump to the tank was dry when disconnected. I was expecting fuel in this line. Is this normal?

The metal b-nut line to the carb was under a bit of pressure and full of fuel, as expected.

[trevarthan]

Fuel pump replaced. Still stalling. The stalls are more frequent now. We have to keep our foot on the gas a little to keep it running now. When it stalls, it's real hard to restart.

Feel like we're making it worse with all of these part replacements. :p

[68 P.O.S.]

Originally Posted by trevarthan

I've tested the vacuum advance on the distributor with a rented hand held vacuum pump. Seems to be working (saw the lever move), but I don't know if it's within spec or not because I don't know the specs or where to find them.

Sounds like the diaphragm is good then. You can find the specs if you know what your distributor came out of. Likely a smogger with too much vac advance though. What are your plugs gapped at? If you don't already know, HEI gap is .045

Originally Posted by trevarthan

Granted, I haven't set the idle yet, but a quick check with the timing light and the vacuum advance disabled indicates 14 degrees before TDC.

The plate under the hood says 4 degrees BTDC, but I see a lot of people on the forum here saying 12 to 14 is fine.

Your timing is good to go. That 4 degrees is a factory safe number to keep warranty and recall issues to a minimum. Gotta protect that pocketbook. 12-14 degrees is a performance number which optimizes engine efficiency. Regardless, total timing is more important to set.

Still sounds like the carb needs a good cleaning and rebuild.

[Sparky dave]

Fuel pressure gauge plumbed before the carb would tell you a lot. Too little or too much pressure will cause you problems. It will tell you if anything at all is wrong especially if you can see it from in the cab while your driving. Well worth the money if you can get one. Otherwise your guessing with fuel.

[Sparky dave]

This has been ongoing for a while now hasn’t it? Seems like you have been over many many possibilities, like trying to find it using pot luck almost. Sorry I’m not meaning to be rude I’d love to help you fix this. Not followed everything you have done but If it was my problem I’d go back to basics which will quickly tell you where your problem is as there is not much to the basic operation of these engines. So here goes back to basics

-Compression test and leak down test ( check your engine is actually capable of running correctly)
-Fuel pressure, no guessing you need to know what it is
-Ignition. The only wiring you need is a +12 wire directly from batt to distributor and a ground wire from distributor to battery. These can be temporary wired in for purposes of proving your wiring system

After these basic steps you will either find your problem or know exactly where your problem lies. Go back to basics until you find your problem then you can get technical when you know where to get technical. Hope this helps you get methodical in your faultfinding. Best of luck, Dave

[trevarthan]

Originally Posted by Sparky dave

-Fuel pressure, no guessing you need to know what it is

I would love to be able to test the fuel pressure. Tell me what to buy and how to install it.

My understanding is that it's kind of a pain in the ass to test pressure with these mechanical pumps because they pulse and your needle will just jump constantly.

I disconnected the bnut from the carb this morning and had my son crank it. Squirted fuel all the way onto the transmission bell in the back of the engine bay. My non-scientific feeling is that the fuel pressure is fine.