Intermittent power loss (need help troubleshooting please)

[LockDoc]

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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

[72c20customcamper]

Originally Posted by trevarthan

Hello,

I've got a 1971 c10 lwb with the 4.1L straight six.

Recently it has started losing power after driving for awhile (random intervals - as much as an hour, as little as a few minutes). Sometimes this is a complete stall, but sometimes it's just a lack of power. Sometimes it stutters when it loses power. Sometimes the engine keeps running when it loses power, but it just won't go.

It seems to be more likely to happen when the engine is warm, but I'm not 100% sure it matters. It'll stall at idle sometimes too if you restart it immediately after a stall and let it idle. Restarts are usually difficult after a stall.

I have a car that did this . Drove me crazy. Finally figured it out. Was vaper locking ,when it dies look into the carb and pump the throttle linkage. If no gas then its vapor locking. The fix for mine was to install a 4 psi inline electric pump . Still would hesitate when it's hot out after sitting in traffic.But has not done it since I switched to non ethanol gas. Ethanol has a much lower boiling point than gas.

[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.

[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 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.

[trevarthan]

Originally Posted by RichardJ

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.

I've started rebuilding the carb today. Appears to be a Rochester Monojet.

Looks to me like that port is plugged inside the carb, so I doubt it was a problem:








At the time I wasn't sure what any of this stuff was or did, so I was a little overwhelmed, but I've watched a rebuild video or two now so I'm feeling a little better about it.

One thing we noticed was the that venturi tube looks battered for some reason:



Also, the rebuild videos say it's a good idea to replace the float because the old plastic floats can start absorbing fuel which changes their buoyancy. My rebuild kit doesn't come with a float, so I'll probably order this brass one: https://www.carburetor-parts.com/Roc...oat_p_403.html

Full album pics of disassembly, if curious: https://imgur.com/a/cmJFOoL

Time to clean it.

[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]

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.

[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.

[trevarthan]

Originally Posted by 68 P.O.S.

What are your plugs gapped at? If you don't already know, HEI gap is .045

I replaced them yesterday with platinum plugs gapped at 0.035, I think. Will this cause problems with the HEI or is it fine? Can't regap platinum without chipping it, I've been told, but I can always toss them and buy regular plugs if those won't work.

[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.

[RichardJ]

There are dozens of versions of the MonoJet so this may help or may simply make it more confusing.

http://www.carburetor-parts.com/asse...ice_manual.pdf

Not plugged, that's a bowl vent. Even with your version of the Vent Valve there should still be a hose nipple.
Have you found the end of the third hose coming from the charcoal canister?

I usually check floats by simply dropping them in a tuna can full of gasoline.

You can test a fuel pump off the engine. I'm not going to tell you it is as good a test as an actual running test with fuel. Mount the pump in a vice and plumb a 0-15 psi gauge to the pump outlet. Operate the pump lever with a vice-grip pliers. It should show 4 - 4 1/2 and the pressure should not drop between strokes. A finger placed over the inlet will show a continuous suction.
This test obviously won't show a leak, but can eliminate a pump with bad valves.

I replaced my 6 cyl pump last Aug. and posted this on an ongoing L6 pump thread.
The old pumps i have pulled off had a tab riveted to the pump arm. I believe it to be a sacrificial metal to prevent wear on the cam, pump lobe. The new Carter pumps don't have the tab. I spent time finding out that Airtex pumps still have the tab. i spent more time and gas getting an Airtex from the Truck Shop. I tested it before installation and found that the pressure dropped between successive pump strokes.

I then bought a Carter from NAPA. It pumped to 4 psi on the first stroke and stayed there on every successive pump cycle.

[trevarthan]

Originally Posted by RichardJ

Not plugged, that's a bowl vent. Even with your version of the Vent Valve there should still be a hose nipple.
Have you found the end of the third hose coming from the charcoal canister?

Hmm. I'll have to look at that vent hole again when I get it out of the parts bath.

I replied earlier regarding the third hose, but I guess this thread is getting long and it's easy to miss. This is the charcoal canister hose layout:

1. to PCV valve
2. to carb on valve cover side
3. to gas tank

There is no vacuum hose free and flopping around for that port in the carb. If anyone can tell me where it should be going I'll be happy to correct it when I put the carb back on.

[Sparky dave]

This is the type of pressure regulator I have ( in the left hand side of the pic) attached to the bulk head. It has a filter in a glass bowl and a gauge on top. Not saying mine is the best it's just what I could get in the uk. You guys have much smaller tidier units available to you, however someone else will have to chime in and tell you where you will find one

[trevarthan]

I'd like to keep a hard line from the pump to the carb, so I guess I'm going to be buying steel pipe bending and flaring tools.

I can't even find an OEM replacement hard line from the pump to carb for this year/engine, so I guess that's really my best bet moving forward as I've already rounded off the b-nut on the carb side.

I'm still cleaning the carb. Think I've got it mostly clean at this point. When my new float arrives I'll take some photos/video and start assembling it.

I'm still not sure what to do about that float vent port. There's no nipple. I'm guessing plugging it is a bad plan too, eh?

[Grumpy old man]

Have you tried Inline tube for the fuel line ?

[Sparky dave]

Yes a fuel pressure gauge t into the fuel pipe just before the carb. It should hold a steady pressure if it jumps about it I struggling to keep up with your engine. Also some pressure regulators come with gauges which would be even better. It can flow with an open pipe ( squirt) but doesn't mean you have correct working pressure when it's connected in. Carbs are sensitive to pressure. If you don't you could be playing with your carb for years and you will never get it right. Interesting post this one!

[Sparky dave]

Are you back on the road yet?

[trevarthan]

Originally Posted by Sparky dave

Are you back on the road yet?

Unfortunately, no.

To recap:

After we changed the fuel pump the engine immediately went from running for minutes to not running for more than a handful of seconds.

We removed the carb.

This was the carb immediately after removing from the vehicle: https://youtu.be/n1xsBgTyjTo

We disassembled the carb. https://imgur.com/gallery/cmJFOoL

Ok, and here's what's happened since the last post:

Immediately upon disassembly we noticed the float sticking. I wish I had filmed that, but you'll have to take my word on it.

I cleaned the parts using a combination of a carb cleaning chemical bucket kit from autozone and an ultrasonic cleaner from harbor freight: https://imgur.com/gallery/r3XIL8I

My son rebuilt the carb (I gave him this truck and he's been driving it for 6 months, so he's the other person when I say "we"). This is what it looked like after I got it back from him: https://imgur.com/gallery/JDuFs9x

Here's a quick video of the new float moving without restriction: https://youtu.be/z3OAUSM7gwg

A few of the bolts on the top part of the carb (the long bolts) were stripped in the bore:




So I replaced those bolts with longer bolts where the bore had some extra room. I actually got creative here and bought longer bolts from ace hardware then cut them slightly shorter with a hacksaw so they would seat properly.

We tapped the air cleaner bracket bolts to a larger size because they were stripped as well:



I didn't get to bench test the carb as much as I should have. I may take it off and do a fuel pressure test soon. I bought a positive and negative vacuum hand pump for this purpose. It'll arrive in a few days. Also, I was watching a motorcycle carb rebuild video today and learned I can do something similar using a gravity fed bottle of gas. I may try both.

I installed the rebuilt carb today: https://imgur.com/gallery/A527iUr

I noticed the gaskets became wet with fuel pretty much immediately:





I've got the screws as tight as I can get them. I'm using a GP Sorensen gasket kit from autozone. Not sure if these are just cheap gaskets or if there is some other problem.

The engine still does the same thing. It runs for a handful of seconds and then dies. I can't even keep it running long enough to set the mixture screw properly.

I think tomorrow I'm going to try to figure out how to temporarily replace the steel fuel line between the carb and the pump with a soft line with an inline pressure gauge. I need to know if the pump is making too much pressure or if we just screwed up the rebuild. It seems like it's flooding to me.

[HO455]

HO's first rule of trouble shooting says "Always look at the last thing you screwed with first". From your last post the symptoms would lead me to check that all your gaskets in the carburetor are the correct ones. There are many variations.

[geezer#99]

There’s a float height and float drop spec.
What’s the spec for float drop setting?
Looks like yours is set way too low.