Annoying Engine problem

[macblazer2]

So the specs:
1971 Chevy C20 Longbed with a 454 big block and TH400 tranny. Aftermarket distro (aka no points) Has a 750cfm Edlebrock carb on it.

I've been having an annoying problem that for the life of me I can't trace down. I'm leaning toward electrical so I'll probably replace the entire wiring harness at some point soon since the original is pretty hacked anyways.

When the weather is warm I see this issue more than I do in the winter when it is cold.

So every once in a while the truck will just suddenly act like it is only running on 4 cylinders (power just drops into the toilet and it starts sputtering and sometimes stalls out) and a lot of times if I just take my foot off the gas and let it coast it will go back to normal.

First I thought maybe it was because I was getting on the gas to hard and that was what caused it, but no...normal driving (not heavy acceleration and it will still do it)

The carb is brand new, the plugs are new, the PCV has been replaced, the distro cap and rotor look fine, etc etc.

"Seems" to do it mainly when it is under load. Meaning I can have it in the driveway and rev it and let it run and it has never done it under those instances.

Any ideas for what people think I should check or maybe haven't through of would be cool.

thanks!

Dale

[LockDoc]

Originally Posted by macblazer2

So the specs:
1971 Chevy C20 Longbed with a 454 big block and TH400 tranny. Aftermarket distro (aka no points) Has a 750cfm Edlebrock carb on it.

I've been having an annoying problem that for the life of me I can't trace down. I'm leaning toward electrical so I'll probably replace the entire wiring harness at some point soon since the original is pretty hacked anyways.

When the weather is warm I see this issue more than I do in the winter when it is cold.

So every once in a while the truck will just suddenly act like it is only running on 4 cylinders (power just drops into the toilet and it starts sputtering and sometimes stalls out) and a lot of times if I just take my foot off the gas and let it coast it will go back to normal.

First I thought maybe it was because I was getting on the gas to hard and that was what caused it, but no...normal driving (not heavy acceleration and it will still do it)

The carb is brand new, the plugs are new, the PCV has been replaced, the distro cap and rotor look fine, etc etc.

"Seems" to do it mainly when it is under load. Meaning I can have it in the driveway and rev it and let it run and it has never done it under those instances.

Any ideas for what people think I should check or maybe haven't through of would be cool.

thanks!

Dale

Hi Dale,

Welcome to the board. You might start off by replacing the rubber fuel hose under the cab and while you have it off check inside of the metal lines for any blockage. It could actually be a multitude of things. Plugged fuel sock on the pickup tube inside the tank, plugged fuel line, plugged add on fuel filter, crack or hole in rubber fuel line, weak fuel pump, non-vented fuel cap when it should be vented, etc...

As far as electrical, bad connections at the battery, junction block on the inner fender, bulkhead connector at the firewall, bad ignition switch, or bad ground connection on the engine. You will probably have to try and isolate it a little more. If you shut it off and restart it when it is doing it does it continue to do it or does that stop it?

LockDoc

[davepl]

I think it's a longshot, but it popped into my head, so: when you load the engine it torques up against its mount. If that's where its primary ground path is, it'll disturb that.

That should only be the case if you don't have proper ground straps. Do you have at least two copper/aluminum braided ground straps from the block to the frame?

Since it's not the original engine, it can be missed.

[hugger6933]

Did you check to see if the center of the rotor is loose? I've seen rotor buttons that looked fine but were the whole problem. By the way welcome to you from AR

[macblazer2]

Originally Posted by LockDoc

Hi Dale,

Welcome to the board. You might start off by replacing the rubber fuel hose under the cab and while you have it off check inside of the metal lines for any blockage. It could actually be a multitude of things. Plugged fuel sock on the pickup tube inside the tank, plugged fuel line, plugged add on fuel filter, crack or hole in rubber fuel line, weak fuel pump, non-vented fuel cap when it should be vented, etc...

As far as electrical, bad connections at the battery, junction block on the inner fender, bulkhead connector at the firewall, bad ignition switch. You will probably have to try and isolate it a little more. If yiu shut it off and restart it when it is doing it does it continue to do it or does that stop it?

LockDoc

Yeah the fuel lines seemed ok. It has 2 fuel filters on it. One on the frame under the cab and another in the engine bay. I replaced both of them and the lines didn't seem plugged. I had thought about the potential of it being a fuel pump issue, but seemed weird it would go from normal to this weak condition almost like flipping a switch. I'm thinking replacing the entire wiring harness since it is pretty screwed up anyways and see if the problem goes away.

[macblazer2]

Originally Posted by hugger6933

Did you check to see if the center of the rotor is loose? I've seen rotor buttons that looked fine but were the whole problem. By the way welcome to you from AR

That is something I hadn't thought of so I'll check that. Been considering replacing the entire distributor anyways because it has an old flamethrower type on it and I tend to hate those.

[macblazer2]

Originally Posted by davepl

I think it's a longshot, but it popped into my head, so: when you load the engine it torques up against its mount. If that's where its primary ground path is, it'll disturb that.

That should only be the case if you don't have proper ground straps. Do you have at least two copper/aluminum braided ground straps from the block to the frame?

Since it's not the original engine, it can be missed.

Don't think that is it, but good idea. I'll probably put new ground straps in when I swap out the wiring harness.

[geezer#99]

When it's under load, it needs more fuel.
Which leads to this---
Do you have a fuel regulator?
Eddy carbs don't like more than 5 psi.

[macblazer2]

Originally Posted by geezer#99

When it's under load, it needs more fuel.
Which leads to this---
Do you have a fuel regulator?
Eddy carbs don't like more than 5 psi.

Could be....but don't think that is it. It has been driven at 80 mph+ up a highway for 30 miles without the issue popping up. I've also seen the issue pop up under a light loud.

But good idea....I know it doesn't have one so maybe I'll throw one in when I either switch it over to an electric fuel pump or replace the current mechanical one.

[1972BlueC20]

May not have anything to do with your problem, but I'd get rid of the fuel filter under the truck. I've heard of issues that can result. Just 1 filter after the pump right before the carb is all you need. Get yourself a fuel pressure gauge too. Edelbrock is very sensitive to pressures over 5 psi or so. Stock pumps can run at 9 PSI if I remember correctly.

[macblazer2]

Originally Posted by 1972BlueC20

May not have anything to do with your problem, but I'd get rid of the fuel filter under the truck. I've heard of issues that can result. Just 1 filter after the pump right before the carb is all you need. Get yourself a fuel pressure gauge too. Edelbrock is very sensitive to pressures over 5 psi or so. Stock pumps can run at 9 PSI if I remember correctly.

Agreed...that is part of my plan. I'm going to be doing the gas tank to under the bed relocation. When I do that I'll redo all the gas lines and switch to a single inline filter and probably replace the fuel pump.

I'll definately take your advice and put in a pressure gauge and put in a regulator like some others have suggested.

[snipescastle2]

didn't see which dizzy you are running, but if it's a points type, those old coils had oil in them, and they do go bad, perhaps the windings are breaking down when it gets hot, if grab the coil and it's real hot to the touch, (and it could be smoking hot too) that might be part of the problem. I also had ignition wires that would ground out intermittently, and I chased my tail trying to find the problem because I ruled out the wires since I had just replaced them, it was only after I used the ohm meter to check the resistance per foot, did I find the problem....one of my "New" wires had been touching the manifolds

I have also seen HEI distributors "Burn/ Arc" the tab off the top of the rotor!
good luck chasing this down!
Ben

[macblazer2]

Originally Posted by snipescastle2

didn't see which dizzy you are running, but if it's a points type, those old coils had oil in them, and they do go bad, perhaps the windings are breaking down when it gets hot, if grab the coil and it's real hot to the touch, (and it could be smoking hot too) that might be part of the problem. I also had ignition wires that would ground out intermittently, and I chased my tail trying to find the problem because I ruled out the wires since I had just replaced them, it was only after I used the ohm meter to check the resistance per foot, did I find the problem....one of my "New" wires had been touching the manifolds

I have also seen HEI distributors "Burn/ Arc" the tab off the top of the rotor!
good luck chasing this down!
Ben

Yeah no points in this puppy...Has cheapy malloy dizzy and coil in it.

But you know the grounding out issue is one I've been thinking about. I've also been down the road of things touching the manifold.

Was bringing a 71 Chevy back home from Houston the one time. Stopped to get a soda and decided that the car wasn't where I wanted it on the flatbed trailer. So I loosened the straps, lined it up better on the trailer, tightened the straps back down and got back on the road. Problem....the car has a drag racing slap shifter in it that was missing the display and what I THOUGHT was park...was neutral. Got about a block, felt a tuck on the truck and had JUST enough time to look up in my rearview mirror to see the car go off the back of the trailer. hehe Pushed it of the road into a parking lot, fired it up and right before I was going to drive it back up on the trailer the motor died. Several days later I was messing with it in the driveway to find that the retard I got it from had ran one of the dizzy wires close to the manifold. When the car went off the trailer, the wire fell on the manifold and while it was sitting there running waiting for me to pull it on the trailer the wire ground and blow the coil out of the HEI dizzy. hehe

[macblazer2]

Update for those that might have been wondering.

I got up underneath it and not only was there that second fuel filter on the frame rail, BUT whoever did it had a loop in the line. Rubber line came down from the tank, looped and then went into the filter and then into the hard line up to the fuel pump.

So I pulled it and put it back to stock with a short section of rubber line from the tank to the hard line on the frame rail.

So far....so go..Every once in a while I can hear it stumble just a tad (but thinking that is just because it REALLY needs a proper tune-up) but it hasn't completely fallen over on itself like it was on hot days like before.

[LockDoc]

Originally Posted by macblazer2

Update for those that might have been wondering.

I got up underneath it and not only was there that second fuel filter on the frame rail, BUT whoever did it had a loop in the line. Rubber line came down from the tank, looped and then went into the filter and then into the hard line up to the fuel pump.

So I pulled it and put it back to stock with a short section of rubber line from the tank to the hard line on the frame rail.

So far....so go..Every once in a while I can hear it stumble just a tad (but thinking that is just because it REALLY needs a proper tune-up) but it hasn't completely fallen over on itself like it was on hot days like before.

Glad to hear you have it so it will at least run.

LockDoc

[1969guy]

Also make sure to check that if your not running the points distribitor to either replace the resistor wire with straight copper or just unhook your resistor wire and run a copper wire through the firewall from the distributor to the unfused male ignition terminals on the fuse block.

I had a truck that wouldn't start with the whole resistor wire intact but stripped it out a little just a foot away from the firewall and attached a copper wire from there to the distributor and it started right up!

[SeniorVerde]

Here's another long shot idea, but one I've seen happen:

Fuel additives, leaded gas substitutes, octane boost- all come in bottles with with foil over the mouth and a plastic disk in the lid. You (or a previous owner) take the cap off, pull the foil, and dump it in the tank.

My Dad had a truck that the previous owner always put in some kind of treatment that came in one of those bottles. One time, the cardboard disk that is usually under the cap stuck to the mouth of the bottle, and into the tank it went.
You could drive for a couple of days, a month, maybe half a year- then every once in a while, that disk would get sucked up against the pickup tube, and make the truck act like it was low on gas. Drove my Dad nuts until he caught a glimpse of the thing floating one time he was filling the truck at the gas station.
Spent the whole evening trying to harpoon it through the filler neck reaching into the behind-cab tank with some kind of hot dog skewer. He eventually got it, and the truck never had a hiccup after that.

If we ever dump any additive in the tank, now we remove the foil all the way, and make sure the cardboard is still under the cap.

Probably not your problem, but stranger things have happened.

[macblazer2]

I'll have to try some of those ideas as well.

Because turns out the problem isn't fixed. Drove it up to North Austin. (about 95 degrees outside) Got almost there and it started doing it again. By the time I finally got to the location it was bogging down bad. When I came back out it wouldn't start up.

Checked everything....finally started back up.

All the wires to the coil felt tight, all the plugs at the distro felt tight, BUT it did start after I checked those wires, so who knows.

Friend suggested vapor lock, but the entire fuel line from the fuel pump to the carb (except for the filter) is covered in that thermo sleeve type hose. But I guess it is always possible the fuel pump is getting hot and is on it's last legs and starting to mess up when it gets hot.

[67chevy_hotrod]

My 64 bel air has the identical problem. It's grounding straps are all junk and when it tourques it pulls on the ground on the block cause of some custom wiring...

[macblazer2]

Mine I'm thinking is heat related some how. It never seemed to have this issue during the winter when it is cold, but soon as it warms up around here it does. Also usually only after it has been driven a bit.

Thinking I'm going to put tracing this one down on hold for a bit and focus on getting the wife's 72 C10 back running and legal again.

After that I'll probably come back to mine, replace the fuel pump, replace the distro, plug wires, and go over the other things and see if that clears it up.

But for now the wife's birthday is coming up in September and I need to get her 72 C10 into the shop and have the valve train gone through and properly adjusted.