Truck running poorly under load, need help.

[E.D.75GMC]

Alright first off, it's a 1975 with a 350 four barrel and TH350. Now the issue, when I'm cruising around and I need to accelerate decently (I.e merging up to a higher speed or passing), the truck starts to stumble and feels like it's about to die. Imagine a bad misfire, that's what it feels like. When you're in neutral or park you can rev it up quite a bit and it runs PERFECT. Idles just fine as well. ONLY when you get into the throttle when in gear does it feel like it's about to die, and I've noticed it's quite a bit worse when you're going up hill. To the point where even gentle acceleration feels like it's going to kill it. Within the past three years I've replaced the fuel pump, distributor cap and rotor, ignition pick up/coil, spark plug wires, and more recently (18 months ago) I've changed the spark plugs and had a rebuilt quadrajet installed. I made sure there are no vacuum leaks, and the fuel filter is clean. I had the shop that I've been taking the truck to since I bought it in Feb 2013 take a look and they couldn't find a source for it. One more thing, I took the original narrow flute opening air cleaner off and tried one that has the cold air duct from the core support but that made no difference.
I'll see if I can record a video of the issue and post it here. But in the meantime, does anyone know what may be the problem? It's been happening for months and I can barely drive the truck anymore.

[rich weyand]

Did you check the timing? If the distributor clamp loosens up (such as due to temperature change) the drag on the rotor will pull the distributor retarded and it will act just as you describe. BTDT.

[nonstop]

Have you adjusted the choke at all?

[E.D.75GMC]

Rich; that sounds like a good idea. I have my buddies timing light in my garage, I'll check it out tomorrow.

Nonstop; No, I haven't messed with the carb at all. I bought the rebuilt carb in December 2014, installed it, took it to the shop I use and they tuned it for me. Hasn't been adjusted since they worked on it but it's been fine for the 5000+ miles I've put on the truck since then.

[Kenfuzed]

Maybe a vacuum leak at the trans modulator? Otherwise I would suspect a timing issue with the distributor.

[77 ChevySport]

Just a long shot; could it be vapor lock?? Not sure what outside temp it was when you had these problems. Been there; done that. These trucks where susceptible doing this especially under load. Chevy made an insulator to go between the carb and int. manifold to solve the problem back then. Larry

[Dead Parrot]

Have you checked the rubber suction line(s) from the gas tank(s) and or tank selector valve? Your symptoms sound similar to what mine did a year ago. After checking all of the usual suspects, thought to check that rubber line. Looked like it could have been the original installed in 75.

It being worse going uphill could be due to needing greater suction to pull fuel from the tank to the pump.

Check for proper gas tank venting.

[Mattchu60]

I would say check the distributor cap. I put a new one on mine just 8,000 miles ago and it started misfiring bad in the spring when under load. I checked the cap and the button was worn all the way down. New cap and rotor solved the issue. Otherwise I would say new plug wires or check your timing.

[friscobob]

Sounds like ignition to me. Is it HEI? This is where I'd be hooking it up to a scope.

[E.D.75GMC]

Originally Posted by 77 ChevySport

Just a long shot; could it be vapor lock?? Not sure what outside temp it was when you had these problems. Been there; done that. These trucks where susceptible doing this especially under load. Chevy made an insulator to go between the carb and int. manifold to solve the problem back then. Larry

I thought it may have been vapour lock as well, so I removed the original air cleaner with the preheater and tube and ran one with a wider opening with the duct that pulls air from the core support. I left the end of the duct open to allow air to be forced directly in, and even with the cool temps we've had up here lately there's been no change

[E.D.75GMC]

Originally Posted by Dead Parrot

Have you checked the rubber suction line(s) from the gas tank(s) and or tank selector valve? Your symptoms sound similar to what mine did a year ago. After checking all of the usual suspects, thought to check that rubber line. Looked like it could have been the original installed in 75.

It being worse going uphill could be due to needing greater suction to pull fuel from the tank to the pump.

Check for proper gas tank venting.

Once the rain dies off I'll slide under and have a look. It's a single tank truck so it should be pretty straight forward.

[77 ChevySport]

Had several friends with this type of truck in the late 70's and when they towed a trailer on a hot day this would happen. GM made a aluminum insulator (shown below) and it cured the problem. The key was to keep heat off the fuel line and bottom of carb. Larry P.S. GM made them for Holley and Rochestor carbs.

[E.D.75GMC]

Finally had a dry day, so I replaced the old original fuel line that goes to the pump. Still having the issue but I'm gonna check out the trans modulator on Friday. The insulator is a good idea, even though I've driven the truck in +30 degree Celsius weather many times over the years and never had an issue I may pick one up for good measure.

[84reject]

If the vacuum line to the modulator is disconnected or leaking the trans would stay in gear much longer between shifts too.

[rich weyand]

Did you ever measure the timing? Five minutes, tops.

[f.monroe]

Sounds like a timing issue(they will always retard like stated above on their own ,never advance) or the spark plug wires,cap,or rotor are breaking down. I have had this happen in the past. Pulling up a hill or accelerating puts a higher demand on the ignition system. Pop the hood tonight and look at the plug wires while it's running. See if they are jumping fire anywhere.
Frank

[E.D.75GMC]

Timing light is showing 9 degrees before TDC at low idle. Does this sound about right? I don't have much experience with this aspect of the ignition system.

[rich weyand]

That is about right, although a little low for a non-emissions (1975) engine, if that is with the vacuum advance disconnected.

You did measure it with the vacuum advance hose disconnected, right?

A better value is between 12 and 16. Until it knocks, more is better.

[E.D.75GMC]

No, I'll have measure it again tomorrow with the line disconnected! Should I plug the hole in the carb where the vacuum advance line connects as well?

[rich weyand]

Then you're way retarded. With the vacuum connected, you ought to read about 30* BTDC. That's where your power went.

People tell you to block the port in the carb when you disconnect the vacuum advance, but you don't need to as long as it runs. It's purely a mechanical measurement.

Just pull the hose off and measure what you have. You're probably around 6* AFTER top dead center without the vacuum advance.

It could have dragged that way if the disti wasn't clamped tight.

You probably want to start with a setting between 12 and 16. Sorta depends on what vacuum advance you have. Set it at 12 for now, and let's see what you get.

[rich weyand]

While you're in there, look down between the distributor cap and the vacuum advance can, and see if you can read any stamped numbers on the arm of the vacuum advance can. If you can, we can look up the can and give better advice on settings.

These are the numbers you're looking for. See that "AS-11" in this picture?

[E.D.75GMC]

Alright, I let the truck warm up then I disconnected the vacuum advance and took another reading at low idle. Still showing 9 degrees before TDC. Seemed odd to me that there was no change, but we'll work with it. Going to a car show tomorrow, so have to get the Camaro cleaned up quick before I can dig in to the truck, but I'll keep you all posted here and take a picture of the code.

[Kenfuzed]

When disconnecting the vacuum line from the distributor you need to plug the port at the carburetor. Your picture shows a piece of open vac hose which should be producing a vacuum leak. Normally you would noticed a change in idle when having this port open. You should also see a change in timing if connected to manifold vacuum and when disconnecting unless the vacuum can is bad.

You can test this by applying vacuum to that distributor can using one of those Mityvac hand pumps while the engine is idling. No change obviously indicates the can is not applying a change to the distributor advance.

[rich weyand]

You don't need to plug the vacuum line on the carb, but you can if you want. I never bother. You can pull all the plugs out and spin the motor with the starter to measure timing, as long as you have the #1 plug wired and grounded. It is a purely mechanical setting.

Yeah, if you were going to drive it around like that, you need to plug it. But not just to measure timing.

The reason your idle timing is the same as your timing with the can disconnected is that you are connecting to ported vacuum. That's part of your problem. Connect the vacuum to the manifold vacuum port. Then set base timing, with the vacuum advance can disconnected, to 12* BTDC.

[E.D.75GMC]

It's all sorted now and the truck is running well! We played with the timing a bit, got that adjusted properly. Funny thing now; the timing didn't fix the issue. When we had the hood open while the truck was running, right after taking the truck for spin to see how the new timing worked out, I could faintly smell burning plastic. It was coming from the distributor. Shut the truck off, popped off the cap, everything looked good. Popped the plastic piece where the ignition module sits and BAM - there lay the problem. Turns out the module was slowly frying itself, and the plastic casing was warped and partly melted. It must have been cutting power to the spark plugs and that's where the stumbling was coming from. Replaced the module and went for a good 40 mile drive and it was 100% perfect. Very happy to have it solved now, and thank you to everyone who commented here. It's always great when people help each other out. One of the many reasons why I joined this forum. I hope this thread helps others out in the future!