The Stovebolt Site Forums The Shop Area Engine Shop Truck hesitating under load

This morning my truck started hesitating while accelerating in 3rd. It was OK as long as I was cruising, but when I gave it some gas, it would hesitate. It sounded like a miss at first, but it got worse. As it happened, I barely made it home. It started doing it in all gears and it got to the point where it would quit at idle and wouldn't restart.
Here's what I've checked out: The needle and seat are clear. The float moves freely. The bowl was full of gasoline. The fuel pump is pumping a lot of gas (I put the line into an empty jar and cranked the engine).
My vacuum advance line is tight. The line to the wiper is connected and not leaking. The carb is tight to the intake manifold.
My ballast resistor is new and shows the correct voltage on both sides. There is proper voltage at the coil. The generator is putting out over 12 volts.
All plug wires are secured, as are wires to coil. I had a spare coil lying around and put it in just for the heck of it and the problem is unchanged.
After letting it sit for a while I was able to start it. It idled nicely and accelerated without any problem. It sounded totally normal. But then when I drove it around the block, it hesitated really badly and also developed a problem idling. Barely made it around the block.
Next, I'm checking the vacuum advance. It's maybe the only original thing left on this truck.
And I don't know if this is related or not, but yesterday I pulled the generator to replace the bearings. It ran fine all day yesterday.
Another thing: I ran it in the rain for the first time last night. Now today I have this problem. But it didn't start until after I'd been on the road for a few minutes.
Any ideas?

checked the dizzy cap for hairline cracks?
try running it in the dark and see if there are any 'leaks'...

I'll do that. Thanks. In the meantime, I discovered that my vacuum advance would not hold a vacuum. So I replaced it and the problem is still there. The engine runs great for the first five minutes or so, then it starts to stumble. I took it out just now and it ran great but after about five minutes, I pulled up to a red light and it quit. I had a hard time starting it again. Put it in first and accelerated and it was bucking like a horse. When I coasted up in front of my house, the engine just quit.
I bypassed the ballast resistor and ran it on 12 volts and it didn't make a difference. When you're sitting still with the trans in neutral and you race the engine and hold it at high RPMs, it stumbles badly.
One weird thing and I don't know if it is connected, but I tried to check the timing and the timing light was so weak I couldn't see the mark on the flywheel. I've never had this problem with this timing light. It's almost like it's not getting enough power.
Thoroughly stumped on this one for now. I just put the truck away and called it quits. But it does seem like it might be heat-related and therefore maybe electrical. Any ideas are appreciated.

Have you checked the points gap?
A bad condenser or coil can break down with heat.

I have not done that. The condenser is new (which doesn't mean it isn't bad). The coil is also new. Plus I switched coils just because I have an extra and it didn't fix the problem.

I ran the engine in the dark and saw no sparks anywhere. I also bypassed the ignition switch and ruled that out. At this point it is difficult to start and will only run for a few seconds. I keep going back to the fact that the timing light was very weak when I hooked it up. So maybe there is an electrical problem causing a weak spark.

didja take a plug out and ground it to see how fat the spark is? do that cold and then hot after the problem starts .... and instead of taking it out for a drive, just keep it in the driveway at a fast idle for a bit and see if the problem develops, might be movement related, like something shorting/grounding out

Bill

When you replaced your ballast resistor did you reset your points? My guess is you need to replace your points.

I haven't checked the spark yet. I'll do that today. And I agree about the running it in the driveway bit. I did not replace the points when I replaced the resistor. Today, I'll have a good look inside the distributor. Seems like this truck has an inexhaustible supply of curveballs it can throw at me. The vacuum advance was pretty much the last piece of old equipment under the hood. Everything else has been replaced or rebuilt. Except the distributor. My money is on bad points/condenser but I'm thinking I should take the whole thing apart and check the bushings and mechanical advance as well.

if the cap hasn't been replaced, replace it

Bill

I'm sure you've checked, but is you battery fully charged/charging, connections clean and tight, including the ground on the frame?

Have you tried cleaning the points?

Also check the plug wires, or replace them. I had a set of new once once, and one of them was bad. It caused no end of grief for me.

Battery is fully charged and charging. I have not checked my ground straps yet. The cap, rotor, points, and condensor were all new about 9 months ago. I haven't gotten out there yet today but am getting ready to. I'll report back. First thing will be seeing what kind of spark the coil gives when I separate the points.

I just tested the spark at the coil wire. Following advice on some earlier threads, I pulled the dist cap off, pulled the wire off the center terminal of the cap, set it close to a ground, and then separated the points with a wooden stick. There is only an intermittent spark when I do this and the spark looks to me to be very very weak.

try a different hi-tension wire on the coil - do you maybe have an internally resisted coil as well as the ignition resistor? wires backwards on coil + and -? also check the terminal at the distributor side to the coil isn't loose or doesn't have a rotten insulator

Bill

Bill, I replaced the distributor cap, points, and condenser and that seems to have done the job. I ran it for quite a while out there and was not able to duplicate the problem. It runs very nice right now and accelerates smoothly. The old points were pitted somewhat. Time will tell whether or not I've solved the problem.
One thing I do notice is that the engine seems to have a very slight miss every now and then. You can barely notice it. This is something that has come and gone with this truck for some time. When the miss happens, the ammeter does a very slight jump toward the discharge side.
Thanks for the help.
I forgot to mention that I have a coil that is meant to be used with an external resistor.

I don't like to think of my truck as that guy Michael from the Halloween movies. But it has crossed my mind. I just took it around the neighborhood and it ran great. No hesitation or bucking. It never stalled out and it idles nicely. I have to go somewhere tonight but I'm not taking it. I'm taking my wife's daily driver. I'm just not trusting this thing very much right now.

hope it keeps working!! but do check that terminal on the side of the distributor, they can get loose and cause erratic problems, and the plastic piece the metal pin goes thru can get cracked with age .... also on the dist cap, they can get almost invisible hairline cracks inside that gather condensation and cause erratic shorts inside, could have been that

Bill

Thanks, Bill. I took the entire distributor apart and cleaned it. There is no side-to-side play in the shaft. The plastic thing is in good shape. I discovered that I had the cap on cockeyed and the rotor was digging into some of the contacts. It actually scored them out pretty good. There was quite a bit of gold-colored dust inside the cap. How I managed to do this, I don't know. Probably just careless and in a hurry. I made sure I got the new cap on right. Nobody had the rotor I need so I'm re-using the old one. I plan to get a new one soon.