Slant won't drive over 25mph without cutting out

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ESP47

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This is a 225 with a Carter BBD, HEI ignition and NOS Mopar distributor.

Drove it to work and everything was perfectly fine. Went to drive it home and it would stumble and die once I'd hit about 25mph. Seems to drive fine under that speed. It also idles the way it should in neutral. I can rev it up in neutral and everything seems smooth but as soon as it's in gear and I get a load on it and get the RPMs up, the thing dies out.

I put a vacuum gauge on it and normally I'm sitting smooth at about 20hg but now the needle is flickering semi rapidly between 16 and 18hg. I can detect a small vacuum leak at the throttle shaft bushing. I had it rebushed a few years back and there's always a small vacuum leak there since the shaft has to spin inside the bushing so I can't quite tell if it's excessive or not. Putting my finger on the outside of the shaft/bushing and plugging it that way makes no difference on the vacuum gauge.

I also noticed that I was getting arcing between the coil posts and center coil wire every couple minutes. I just ordered a tune up kit to correct that.

Any idea what may be going on here? I've had vacuum leaks in the past where I was only at 15hg on the gauge and it would cause a high curb idle but it never caused this issue where it wouldn't go over 25mph.
 
Arcing generally indicates an open or damaged coil/ plug/ other high tension. Might even be a damaged rotor. Anything that inhibits the path of the coil secondary voltage will create a "lightning" like condition, where spark is looking for a path to ground

Pull the wires, inspect, and check for continuity, including the coil wire. "Old school" use to be max of 1000 ohms per foot of wire. I've never seen a good wire anywhere near that high. Also pull the cap and the rotor and inspect both, and if it seems to have been dragging, or damaged, check the dist for shaft wear or out of true

It is not uncommon, with grease, cooked on debri, or a "swollen" cap that the cap fits overly tight and may have been cocked a little bit
 
I’ve seen brand new plugs, wires “bleed” electricity.

How’s the fuel filter and fuel pump?
 
Arcing coil wire post is most likely the problem. Both coil & wire should be replaced. Also worth inspecting the rest of the wires, cap, rotor etc.
 
Thanks guys, I've got plugs, wires, cap, rotor and a coil on the way. Spark plug gap is .040 with the HEI ignition and MSD Blaster 2. Been running it this way for over 10 years with no issues.
 
rumblefish360 said:
I’ve seen brand new plugs, wires “bleed” electricity.

How’s the fuel filter and fuel pump?
Click to expand...

I'm getting a full shot when I pull on the linkage but there's no harm in changing out the filter just in case.

Would either of these issues rear its ugly head only in gear vs. at idle? Because I can rev it up to whatever I want in neutral without a hiccup. It's only in gear when the problem arises.
 
A clogged or semi clogged filter will limit a lot. A work out fuel pump will do the same. I’ve seen big block fuel pump pushrods get worn down decreasing the pumps ability.

This is just the first step in poking around to see what’s what.
 
I agree with @rumblefish360 in that it does sound like a fuel issue. Diagnostics over the internet though is a guess at best.

You could get a small gas can and long hose and put the gas can in the floor, run the hose to the fuel pump inlet and drive it. If it drives like it's supposed to, you know you have some trash stopping up the fuel flow.
 
Thanks guys, I'll give it a shot while I'm waiting for the other parts to come in.

I did notice that the cap had excessive wear on a couple of the terminals, which was odd.
 
That is my next (should be first thing) to check. But I think it was mentioned. A good ignition with fresh parts! Worn out stuff will do what is going on. When the car is under load, driving and depressing the gas pedal making demands on the ignition system, poor &/or bad parts will show up as you described.

You’ll fix it. Keep at it. Freshen everything up one step at a time. You’ll get there.
 
Had something similar on my mom's old 84 5th avenue
It ended up being a plugged sock in the tank.

A neighbor has a 70 f100 that his dad bought new. Had a similar issue twice. First time was a collapsing rubber line right off the tank, (which was original from when the truck was New) the 2nd time was a condenser gone bad
On my old 79 D100 I had it do something really weird (and this one was a /6 besides)
Back when I still lived at home with mom and dad, I came home from work and hit the shower. It ran great on the way to work and again when I went home at the end of the day. Went in the house took my shower and went to leave. It wouldn't fire, crank and crank and crank. I burnt up a starter trying to figure it out.
When I went to hook up the battery and try it again, it was about 1/2 dark. I cranked it with the hood still up and then I saw it. A huge arc from the coil wire (about midway along it's length) to of all things a heater hose, also about the middle of the hose. . I couldn't believe it. Still have a hard time understanding that one 30 years later.
I moved the coil wire over a couple of inches away from the hose and the damn thing fired right up and only had to crank over a couple of revs and it was running.
A few other things for you to check.
 
Is it possible that when you went to work it was in low humidity conditions, but coming home it was raining (or otherwise high humidity conditions)? Water conducts electricity (at least water with "stuff" in it like rain water or water splashed up from the road). You may have a fuel system issue, but that electrical arcing at the coil delineates a point of first attack.

For tune-up parts, I have had excellent results with NAPA Belden and Blue Streak cap & rotors. They use brass terminals; much more durable than the aluminum terminals found on most (even OEM) caps. Second, ignition wires leak. The best wires I've found are Granatelli solid core wires. They use a ferrite ring that suppresses EMI. I use them on our ECU controlled Grand Cherokee.

Chryco used Champion Spark Plugs from the factory. Therefore, there are diehards that swear by them. Understand, today's Champion company is not the same as it was 40 or 60 years ago. I swear by Pulstar Spark Plugs, as they incorporate a "peaking capacitor" in the plug to generate a mild plasma spark.

I also found a web site with an informative dissertation on Ignition Systems that I believe may be of use. Like I said, you may have fuel system issues, but arcing coils would get my attention first.
 
I tried Pulstar plugs in my GTO. Noticeably smoother idle. But then one failed......
 
Bewy said:
I tried Pulstar plugs in my GTO. Noticeably smoother idle. But then one failed......
Click to expand...
Pulstar is on I think Gen 4 of their plugs. I got hooked about 14 years ago. They went to Platinum and Iridium, which totally sucked! They recently (within the past 5 years) did another revamp. I have the current generation in our Jeep GC and like them. Standard plugs kept fouling out cyl #1. These keep firing.
 
volaredon said:
Had something similar on my mom's old 84 5th avenue
It ended up being a plugged sock in the tank.

A neighbor has a 70 f100 that his dad bought new. Had a similar issue twice. First time was a collapsing rubber line right off the tank, (which was original from when the truck was New) the 2nd time was a condenser gone bad
On my old 79 D100 I had it do something really weird (and this one was a /6 besides)
Back when I still lived at home with mom and dad, I came home from work and hit the shower. It ran great on the way to work and again when I went home at the end of the day. Went in the house took my shower and went to leave. It wouldn't fire, crank and crank and crank. I burnt up a starter trying to figure it out.
When I went to hook up the battery and try it again, it was about 1/2 dark. I cranked it with the hood still up and then I saw it. A huge arc from the coil wire (about midway along it's length) to of all things a heater hose, also about the middle of the hose. . I couldn't believe it. Still have a hard time understanding that one 30 years later.
I moved the coil wire over a couple of inches away from the hose and the damn thing fired right up and only had to crank over a couple of revs and it was running.
A few other things for you to check.
Click to expand...

Apparently rubber hoses have a good amount of carbon in their make-up. I found this out a long time ago when I decided to play a trick on a fellow young mechanic. We would screw with each other's cars, trying to sabotage it in a clever way that would hopefully make him take a little time to figure out. I took a length of black vacuum tube and replaced his coil wire with it. I used two old coil wire boots on it, and it looked exactly like a regular coil wire. I was going to get him good with this one, I thought. Well, he jumped in the car and the damn thing fired right up ! We couldn't believe it. He backed up and was pulling out of the parking lot, when we yelled at him to come back. I found out later about the high carbon content. I think we put grease under his door handles the next day to make up for it !
 
If the engine just suddenly dies, it is likely electrical. If it slows and stumbles first, could be a fuel problem. Check that the rubber fuel hoses on the suction side are firm. Mine was soft and collapsing at the fuel pump inlet in our 1969 Dart 225 during a cross-country drive in 1991. The engine would start stumbling above 55 mph on a warm afternoon. Probably due to getting the then-new ethanol gas. Best to change all rubber fuel hoses to fuel-injection type. Even better is "for bio-diesel" which has a Viton liner, like Goodyear Barricade.
 
If No one had mentioned it yet, I know it's kind of a long shot, But Check / Clean the Firewall Bulkhead electrical connections - be sure they are clean - I have had some difficult to diagnose issues that ultimately resulted in the firewall bulkhead connections having high resistance and corrosion.
 
Just to throw out an update. It now runs full speed after changing the plug wires. Must have been too much resistance in one of them. They all look fine visually. Might put an ohm meter on them and see if there's anything noticeable there.

The vacuum is still low so it seems like I have having two separate problems. Still haven't figured this one out yet. Going to spray soapy water on the engine and push some air into the system and see if I can find any bubbling. The carb cleaner is telling me the only leak is at the throttle shaft bushing but I suspect there's another leak somewhere that I just couldn't find with that method.

As always, I appreciate it anytime you guys take time out of your day to give advice.
 
Check slop in your timing chain. That would cause low vacuum. Many /6's had those plastic covered cam gears. The plastic gets brittle and breaks off, causing cam timing issues. A worn cam lobe would hurt vacuum as well. Since they removed the zinc oxide from engine oils, more older engines are suffering from worn cam lobes. You didn't mention what year this is, but a clogged exhaust (like a cat) would also kill upper rev HP and give you low vacuum readings.
 
I know you said you were only running 0.040" spark plug gap. Stock is 0.035". Normally that number shouldn't cause misfire if everything else is up to snuff. By increasing the spark plug gap, you are increasing the resistance for the spark to jump from one electrode to the other. But electricity is lazy and likes to travel the path of least resistance, so if there is another easier path it will find it.
Sometimes low vacuum is a function of too lean a mixture. Which could be from a vacuum leak. I would try a leak down test, if the leak is not otherwise obvious like a leaky power brake hose or vacuum advance hose.
Can you be more specific on the idle vacuum reading? What number is it? does the needle fluctuate? Fluctuate in a consistent, regular pattern? Does the needle "float"?
 
mpgmike said:
Check slop in your timing chain. That would cause low vacuum. Many /6's had those plastic covered cam gears. The plastic gets brittle and breaks off, causing cam timing issues. A worn cam lobe would hurt vacuum as well. Since they removed the zinc oxide from engine oils, more older engines are suffering from worn cam lobes. You didn't mention what year this is, but a clogged exhaust (like a cat) would also kill upper rev HP and give you low vacuum readings.
Click to expand...

I'll check this as one of my last resorts since it's more difficult to get to compared to a handful of other things I need to check. 71 Duster so no cats here and I removed all the emission stuff years ago.

rustycowll69 said:
I know you said you were only running 0.040" spark plug gap. Stock is 0.035". Normally that number shouldn't cause misfire if everything else is up to snuff. By increasing the spark plug gap, you are increasing the resistance for the spark to jump from one electrode to the other. But electricity is lazy and likes to travel the path of least resistance, so if there is another easier path it will find it.
Sometimes low vacuum is a function of too lean a mixture. Which could be from a vacuum leak. I would try a leak down test, if the leak is not otherwise obvious like a leaky power brake hose or vacuum advance hose.
Can you be more specific on the idle vacuum reading? What number is it? does the needle fluctuate? Fluctuate in a consistent, regular pattern? Does the needle "float"?
Click to expand...

Yeah the plug and gap combo I'm running is what has been recommended on slantsix.org for a long time. I've been running this combo for about 15 years now with no issues. I see what you're saying about resistance and it makes sense but I don't think my issues are due to the gap itself.

A leak down test is definitely in order. The needle on the vac gauge semi rapidly flickers back and forth between 16 and 18hg where it used to just hold a solid 20 or so. Makes me wonder if it could be something in the valve train and now just a simple leak.
 
"A leak down test is definitely in order. The needle on the vac gauge semi rapidly flickers back and forth between 16 and 18hg where it used to just hold a solid 20 or so. Makes me wonder if it could be something in the valve train and now just a simple leak".[/QUOTE]

That particular symptom indicates worn valve guides
 
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