360 running on 4 cylinders

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Bl1zzard

Bl1zzard

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We decided to take my 74 Dart out to my sisters acreage for a BBQ. About 60 miles into the journey, I noticed that there was a rougher sound to the engine. We pulled into the nearest town and it wouldn't idle, I had to 2 foot it into a gas station. Did a quick check under the hood and couldn't see anything obvious. We limped it home and I've been troubleshooting it all morning. Here's what I found. I have good spark to all cylinders, pulled the valve covers and all the valves appear to be opening and closing without noise or clatter with the right amount of lift. Pulled the top off the carb and the bowls are clean with the right level of fuel. Pulled the distributor cap and it's nice and clean, no carbon or anything. I cranked up the idle speed and measured the heat on the exhaust headers and here's what I'm seeing.
1 - Firing
2 - Not firing
3 - Not firing
4 - Firing
5 - Not firing
6 - Firing
7 - Firing
8 - Not firing

Here's the spec's of the engine. 360LA with Edelbrock Performer 2176 Intake, Carter AFB 650CFM, MSD coil and plug wires, stock ignition, all stock bottom-end (maybe a purple cam according to the PO), stock fuel pump.

It seems to be directly related to fuel and the cylinders that aren't firing are all fed by one side of the dual plane manifold, could the carb have plugged up on one side somehow? It starts with a flick of the key, but it sounds like it wants to come apart at idle and really rough off idle. No backfires or valve noise.
 
Easy. Carb problem on one side or massive vacuum leak into one half of the manifold. We had one member here with a cracked intake runner but that would be rare.
 
67Dart273 said:
Easy. Carb problem on one side or massive vacuum leak into one half of the manifold. We had one member here with a cracked intake runner but that would be rare.
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Forgot to mention, I hooked up a vacuum meter to it and it was all over the map. I think it's the carb. I've rebuilt the damn thing twice in the last 6 months and I really think it's not going to work out. My cousin is sending my an old Holley 750 he isn't using anymore, so I'll bolt that on and see what gives.
 
First thing to do is see if/ which idle mixture screw is effective. Then try removing the opposite one and put your finger over the hold with it running. If that doesn't change anything, squirt som carb cleaner and then an air hose into the hole. It might not "fix" it but it might change things enough to insure that you are on the right "trail."

Back in the days when I had a SB Mopar in an old beater FJ-40 Landcruiser, the tank had a lot of rust issues, very fine powdery rust. Hell I ran three fuel filters on that thing sometimes, the powder would go right through them. There were "a few" times out in the woods when I had the top off the carb trying to get it clean. Used to run an AC sediment bowl filter at the carb. You cannot even buy those I don't believe, anymore.
 
67Dart273 said:
First thing to do is see if/ which idle mixture screw is effective. Then try removing the opposite one and put your finger over the hold with it running. If that doesn't change anything, squirt som carb cleaner and then an air hose into the hole. It might not "fix" it but it might change things enough to insure that you are on the right "trail."

Back in the days when I had a SB Mopar in an old beater FJ-40 Landcruiser, the tank had a lot of rust issues, very fine powdery rust. Hell I ran three fuel filters on that thing sometimes, the powder would go right through them. There were "a few" times out in the woods when I had the top off the carb trying to get it clean. Used to run an AC sediment bowl filter at the carb. You cannot even buy those I don't believe, anymore.
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I'll give that a try, thanks. We put in a new tank last winter, but didn't change the fuel lines, I'm probably going to regret that. I have a new fuel pump, filter and wating for the Holley to arrive, when it does I'll change it all out and hopefully she'll come alive again.
 
Did you check your plugs? May have fouled the cyls fed by that side of the carb.
 
Nothing to worry about....Engines nowadays typically run on 2, 4, 6 or 8 cylinders depending on load. Your engine is just jealous and it's trying to imitate the newer motors???......:rofl::poke::steering:
 
Bl1zzard said:
Yeah, the plugs are good nice light gray, not wet or fouled at all.
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Interesting that your spark / no spark pattern mimicks your firing order : spark, none, spark, none, etc. Do you have a true dual plane with no center notch ala LD-340? Run a open spacer. That would have gotten you home but may have melted a few pistons running very lean! What carb? I'm thinking an AFB just because the bowls are separated side to side. Plugged main jet.....
 
If you have compressed air, pull the idle mixture screws out and blow some air into the holes they were in. Just a blast in each one should do it. See if it will idle good now. If so, you need to clean the carb out again. It doesn't take much to clog the idle circuit on one side. The Holleys were even worse than the Carters for that IMO.
 
Carb has a low-speed circuit problem , on the side feeding; 2358 runners.
Remember; the carb doesn't suck.
Atmosphere, when allowed into the floatbowl,pushes fuel up the fuel wells, over the top, and into your engine thru appropriate orifices and ports; in response to the reduced pressure at said orifices/ports.

So it needs 3 things; fuel in the bowl, a reduced pressure at the orifice/port, and an unobstructed fuel path.
Air bleeds are installed at various places to break up the fuel into tiny enough droplets so it will catch fire and burn more completely , and to fine tune the strength of the pull-over.

When all-else fails; check and compare the left/right primary throttle valve position, relative to the transfer slot exposure
 
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We replaced the Carter AFB for a known good Holley 750 from my cousin, also replaced the fuel filter and fuel pump at the same time. Car started fine and as it warmed up I opened it up wide open and it sounded great. We decided to take it for a spin and as it came up to temp, it started running bad, wouldn't idle again, no power when I stepped on it. Limped it home and are back to square one. Could it be a spark problem? I seem to remember from my younger days when a coil is starting to go that it loses spark when it gets hot.
 
Yeh but a coil won't fire on just "half" the engine.
 
Sorry that was a rabbit hole. We measured the header temps again off idle and they all warmed up to about 170-200degrees. Seems the hooker ceramic headers run really cool at idle on those cylinders.
 
That doesn't make sense----that 1/2 the cylinders would be that much colder than the others. So I don't know, start over? the basics, compression. leak down tests. Devise a backyard cylinder balance test. That is, at idle, devise a way to short one cylinder at a time and look for uneven firing or non firing cylinders. What I usually do is loosen the boots on the dist and pull the wires mostly out. Use a grounded probe and a pair of insulated fuse pullers to remove a wire and short the dist. tower. Look for uneven RPM drop.

Vacuum leak/ broke plug/ bad wire/ fouled plug, you been over-revvin it? Bent pushrod damaged valve broke spring the list goes on
 
Here's a thought; IDK how to apply itto your situation, but I just thought it worth mentioning;
when the lash on a single cylinder tightens up as the temperature increases , and either valve fails to close, or closes waaay too late, the engine stalls.
When you try to restart it;
If it's the exhaust valve, the engine will pull air in from the exhaust pipe, which has no fuel in it, and it will be impossible to restart.
If it's the intake, the piston will pull in a fuel charge ok, but then on the compression stroke, push a bunch of it back into the intake, Then when it lights off, the fire goes back into the intake and pops it off in there, and it still won't start. But when it cools off, it starts and runs normally....... until it warms up again.

Like I said, In your case, I don't see this as applicable.
But now that you have experienced this with two different carbs......and the wacko manifold vacuum, IDK, it's becoming plausible. I would at least do a compression test on those 4 cylinders, first cold and then right after it stalls.
 
Thanks for suggestions guys, I'm off tomorrow for a week so I'll have some time to do some serious troubleshooting. I'll try all the steps mentioned again and see what kind of results I get. I really don't think it's a valve problem though after inspecting with the covers off, no clatter, no mis-fires. I really thought it was a fuel problem with that old Carter, but now am leaning towards spark, especially with the problem occurring after it reaches operating temperature.
 
I spent the day troubleshooting and am no further ahead. Could it be valve timing?

I pulled plugs and measured compression, it varies from 115-125 across all cylinders. Not great, but not bad. It's a stock bottom end so it will have low compression.

The plugs are all black now with the Holley installed, running really rich. Tore it apart, cleaned it up with carb cleaner, set the floats and idle mixture to factory recommended settings, ran a little better but still rich as hell (I now have a headache). I'm not convinced it's a good carb. Anyway, installed a fuel pressure meter and it's running about 6psi as expected at idle.

Measured spark gap, it's about a 1/4 inch maybe more.

Put on a timing light just to see if it moved for some reason, nope. Initial 12 degrees all in 32 degrees.

Sprayed water all around the intake manifold, no change to idle.

So here's the symptoms again. Car was running fine since the spring, took it out for a few short highway trips around 100 miles round trip. No issues. Decided to take a longer trip and about halfway there noticed the sound of the engine changed, it had a bit of a shake at highway speed. Pulled into nearest town and it wouldn't idle. Limped it home, it still maintained highway speed ok but no power. Thought it was the old Carter since half the engine was cold when idling (180 degree dual plane intake). Replaced the Carter with a Holley 750, same problem but now with added bonus of running too rich.
 
Since it appears that the carb isn't the culprit, I'd clean up the Carter and put it back on so it doesn't run so rich. I'm assuming the spark gap test you made is where you hold the end of a spark plug wire or coil wire close to a ground to see what kind of gap it will jump? Hopefully it not the spark plug gap! :lol: I don't remember, are you running stock electronic ignition? Do you have an extra module and/or distributor to try? Is your MSD coil one that uses a ballast resistor or not? Try a stock coil? When it only runs on 4 cylinders, are the dead cylinders getting spark? (you can test that with a timing light)
 
Spark gap 1/4 inch? What does that mean exactly?
 
IMO, it has to be connected to the dual plane, or to the right half of the carb. Or to anything connected to that side of the intake.
Like maybe;
a ruptured booster Diaphragm?
Or the floorjet is sucking air.
or the intake plenum, on that side, is stuffed full of rags,lol.
IDK

I just cannot see it being related to anything else. I doubt you have an intake leak at BOTH heads., and I doubt both carbs have the same problem, that is just too coincidental.

But there is one thing; I have seen plug insulators crack, and fire fine in air, but not when screwed in.But I can't figure why 4 would puke at the same time, on the same plenum, while cruising. That would be the ultimate coincidence,lol. Still, I would swap them around just because.

I'm assuming your compression test was done cold, or warm, but not HOT while in fail-mode; so IMO, it means very little. However, I'm not thinking your issue is compression related; the only thing that ties those particular 4 cylinders together, is the dual-plane plenum.
 
The odd thing is that it runs ok cold. Something is changing drastically once it warms up........and to just those 4 cylinders.
 
Compression, Ignition, Fuel

Have to go through all 3 of these systems and make sure each one is doing its job properly.

That includes fuel supply. Suspect that the '74 still has the original gas tank in it. You would be surprised how much sediment rust and gum is in the tank after 46 years.

Time for a new tank, sending unit and screen, fuel pump, and fuel filter for starters along with a fresh fill of Non-Ethanol gasoline.

That Ethanol gas really draws moisture and water in humid conditions, messing up your fuel system.

Not going to run right if you don't have clean fuel.
 
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