Rough idle but drives very smooth.

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Mako21

Mako21

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My slant six is having a persistent issue. It idles very rough (on three cylinders) but drives very very smooth. Black soot out the exhaust and strong gas smell when idling only. I am fouling plugs with carbon constantly.

When I hook up a timing light, cylinder 1,2 and 4 have a very random/unsteady firing while the others 3,5,6 are steady consistent. When revved, all cylinders have a good consistent firing.

I’ve replaced the cap and rotor with two new sets, the plug wires three times and spark plugs countless times experimenting with wider and narrower gaps. I also replaced the coil with a new one. I purchased a new battery as well. I completely disassembled and rebuilt the carburetor. Valve lash done to spec three times with engine hot. Fuel vapor mod completed with new tank and all new R12 gas lines and good filter.

What am I missing? Could it be a bad distributor? My car is a stock 1976 slant 6. Still running ignition factory parts except what I mentioned above. I’m thinking something is up with the ignition. Originally I though maybe a carb issue running too rich and fouling plugs, but now I’m thinking ignition.

Any ideas? Thanks for your help.
 
It sounds like carb issue. What is the choke situation? It sounds like it is choking itself, but at high rpm that would disappear. Make sure the choke plate is fully open when at operating temperature.
 
I was thinking it would be a failing ignition control module. Do these cause problems as they slowly begin to deteriorate?
 
Mako21 said:
I was thinking it would be a failing ignition control module. Do these cause problems as they slowly begin to deteriorate?
Click to expand...
what carb is on it?
What year car?
sounds like its running damn rich to me, fouling plugs and such. But when you rev it up it gets enough air to clear out and start firing the other three cylinders.
EDIT: you said its a 1970, but then said you suspect a faulty ignition control module. I thought electronic ignition didnt come on cars until 73?.
correct me if i'm wrong.
and also, pictures please
 
It’s a 1976 year with factory HEI. Carb is the factory Holley 1945. I don’t think it’s a choke or rich running issue.

The reason I rebuilt the carb was because this same stalling issue was occurring prior to the rebuild and after the rebuild still have the same issue.
 
I hate to keep throwing money at it but at this point am wondering if it’s the ignition control module or distributor itself worn out.
 
I have never had a pickup coil come and go like that...(idle bad vs driving good) When they go...they typically die entirely, then magically work an hour later (in my experience)

how do you know you have a misfire at idle? just a weak timing light signal? have you physically looked at the spark with a plug grounded on the engine?

wouldn't shock me if the mechanical advance in the 60 year old distributor was getting hung up maybe? or the shaft was sloppy...or the block bushing.



Bad ECU or grounding?

I'm not saying throwing parts at it either, but you're under $100 to attack the distributor and ECU combined, or less if you do a module instead of the Dist...but make sure the distributor shaft isn't a wobbly mess. or even the cam bushing / gear.

Complete distributors looks to go for around $70
1976 DODGE DART 3.7L 225cid L6 Distributor | RockAuto

or if you want to try a module first, those are $15
1976 DODGE DART 3.7L 225cid L6 Distributor Pickup Coil | RockAuto

ECU $18-40
1976 DODGE DART 3.7L 225cid L6 Ignition Control Module (ICM) | RockAuto
 
First make sure the firing order is correct, and if it is, then;
With the engine idling, Go put your hand on the tailpipe so as to nearly choke it off. If it feels like every so often the engine is sucking your hand in, then one or more of your exhaust valves may not be sealing.
It could just be carbon on the seats, but it could be other things as well.
 
Mako21 said:
My slant six is having a persistent issue. It idles very rough (on three cylinders) but drives very very smooth. Black soot out the exhaust and strong gas smell when idling only. I am fouling plugs with carbon constantly.

When I hook up a timing light, cylinder 1,2 and 4 have a very random/unsteady firing while the others 3,5,6 are steady consistent. When revved, all cylinders have a good consistent firing.

I’ve replaced the cap and rotor with two new sets, the plug wires three times and spark plugs countless times experimenting with wider and narrower gaps. I also replaced the coil with a new one. I purchased a new battery as well. I completely disassembled and rebuilt the carburetor. Valve lash done to spec three times with engine hot. Fuel vapor mod completed with new tank and all new R12 gas lines and good filter.

What am I missing? Could it be a bad distributor? My car is a stock 1976 slant 6. Still running ignition factory parts except what I mentioned above. I’m thinking something is up with the ignition. Originally I though maybe a carb issue running too rich and fouling plugs, but now I’m thinking ignition.

Any ideas? Thanks for your help.
Click to expand...
Well, 1-5-3-6-2-4-1-5-3-6-2-4-1-5-3-6-....
See a pattern here?
Check to see if there is something bent or wobbling, or damaged/worn teeth on the dizzy gear.
Or the cam sprocket may be starting to lose the plastic teeth on one half...
 
Last edited:
Clue: first 3 lines of the post.

Black soot, gas smell.

It is a carb problem. Maybe a leaking float, high fuel level, causing extra rich idle. With throttle opened for more air, extra fuel gets used up & runs smooth.
 
Well I bought all new spark plugs and change them out and the car is running normal.

I do have a slight popping coming out the exhaust and barely noticeable stumble interrupting the otherwise smooth idle.

So now thinking it’s a carb issue, I closed the idle air mixture screw all the way in (Holley 1945) and she increased idle, idles better, and is much more responsive while driving. So now I have the idle air mixture screw fully seated clockwise in.

Not sure what’s happening here.
 
It’s overly rich. Probably from too high a float level or needle and seat leaking. Take off the air cleaner, start the engine and with it idleing look down the carb. Do you see fuel dripping anywhere?
 
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