Why is my slanty chugging? (Problem solved, for now anyway)

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cruiser

cruiser

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Hi All: Here's one for you expert slant six tuners. I have an interesting thing happening with my 225 in my 1974 Duster (torqueflite auto). First of all, I just love the motor. It runs great and has plenty of power for my car. I've restored it completely stock. The replacement 1974 model year motor has 100K miles on it and has good compression. The head and valve train were overhauled 5K miles ago. The exhaust system is totally stock, including the resonator which came automatically with the Noise Reduction Package. The carb is a rebuilt stock one barrel Holley 1945, stock choke setup. New clean NGK ZFR-5N plugs with gaskets removed, gapped to .035" as the factory shop manual recommends. Here's the problem, if it is in fact a problem at all. The car has a very nice butter smooth idle when warmed up. Idle RPM set to the factory recommended setting. If I stand by the exhaust pipe and listen to the exhaust, it "chugs" or "puffs" as it idles - maybe one or two chugs every ten seconds. The engine doesn't seem to slow down when it's going this. I've adjusted the idle mixture screw, which will increase or decrease the amount of "chugging", but I can never find that sweet spot where it completely goes away. I'm baffled. Do these individual "puffs" represent a failure of a cylinder to fire at that moment? if so, it doesn't seem to slow the engine down. Or is this "puffing" actually normal with a resonator equipped exhaust system, and it's not really missing at all. I just can't figure it out. Ideas? Slant Six Dan, if you're out there, please weigh in. Thanks, everybody!
 
How old are the ignition wires? I have seen wires that will occasionally arc even though they ohm out OK. This is due a breakdown in the insulation by antifreeze soaking. Try looking at your motor running at night to see it.
 
Keep an eye on the exh valves burning on no.4/5.

Could be muffler collapsing inside.

Does it suck a peice of paper flat at the tail pipe when running?
 
cruiser said:
Do these individual "puffs" represent a failure of a cylinder to fire at that moment?
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Yes. That splutter/chuff/pluff sound is a single-cylinder misfire. A less primitive ignition system, aside from its other benefits, will likely reduce or substantially eliminate the misfiring—HEI upgrade with attendant upgrades to the spark plug wires and a carefully optimized distributor cap and rotor (per the suggestions in this long but worthwhile thread).

But keep in mind we are talking about a car most of 50 years old, with a not-very-well-designed carburetor known, especially in its first couple years ('74-'75) for poor mixture distribution. If the only symptom is the audible misfiring, you'd be perfectly well within reason to just quit putting your ear near the tailpipe.
 
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Murray said:
How old are the ignition wires? I have seen wires that will occasionally arc even though they ohm out OK. This is due a breakdown in the insulation by antifreeze soaking. Try looking at your motor running at night to see it.
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Ignition wires are new.
 
slantsixdan said:
Yes. That splutter/chuff/pluff sound is a single-cylinder misfire. A less primitive ignition system, aside from its other benefits, will likely reduce or substantially eliminate the misfiring—HEI upgrade with attendant upgrades to the spark plug wires and a carefully optimized distributor cap and rotor (per the suggestions in this long (but worthwhile) thread).

But keep in mind we are talking about a car most of 50 years old, with a not-very-well-designed carburetor known, especially in its first couple years ('74-'75) for poor mixture distribution. If the only symptom is the audible misfiring, you'd be perfectly well within reason to just quit putting your ear near the tailpipe.
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Dan - the audible misfire at idle is the only symptom. The engine runs really well at highway speeds. If it's misfiring on the highway, I certainly cannot tell. Maybe I'm just thinking about it too much. Your point about a nearly fifty year old car is well taken.
 
Bad fuel distribution or incorrect a/f.
Be it Carb or intake.
Unless the spark or timing is unstable...it should fire every cyl like clockwork with a stock cam.
 
It's entirely possible the car did it when new. My 170 did it. I just drove heck out of it.
 
I would get rid of that factory exhaust, and go with 2 1/4 pipe. She will run even better. Maybe it's a little vacuum leak
 
Okay, I just replaced all six plugs with new NGK ZFR-5N plugs, and the problem went away.This car does seem to go through a lot of spark plugs. My maintenance records that came with the car (going back to 1973) reveal the same thing. Lots of spark plug changes. Anyhow, thanks to all who weighed in on the problem - it's much appreciated!
 
How often are you changing the plugs? Used to be every 12 thousand miles back in the day I'm thinking...I remember my mom changing points/plugs every fall for winter in Pa on a points car 66 Chevelle with 283 .
 
the plugs on my HO367 were new in 1999, over 100,000 miles ago. Car still "misses" same as it always has...... lol, due to the size of the cam no doubt..............
@cruiser take a good look at the porcelains of those plugs; I'll bet you will find a cracked insulator. It may look like a hairline pencil track.
When that happens, then at idle, the spark can leak to ground, but when you increase the load and/or rpm, two things happen;
1) the spark energy is increased, and
2) the time to leak is reduced.
So it is entirely possible for this to fit your symptoms.
As the crack gets larger, and fills with soot, the symptoms get worse, and eventually, if left too long, that porcelain my rupture and cast off very hard glass bits..
The crack is usually caused by detonation, so if this is happening on an on-going basis, you better review your ignition timing program, or your lugging habits.
But good news, it may not be a crack at all, but just a carbon-track. You'll have to figure out which it is.
 
15k miles with leaded gasoline, 30k with unleaded was the standard in the '70s. We have better spark plugs now, and cleaner-burning gasoline, so you should be able to go a long time on a set of plugs. If putting in new plugs stopped the misfire, it reinforces that upgrading your ignition system (as a whole) will likely knock down the car's seeming need for brand-new plugs to run without misfire.
 
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slantsixdan said:
15k miles with leaded gasoline, 30k with unleaded was the standard in the '70s. We have better spark plugs now, and cleaner-burning gasoline, so you should be able to go a long time on a set of plugs. If putting in new plugs stopped the misfire, it reinforces that upgrading your ignition system (as a whole) will likely knock down the car's seeming need for brand-new plugs to run without misfire.
Click to expand...
Thanks Dan!
 
Plugs might be getting carbon coated from a rich mixture or poor oil control in the chamber. Plugs then misfire & new plugs 'cure' the problem.
 
Well, it ain't like plugs for these engines are all that expensive or all that tough to put in, so what's the big deal? I change mine even on my newer vehicles every 30k miles whether they need it or not.(1 each... 3.9, 318 magnum,360 magnum)
 
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