What's the best way to find a dead cylinder?

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cruiser

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Having rough idling/driving issues. Suspect that the engine (1974 225 bone stock everything) is missing on one cylinder. So what's the best way to determine which cylinder isn't firing? How about pointing one of those fancy temperature guns at the exhaust manifold adjacent to the suspect cylinder while the engine is running. If it's cooler than the rest, this may indicate a dead cylinder. Any other ideas out there? Thanks, all!
 
Pull one plug wire at a time with the engine idling the one that doesn't change is not contributing. You may want to do a valve adjustment that may fix your problem. Use insulated pliers or ones designed for this.
 
We used to pull a plug wire until we found the culprit. You can use a temp gun also but be aware that its normal to have some swing between cylinders. A dead cylinder should show a big difference.
 
As easy as the plugs are to get to, I'd just run a compression test.
 
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Me, I would start by removing spark plug wires one at a time to isolate the dead cyl. Then go from there to find out why it is dead.
 
Charrlie_S said:
Me, I would start by removing spark plug wires one at a time to isolate the dead cyl. Then go from there to find out why it is dead.
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Hi All: Thanks for all the helpful replies. I went with the recommendation to pull the plug wires individually while the engine was running. As I did so, I observed the engine RPM on my shop tach. Every plug that was pulled showed an RPM drop, then an increase when the plug wire was re-installed. So I was wrong about having a dead cylinder. All the cylinders are firing, which I've confirmed. Something else, probably more serious, is going on as the engine is running just like it has a dead cylinder. I'm puzzled by this and cannot find the cause. The choke plate is fully opened when warmed up. No vacuum leaks. No exhaust leaks that I can find. New plugs. Rebuilt (stock) carb. Maybe I've got something bigger going on. Had the head re-done (grind valves, new valve seats and guides, etc.) about 15K ago. Think I might need some professional help on this one. This motor should drive and idle smoothly and it doesn't. Very frustrating, but I'll keep searching.
 
Compression check will give you an indication of engine health for starters.
 
Rough engine operation could be due to a common part [ say, carb that feeds all 6 cyls ]....or....one or more cyl going out due to, say, worn rings causing plugs to oil up.

This is the BEST way to find the problem: a digital tach is very handy for this; many DVMs come with this as an attachment.
Simply hook up the tach to the coil lead. Engine idling, short out each spark plug one at a time. Note the rpm drop. If the rpm drop is about the same for each cyl, then you have a 'general' problem that affects all cyls. If one cyl has a rpm drop noticeably larger than the other cyls, then obviously that is the culprit. Problem could include head gasket, burned valve, wiped cam lobe etc, bad plug lead or spark plug, etc.
 
Id be looking at bad wires or bad distributor cap.
Remember that especially these days new doesn't necessarily mean "good".
Which is why I buy new but old parts from swap meets, eBay, etc. stuff that was made when craftsmanship and pride in ones work meant something and you didn't have to ask " was this made in the USA?". There's plenty of "common parts" like ignition parts still floating around.
I have enough of that kind of stuff stockpiled that will last my lifetime.
 
time to pull the plugs and give them a hairy eyeball. they'll whisper secrets if you ask.

run a compression test and if you have the means a leak down test.

if your cam is solid lifter, i'd be checking lash too.
 
A firing plug just means the plug is firing, not weather the cylinder is dead or dying.
As other above have said, a compression check is the easiest best test. Pull all the plugs, wire the carb open, make sure the battery is all the way up, then have at it.
The numbers you get (write em down) will tell the story.
Edit: a very weak cylinder or two, time to pull the valve cover and check rocker movement, and lash, (if a solid cam).
 
Check/listen for a vacuum leak, plug any hoses to carb.
Wiggle carb for loose bowl/base, tighten base gskt bolts .
Good luck
 
Bewy said:
Rough engine operation could be due to a common part [ say, carb that feeds all 6 cyls ]....or....one or more cyl going out due to, say, worn rings causing plugs to oil up.

This is the BEST way to find the problem: a digital tach is very handy for this; many DVMs come with this as an attachment.
Simply hook up the tach to the coil lead. Engine idling, short out each spark plug one at a time. Note the rpm drop. If the rpm drop is about the same for each cyl, then you have a 'general' problem that affects all cyls. If one cyl has a rpm drop noticeably larger than the other cyls, then obviously that is the culprit. Problem could include head gasket, burned valve, wiped cam lobe etc, bad plug lead or spark plug, etc.
Click to expand...
Wouldn’t the offending cylinder have LESS R.P.M. drop due to its lesser contribution to the cycle?
 
You mentioned that it runs like it has a dead cylinder. What does that mean? No power? Can you describe the symptoms a little more?
 
It's just gotta be the ballast resistor
(The smilies didn't show up huh)
 
A dead cylinder would have little to no change at all not much of an rpm drop. You technically don't need a tach you would pull wires until you got to the cylinder that didn't respond at all when the wire is pulled. You could use an indication timing light on each wire to check erratic spark or bad wires
 
One that fires.. you can drag the plastic handle of a screwdriver on the corresponding exh manifold runner nearest the head and it'll slide like melted butter ... if its a dead cyl, it'll stick and or be like draging it on asphalt.
 
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