Solving a Misfire

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John Martin

John Martin

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So I got my new to me 1970 Dart in January and drove it back from Albany, Oregon to Chicagoland. 225 slant six with a TorqueFlite. It had 106k on it. It was running fair on the trip home but it got slightly worse and worse. It has been daily driven almost since it got back.

The car is totally unrestored, engine has not been touched, until I bought it, since Mound Rd. put it together. It had a leak between cylinders 5 & 6 so the head gasket was redone, it had an exhaust leak at 6 that was fixed as well. Many maintenance items have been taken care of, including a lash adjustment to .010” intake and .020” exhaust hot while running, carburetor rebuild, timing is at 5° BTC but will not go any more advanced.

New:

PCV
Spark Plugs
Cap
Rotor
Condenser
Points
Fuel filter

Spark plugs are sooty but dry, lifters and pushrods all spin when engine is running. I am about to pull my hair out because it seems no matter what I do this misfire will not go away.
 
Verify that the timing order is correct.

If correct, try another original coil.
 
John Martin said:
So I got my new to me 1970 Dart in January and drove it back from Albany, Oregon to Chicagoland. 225 slant six with a TorqueFlite. It had 106k on it. It was running fair on the trip home but it got slightly worse and worse. It has been daily driven almost since it got back.

The car is totally unrestored, engine has not been touched, until I bought it, since Mound Rd. put it together. It had a leak between cylinders 5 & 6 so the head gasket was redone, it had an exhaust leak at 6 that was fixed as well. Many maintenance items have been taken care of, including a lash adjustment to .010” intake and .020” exhaust hot while running, carburetor rebuild, timing is at 5° BTC but will not go any more advanced.

New:

PCV
Spark Plugs
Cap
Rotor
Condenser
Points
Fuel filter

Spark plugs are sooty but dry, lifters and pushrods all spin when engine is running. I am about to pull my hair out because it seems no matter what I do this misfire will not go away.
Click to expand...
Good work on the tune up and lifter adjustment, but what else can you tell us about the mis fire?
Is it at idle only? Does it change as the motor warms up, is the mis fire out the exhaust or back through the carb?
Also, can you do a compression test. Many auto part stores have loner test gauges.
Also, with the timing light attached, and the vacuum advance hose to the distributor plugged, when the engine is rev’ed, how much does the timing bounce around. Old slants typically have worn timing chains,,
And one more item, your changed part list does not include spark plug wires.
On the timing adjustment, on the bottom of the distributor there is a bolt that when loosened allows an additional range of motion to the distributor timing in addition to the bolt that goes into the block.
 
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my68barracuda said:
Good work on the tune up and lifter adjustment, but what else can you tell us about the mis fire?
Is it at idle only? Does it change as the motor warms up, is the mis fire out the exhaust or back through the carb?
Also, can you do a compression test. Many auto part stores have loner test gauges.
Click to expand...

It starts immediately when engine is running. It seems to be less noticeable on fast idle, but on curb idle it is obvious. Once it’s warm it has “puff-puff” out of the tailpipe.

It does not seem to be an issue while cruising. I do not hear of feel a miss if the car is moving.

I was going to do a compression test but I was too late to the parts store and they closed on me. I will get a compression test done over the weekend.
 
Where is the dwell set?
 
John Martin said:
It starts immediately when engine is running. It seems to be less noticeable on fast idle, but on curb idle it is obvious. Once it’s warm it has “puff-puff” out of the tailpipe.

It does not seem to be an issue while cruising. I do not hear of feel a miss if the car is moving.

I was going to do a compression test but I was too late to the parts store and they closed on me. I will get a compression test done over the weekend.
Click to expand...
And see how much the timing is bouncing around, if there is a lot of slop in the timing chain that will affect the ignition - valve close timing.
Another thought is to try to isolate the mis fire by with the engine running and in the put put mode, pulling spark plug wires one at a time see how that affects rpm and the misfire.
 
RustyRatRod said:
Where is the dwell set?
Click to expand...

You mean the gap in the points? I would imagine it is set to spec. The guy who did the head work changed the point for me. I need to do engine mounts so I can pull the distributor since it is in the way anyways.
 
my68barracuda said:
And see how much the timing is bouncing around, if there is a lot of slop in the timing chain that will affect the ignition - valve close timing.
Another thought is to try to isolate the mis fire by with the engine running and in the put put mode, pulling spark plug wires one at a time see how that affects rpm and the misfire.
Click to expand...

I have tried this, it gets worse when the spark plugs are pulled for every cylinder except for 6. It either has no change in the vibration or it gets better.
 
John Martin said:
I have tried this, it gets worse when the spark plugs are pulled for every cylinder except for 6. It either has no change in the vibration or it gets better.
Click to expand...
Sounds like the issue is at #6,,
Try a new spark plug wire there, if that does not help the compression test should show something,,
 
my68barracuda said:
Sounds like the issue is at #6,,
Try a new spark plug wire there, if that does not help the compression test should show something,,
Click to expand...
I already have, I have swapped the spark plug wires on all six, and the cap and rotor again. Overall slightly better but it always comes back.
 
Could be compression, burned valve, misadjusted valve, valve sticking/ bent

Could be ignition--even a bad plug.

Could be something AKA leak causing "lean" in one cylinder. Pay particular attention to "how even" plugs look

Here is what you do

Pull the plug and coil wires and check them for continuity with an ohmeter. Try new plugs. Plugs CAN go bad with no apparrent external "look"

Beg borrow or steal a compression gauge. Google "leak down tester" you can actually make one if you are handy, and use your compression gauge for "parts" for the leak down tester. Lacking that, buy or make an air fitting.......or use the parts of your compression gauge........so you can inject air into each cylinder and listen at exhaust, carburetor, etc for leaks

Next do a power/ load test. This will show what is weak and maybe if there's an ignition problem You really need a tach (tach/ dwell meter) and you can find these used dirt cheap

"Rig" a small probe, screwdriver, with a ground wire. Pull the plug wires up a bit off the distributor so they are loose and so the boots are up off the distributor towers Leave the wires in enough they will still work OK. Another thing you could do here is insert small brads/ finishing nails, pieces of wire/ paper clips down alongside each wire. Now go around the cap with your probe and short each tower. Listen for the drop in RPM it should be even with each cylinder. Also watch the spark as you approach and ground out each tower. The spark should be "hot" and even at each cylinder.
 
John Martin said:
You mean the gap in the points? I would imagine it is set to spec. The guy who did the head work changed the point for me. I need to do engine mounts so I can pull the distributor since it is in the way anyways.
Click to expand...

Never "assume".
 
67Dart273 said:
Could be compression, burned valve, misadjusted valve, valve sticking/ bent

Could be ignition

Could be something AKA leak causing "lean" in one cylinder. Pay particular attention to "how even" plugs look

Here is what you do

Beg borrow or steal a compression gauge. Google "leak down tester" you can actually make one if you are handy, and use your compression gauge for "parts" for the leak down tester. Lacking that, buy or make an air fitting.......or use the parts of your compression gauge........so you can inject air into each cylinder and listen at exhaust, carburetor, etc for leaks

Next do a power/ load test. This will show what is weak and maybe if there's an ignition problem
Click to expand...

All the plugs are dry but sooty. I know it means it’s rich, but the dry part is good. The last slant I owned had a wet plug on 1 all the time and it turns out it had both compression rings broken.

Valves have been adjusted. I just rechecked them while the valve cover was off, and they have stayed the same.

I was going to compression test it tonight but ran out of time. I would do a CLT but a chipmunk chewed through my air hose. He is dead for his destructive acts.
 
RustyRatRod said:
Never "assume".
Click to expand...

I know, but he surprised me as I had not asked for the points to be changed, and the car ran fine except for the issue we are talking about now. This weekend will be a whirlwind of checking every, believe me.
 
Is the center contact of the dist cap good looking?

I had a dist that had excessive up and down movement. In 50 miles it destroyed the center contact. And was misfiring all over the place. Replaced dist and no more misfire.
 
Dana67Dart said:
Is the center contact of the dist cap good looking?

I had a dist that had excessive up and down movement. In 50 miles it destroyed the center contact. And was misfiring all over the place. Replaced dist and no more misfire.
Click to expand...

Seems fine but I am looking into getting it checked out and rebuilt if needed.
 
Mine seemed fine too, I couldn't figure out how 1/4 inch of play could cause the issue. All symptoms pointed to the dist movement. My summation was that the shaft was moving up and down making and braking the center contact, burning the contact and causing misfires
 
John Martin said:
You mean the gap in the points? I would imagine it is set to spec. The guy who did the head work changed the point for me. I need to do engine mounts so I can pull the distributor since it is in the way anyways.
Click to expand...

You can set points by gapping them or if you have a dwell meter you crank the engine and gap points to correct dwell setting. When I had dual points in the day I would leave a matchbook cover in one set of points a set each point to say 16 each giving me a dwell total of 32. (Cant remember actual setting # just using 32 for example)
 
In my experience 10/20 valve-lash is too tight. I set mine .013/.023 hot; it will idle waaaaay smoother.
While the cover is off Put #6 at TDC compression and then rool the engine backward to pull the piston down a bit, then tap the rocker arms to pop the valves, listening to their sounds. They should make nice happy clucking sounds as they slam back onto the seats.
 
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AJ/FormS said:
In my experience 10/20 valve-lash is too tight. I set mine .013/.023 hot; it will idle waaaaay smoother.
While the cover is off Put #6 at TDC compression and then rool the engine backward to pull the piston down a bit, then tap the rocker arms to pop the valves, listening to their sounds. They should make nice happy clucking sounds as they slam back onto the seats.
Click to expand...

Factory spec is too tight? Mine likes it, and it idles fine except for one cylinder. I don’t think there is any better way than to do it running. I had it clattering like a diesel and then it went quiet at the factory specs.
 
Check the pads on your rockers.
They might have ruts in them the width of the valve stems. If they do, and you use a standard width feeler gauge, then; the standard feeler will span over the rut, and add it's .010 to the depth of the rut. The result is; who knows what you actually set the lash too.
Solid lifter slantys have to be slightly noisy. If you set the valves too tight to make them quiet, then the valves will not get enough seat time and you risk burning them up. Or, to avoid that, you can just run the carb rich to provide cooling. So they run ok, but the engine sucks gas.
Or you can put a 160 stat in it. Now 10/20 is adequate, but power? where did it go. Oh into the rad, I get it.

Maybe I should have been more specific; lemmee restate;
I run my slanties HOT, with a minimum coolant temp of 195. And so
AJ/FormS said:
In my experience 10/20 valve-lash is too tight. I set mine .013/.023 hot; it will idle waaaaay smoother.
Click to expand...
; make more low-rpm torque, get better fuel mileage, and the valves will last waaaay longer. My head is pushing 500,000 kilometers/300,000 miles.
but I suppose my tune might have something to do with that.

Bottom line is .010/.020 compared to .013/.023 will not make any more power, and the benefits of .013/.023 are many;
so why run them tight...

could it be that #6, is the only one that is actually at .010/.020..... and all the others are unknown because of the ruts? IDK but it's something to check out.
 
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AJ/FormS said:
Check the pads on your rockers.
They might have ruts in them the width of the valve stems. If they do, and you use a standard width feeler gauge, then; the standard feeler will span over the rut, and add it's .010 to the depth of the rut. The result is; who knows what you actually set the lash too.
Solid lifter slantys have to be slightly noisy. If you set the valves too tight to make them quiet, then the valves will not get enough seat time and you risk burning them up. Or, to avoid that, you can just run the carb rich to provide cooling. So they run ok, but the engine sucks gas.
Or you can put a 160 stat in it. Now 10/20 is adequate, but power? where did it go. Oh into the rad, I get it.

Maybe I should have been more specific; lemmee restate;
I run my slanties HOT, with a minimum coolant temp of 195. And so

; make more low-rpm torque, get better fuel mileage, and the valves will last waaaay longer. My head is pushing 500,000 kilometers/300,000 miles.
but I suppose my tune might have something to do with that.

Bottom line is .010/.020 compared to .013/.023 will not make any more power, and the benefits of .013/.023 are many;
so why run them tight...

could it be that #6, is the only one that is actually at .010/.020..... and all the others are unknown because of the ruts? IDK but it's something to check out.
Click to expand...

No ruts in the rocker arms what so ever. They were all cleaned, and put back in order. The valves were adjusted with the engine running. I tried to run it looser and it was clattering badly.
 
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