Misfire in 77 New Yorker 400

Rusell Petry

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Hi, Everyone!

I have a1977 NY with a 400, 60,000 original miles. I have a miss in the engine, it get worse when it is warmed up. I have new plugs with the correct gap, the distributor cap and rotor are clean like new. The wires are all popped on correctly. I cannot seem to figure it out. Also the spec label at the radiator says 20⁰ btc for timing, is that correct for this car? It's at the very bottom of the timing marks next to the flywheel. Thanks in advance for any pointers.
 

CBODY67

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IF the car still had the orig Lean Burn system operational, that base timing spec on the decal can be correct.

As for the "miss", check all of the vac hoses for leaks as that can lead to a "lean misfire" situation. With the choke operating, that lean situation can be covered by the choke's enrichment until the choke comes off as the engine warms up and gets to operating temp. After dark, in a dark area, look for jumping sparks from the plug wires, sparks jumping to ground, too. A leaner mixture is harder to fire, so the spark goes to the point of least resistance, as in a crack in the wire itself. You might also hear a "SNAP" each time the spark jumps to ground.

Do gently check the carb base mounting nuts to see if they are all snug.

Just some thoughts,
CBODY67
 

Rusell Petry

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IF the car still had the orig Lean Burn system operational, that base timing spec on the decal can be correct.

As for the "miss", check all of the vac hoses for leaks as that can lead to a "lean misfire" situation. With the choke operating, that lean situation can be covered by the choke's enrichment until the choke comes off as the engine warms up and gets to operating temp. After dark, in a dark area, look for jumping sparks from the plug wires, sparks jumping to ground, too. A leaner mixture is harder to fire, so the spark goes to the point of least resistance, as in a crack in the wire itself. You might also hear a "SNAP" each time the spark jumps to ground.

Do gently check the carb base mounting nuts to see if they are all snug.

Just some thoughts,
CBODY67
Thank you for those tips. The lean burn system was disabled decades ago. Which gets me to thinking, if that was the setting with lean burn active, then maybe it is too low without it?

I will check out the wires in the dark. Thank you again.
 

CBODY67

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The factory Tune Up Specs label is ONLY for the factory-produced vehicle. IF the LB system was removed, then you'll need to determine how much mechanical advance is in the distributor and set the initial timing so that you end up with about 36-38 degrees BTDC total advance.

Finding out what's in the distributor? You can use a dial-back timing light and a dwell tach that goes to 4000 rpm. Just hook up the timing light, have an assistant run the carb linkage up to about 4000rpm, and see where the total advance comes out to be. Might end up with an initial base timing of 5-12 degrees BTDC?

Or, if somebody like @halifaxhops has a known good electronic distributor and knows how much advance is in the distributor, you might do a swap so you know what you have and can deal with. Or send him your distributor and let him check it out.

If you look at the listings in places like RockAuto for reman distributors, you'll find a large number of factory part numbers their ONE distributor covers. Which means "will fit" rather than "specific fit", so the base timing will need to be adjusted to match what's in the distributor rather than following factory settings, to me.

Or . . . you could set the base timing to about 10 degrees BTDC and see how it works. If it does not ping on accel with the fuel you desire to use, then all can be good. Or you might edge it up to 12 degrees, if desired, with no clatter. Or if it clatters, back the timing back until it doesn't and slightly increase the hot base idle speed to keep it at about 700rpm in "P". Different little ways to deal with the situation.

Enjoy!
CBODY67
 

halifaxhops

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Thanks Cbody. I know you probably do not have a cyl kill meter or a scope, but try to figure out which cyl is missing and go from there, can be a bunch of things that can cause a miss. I hvave no idea of what you know about cars. There is a redneck way to isolate a cyl by pulling wires one at a time and if the miss has no change that is where to start. The good ones pulled will make it worse. DO IT ONE AT A TIME WITH THE ENGINE OFF so you wont get juiced. Just an Idea.
 

halifaxhops

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If you look at the listings in places like RockAuto for reman distributors, you'll find a large number of factory part numbers their ONE distributor covers. Which means "will fit" rather than "specific fit", so the base timing will need to be adjusted to match what's in the distributor rather than following factory settings, to me.

Or . . . you could set the base timing to about 10 degrees BTDC and see how it works. If it does not ping on accel with the fuel you desire to use, then all can be good. Or you might edge it up to 12 degrees, if desired, with no clatter. Or if it clatters, back the timing back until it doesn't and slightly increase the hot base idle speed to keep it at about 700rpm in "P". Different little ways to deal with the situation.

Enjoy!
CBODY67

Have to agree on the distributors from parts stores. They basically will get the car going but do not have the specific curve for your car. They are set up to work over a spread of years and set kind of in the middle.
 

3175375

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Thanks Cbody. I know you probably do not have a cyl kill meter or a scope, but try to figure out which cyl is missing and go from there, can be a bunch of things that can cause a miss. I hvave no idea of what you know about cars. There is a redneck way to isolate a cyl by pulling wires one at a time and if the miss has no change that is where to start. The good ones pulled will make it worse. DO IT ONE AT A TIME WITH THE ENGINE OFF so you wont get juiced. Just an Idea.
Removing the plug wires with the engine off takes all of the fun out of it!

Otherwise,

^^^^ what he said.
 

Rusell Petry

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The factory Tune Up Specs label is ONLY for the factory-produced vehicle. IF the LB system was removed, then you'll need to determine how much mechanical advance is in the distributor and set the initial timing so that you end up with about 36-38 degrees BTDC total advance.

Finding out what's in the distributor? You can use a dial-back timing light and a dwell tach that goes to 4000 rpm. Just hook up the timing light, have an assistant run the carb linkage up to about 4000rpm, and see where the total advance comes out to be. Might end up with an initial base timing of 5-12 degrees BTDC?

Or, if somebody like @halifaxhops has a known good electronic distributor and knows how much advance is in the distributor, you might do a swap so you know what you have and can deal with. Or send him your distributor and let him check it out.

If you look at the listings in places like RockAuto for reman distributors, you'll find a large number of factory part numbers their ONE distributor covers. Which means "will fit" rather than "specific fit", so the base timing will need to be adjusted to match what's in the distributor rather than following factory settings, to me.

Or . . . you could set the base timing to about 10 degrees BTDC and see how it works. If it does not ping on accel with the fuel you desire to use, then all can be good. Or you might edge it up to 12 degrees, if desired, with no clatter. Or if it clatters, back the timing back until it doesn't and slightly increase the hot base idle speed to keep it at about 700rpm in "P". Different little ways to deal with the situation.

Enjoy!
CBODY67
Thank you, this helps. I will try this approach on the timing.
 

halifaxhops

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Watch out with the dial back if it has a knob and dial scale, notorious for being wrong, digital are right on. Timing tapes are the way to go IMO. You can print one out for a one time need, forget where I found them. Just FYI
 
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Milo

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Like one of the first posters said if all other things checks out Check for vacuum leaks you can disconnect and plug the lines that go to the heater A/C switch and see if it makes a difference also brake booster but be careful the pedal will be a lot harder without the booster. Also check around the intake manifold gaskets for leaks. you can hook a tach up and spray some carb cleaner around the gaskets and see if the rpms change. Just another thought Hope you get it figured out
 

CBODY67

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Watch out with the dial back if it has a knob and dial scale, notorious for being wrong, digital are right on. Timing tapes are the way to go IMO. You can print one out for a one time need, forget where I found them. Just FYI
Source or link to the digital versions?
 

Rusell Petry

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Thank you everyone! I started looking deeper into the system over the weekend and found a bad spark plug cable. I put a new set on the car and it runs smooth again. sometimes the simplest things get overlooked, but glad it wasn't anything major.

CBODDY67, your idea was a good one. I set the timing at 10 degrees BTDC and it it now runs a lot better. Thank you everyone for the ideas and tips.
 
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