Engine misfiring

[Luke85]

I have an 85 GT (5.0, 5-speed, carb) that is misfiring. The suspect cylinders are 1, 2, and 6 as those exhaust pipes are warm while the others are hot, and those plugs will foul if I run it long enough.

What can cause 3 cylinders to be sub-par?

A little background. The car has been in storage for around 30 years, having been my daily driver before that. I am the original owner and the car is in very nice, original condition, with 79,000 miles. The car ran fine when I first started to resurrect it in 2019 (new vacuum lines, cooling hoses, etc.) and was okay after all that. This missing seems to be recent, though I am unable to pinpoint the exact day since I only 3 two weeks ago got a license plate for it and was not started all winter.

What is new: cap, rotor, plug wires, spark plugs (ACDelco CR43TS), and ignition coil. I replaced all the underhood vacuum lines (those that I could find, anyway), and as far as I know I did that right.

The carburetor gives a good clean squirt from the accelerator pump.

I swapped the # 5 & 6 plugs to see if the problem moved to the #5 cylinder. The #6 cylinder remains warm, so the plugs don’t seem to be the problem. I can smell unburnt fuel, presumably from the three warm cylinders.

A timing light shows activity on all 8 wires, so spark would seem to be okay. The distributor rotor is in about the right place in the rotation when cylinder no. 1 is TDC.

Firing order, as per the manual, is 1-5-4-2-6-3-7-8. This differs from the underhood decal which states the order should be 1-3-7-2-6-5-4-8. I tried that and the car wouldn’t even start. Can someone tell me what they use on their 5.0?

Compression is good on all 8 cylinders, ranging from 145 to 160 psig.

I'm stumped. Any ideas?

[Four Eyed Fox]

if it is a dual plane intake does the carb feed those cylinders? I am thinking its possible most dual planes run in as y pattern oin the intake runners, could be lean or rich on that side of the 4 barrel. Only other thing might be intake gaskets?

[Luke85]

The intake is whatever came on the car from the factory, and I don't know about it being dual plane. Cylinders 1 & 2 are on the passenger side; #6 on the driver side. When revving the motor I'll get spits and pops from the carb. Seems like an ignition issue vice fuel but I'm uncertain.

[Four Eyed Fox]

Pops from carb possibilities can be timing setting, the cam timing(worn timing chain) or valvetrain /camshaft issues. OR a crossed spark plug wire,I'd double check firing order fist then go from there. Its easy to cross wires doing a tuneup.

[Luke85]

I agree it's more likely something simple like a plug wire type of issue, since it seems to have a rather sudden onset vice gradual degradation. Firing order verified as 1-5-4-2-6-3-7-8, though I'm concerned about the underhood decal telling me it should be otherwise. I'd still like to hear from other 5.0 folks what they're running.

[Four Eyed Fox]

keep in mind the 5.0 has one firing order, and the 5.0 HO has a different firing order.

the H.O firing order is: 1-3-7-2-6-5-4-8

[Luke85]

Now THAT'S a distinction I didn't know. I'm using the wrong firing order. Back to the garage...

[Luke85]

It doesn't even start with this firing order. Can you verify the cylinder numbering order? I have 1-2-3-4 running front to back passenger side, 5-6-7-8 driver side.

[wraithracing]

Yes you are correct in your cylinder number. Make sure you wired the distributor cap correctly in regards to firing order and direction of rotation for the distributor. See the picture below to help. You will also want to make absolutely sure where on the distributor your #1 cylinder is located. Usually pulling the number one plug and bumping the engine over onto the compression stroke and then checking the distributor for where the rotor is point will do the trick. Good luck!

[Four Eyed Fox]

ok your cylinder numbering looks correct, how about "distributor rotation" is the firing order on the cap set for a counter clockwise rotation?

[Luke85]

CCW yes. The picture suggests the #1 cylinder is at the center back of the cap. Is that a correct read? If so, mine is set up wrong.

[Four Eyed Fox]

yes number one should be at about "12 oclock" when looking down at the cap from the front of the car, there also should be a number 1 embossed on the cap in the plastic.

[gr79]

Redo the wire order on the cap. #1 has to be on the correct cap post.
Engine decal data is the one to use. New vac line threw the ign timing off?
I like to temp number id sp wires at both ends with tape or something.
A stumper for sure being the only owner and know the history.

[Luke85]

Well, that was it. I had the #1 plug wire (so, all the rest as well) in the wrong place. That, and the non-HO firing order. I'm not sure I want to know how I hosed that up so bad. [Note to self: check for sufficient brain wave activity before working on car. Especially when it's been parked for over 25 years.]

Thanks for the help, all.

Also, here's the car (post test drive, of course):

[Four Eyed Fox]

Nice !��

[gr79]

Always good to hear a success story like that.
Something simple can mask as major. Interesting how the engine changed its tune over time.
Great calendar like pic scenery from NE. Local pics with cars always interesting to view.
Congrads on holding onto car so long. They are fine cars to drive and rely on when left stock or close to.
Now to reinstall the antenna and enjoy its second life they feel a bit different in a good way second time around.

So true to get in right frame of mind if possible when working on anything, cover the details, do it right. Errors happen.
Changed the cap and wires one time on my first Ranger 2.3. Did that task many times before. No start. Wired it 180 off.
Not pressed for time, under ideal conditions too. Going thru everything and overlooking same things over and over.
Learned long time before that to delay partying until after job is done. Breaks can help or hinder progress.

Am in awe of all the major projects undertaken on this forum. Posting good or bad followups are equally important.

[tinyprins.prinsloo]

Ive had the same issue swopped new set of plug wires in and she purrs like new
Went thru a list of things
Carb change
Manifold
Plugs
Distributor
All was no good
Plug wires ..set off platinum ones like new now

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