Technical Ford 260/289 cylinders 5 and 8 won't fire

I have this problem which someone may be able to help with. On my 289 cylinders 5 and 8 don't seem to fire (particularly no. 5, never got more than 212°f on the pyrometer, plug is wet). I just did all the valve stem seals and changed the timing chain so my initial thought was I messed up the lifter preload or timing, even though no popping or spitting back, the engine starts easily and the dots face each other. Did a check and 0° on the balancer tallies with No. 1 at TDC (looked with endoscope, made sure valves were closed). Checked valve preload on both 5 and 8, nothing unusual. Did compression (engine hot, throttle open) and got reasonable values (145 and 142, more or less within the 10% rule). Valves seem to open and close as they should. I did install a new fuel pump and it was delivering too high a pressure (8 psi) so obviously way too rich, so I put the old one back (4 psi). Carb is a 240 CFM Ford 2100 with 0.045 main jets. Checked (fairly new) distributor cap for cracks - none. Swapped plugs from cylinders which were working as well as plug lead, still no change (and there was always spark when checked against the block) - did anyone encounter such weird situation? I've heard about plugs - even brand new ones - going off on a rich running engine but I'd appreciate some opinions before starting with the parts cannon. Links to vids of the engine running and valve operation (it galops because only no. 5 plug is out) below.

Thanks in advance for any ideas!

20260914 170832 - YouTube
20260914 175438 - YouTube

 

Well I have experienced this before . I’m not sure the answer , I changed cam and firing order to 351W and cured my issue . You could remove plug wire to number 5 cylinder and there wasnt one bit of mis fire sound or action take place in a running engine . Spark would jump from plug wire to boot to block on the running engine . I never was able to determine the cause , my engine was fresh and new . But as I said different cam and firing order cured my issue . I changed plugs from a cylinder to number 5 ran through the valve adjustment numerous times , compression test , no known reason for me . There was not one reason mine why yours is dead on number 5 . It’s a mystery to me to this day . It will be great to know why if you determine the cause on your engine . I often wandered how many were running this way and no one even knew it has the issue .

 

Accidently swapped the leads between the dead cylinders?

I would think fuel consumption would make it clear something's wrong rather quickly.

 

In my case plug lead was 100% correct. See also timing marks facing each other on crankshaft/cam. Carb has been gone over, power valve is not leaking. But maybe there's a crack I cannot see?

 

I would have been in the switched plug wire camp. If not, that then it sure sounds ignition related, cap would be my guess since it sounds like you verified the plug wires by switching them around.
You are using the 15426378 firing order correct?
Ford had a different firing order for later HO engines if I remember correctly.

 

15426378, pretty sure I did not get the wires wrong...

 

Something similar. No fire from a number of cylinders due to arcing on the HT leads. Not obvious at all until you looked under the bonnet in the dark with the engine running and you could see the arcing.

 

How are your plug wires routed? It's common for consecutive firing cylinders to stumble/misfire if the wires are run parallel/close to each other, 5-4 and 7-8 are the usual culprits for sbf. The magnetic field from one wire will cause a mistimed/early spark in the parallel wire. Keep them separated.

 

There was no arcing on my engine , only a thing I experienced was , when I would take the plug wire off number 5 and allow to hang near the head . It would jump spark from the boot to the block . Plenty of spark to fire the plug . As I said I could remove the spark plug from a known good operating cylinder and swap with number 5 , it still remained dead . My engine was a 4 barrel intake and Holley carb. . Strange thing it never sounded as it had a dead cylinder or a misfire . It ran rather well . I was just checking header tubes with the heat gun and located the issue . To this day I don’t have a clue to an answer . I changed cam , which changed firing order used same carb intake and ignition , adjusted the valve train the very same way as I did on the old cam numerous times and the issue was no longer present . Engine seemed to run the same and did not produce more power with the new cam and number 5 firing . I remain lost to this moment as to what the answer is to the question. One of life’s great wonders to this day for me .

 

My wires (fairly new, not that it means much these days) are routed just like the factory intended. But I definitely do a check with hood down and no light.

 

I'm just spit balling here but 5 and 8 share a plenum along with 2 and 3 on dual plane intakes. How do 2 & 3 look? Are they rich looking, too?

And did you check vacuum with a gauge? Vacuum readings could help with diagnosis.

Sorry I don't have any answer but maybe these ideas might trigger something.

 

2 and 3 were working but you may have a point there. Vacuum just about to go into the green scale (15), did play with the distributor but not much difference.

But ***uming the carb is running rich on one side (and looking inside with the engine working, it did not look as if gas was just pouring in), there should still be combustion, no?

 

Worn points, if you still use them or the distributor itself. The shaft from age can get play in them causing irregular running. Diving deeper, a sticking valve. Engine have a lot of miles?

 

Points have maybe 1000 miles on them. There was visible, regular (and painful!) spark at the plug. Engine is high mileage, yes, but not worn out. There's a short vid of the valvetrain working below, it looks ok to me?
20260914 175438

 

Take plug wire loosely when idling so that you hear spark jumping to spark plug, does it make difference? If so, you have weak spark.
This test sound weird but its making the coil put much higher voltage in that cylinder.

 

As @onetrickpony mentioned above, a vacuum gauge can be helpful for this kind of work. Even a "stockish" engine and cam with a decent tune-up should probably pull 16" to 19" at idle. More ignition timing, up to a certain point, will increase vacuum as well. Make sure the gauge is connected to a source of full manifold vacuum, not ported vacuum or some other source above the throttle plates. Higher vacuum readings are good, but steady readings are even better. There's lots of good info online explaining the various readings.

I really don't know the specifics of SBF valvetrains. But can you easily swap rockers and/or pushrods between cylinders. Might tell you something if the problem follows the swap. Do Fords use similar but different rocker arms for intake and exhaust?

Beyond that, did you find any su****iously lower temps at the exhaust manifolds? That might indicate something like an exhaust valve not opening fully for one of several reasons.

 

I hear you - makes sense.

 

I had an older OT 4cyl car start missing on a cylinder. Found point cam was lower on that cylinder. Opened up the point gap a little and it then ran on all cylinders. Maybe??

Also as said above, maybe same thing happeneing from a worn distributor shaft bushing?

Dave

 

One more thing to check...

 

What does the inside of the distributor cap look like? Any carbon arcing? Another place where a scope would be a huge help.

 

I had that happen once after I installed a new set of plug wires. Turned out one of them was bad.

 

You sound pretty thorough and knowledgeable. Just curious, what is the ignition timing set at? If you put the number 5 or 8 cylinder at TDC on compression stroke, is the distributor rotor pointing relatively close to the appropriate spot to the cap?

 

Does this engine have an EGR valve?

 

I had a problem like this in a Y-block. I put in a Pertronix to see if it would go away and it did. I think the Pertronix is a bit more forgiving than the points.

I'm thinking it is worn distributor bushings. I Can move the distributor shaft quite a bit in the housing. I bet .015 or more.

 

They do. If it's the dizzy then as some pointed out above it might be general wear...

 

Nope, just a simple PCV.

 

I’m laughing as this is not the case I’m sure but can’t you drop a 3.8 distributor into a v8 with a v8 cap and it won’t fire on two cylinders? I’ll ****** off now just poking at fun..

 

I'm dumb but not that dumb lol. This is the old distributor in there. In light of what people commented above it may get replaced soon though.

 

Yes, sound like ignition. However, with a dual plane intake, the 5 and 8 (driver's side F and R) are fed off one side of the carb. Use your hand, a rag or brake clean and see if richening it up on that side changes things. Quick and easy.
Had a 'smart 'scope' at one place I worked. On situations like this, it would always spit out "fuel induction/intake/valvetrain issue" once compression and spark were confirmed. Which left the tech with not much and the customer unhappy.

 

I also recently experienced the same problem on a 302 with points . Distributor shaft was moving enough to change the gap.