Technical Two possible "dead" cylinders

This is on my newly acquired 41 Ford Coupe with later 49 Merc motor. On another thread I fixed the driver side with double head gasket problems Etc. The car seems to run fine but once in awhile it starts acting like it's firing like it wasn't before. Checked a bunch of stuff and all cylinders have about the same compression 105 to 1:15. They all have spark. The thing is on number two and three cylinders I can pull the spark plug wires completely off and makes no difference at all. I put on a different set of wires just rule out bad wires and that didn't make a difference did it compression check on those two cylinders again and the compression is good. Spark is good. Plugs look good. So if I pull a spark plug while running on any other cylinders it dies down as it should but I'm number two and three nothing happens. Wondering if I've got a blown head gasket between the two cylinders. It's like nothing's happening in these two cylinders as I can put a heat gun on the spots of the heads where the compression should be happening and it is way way colder on these two cylinders. Firing order is correct , I can even cross number two and three and makes no difference . Thoughts? Probably going to pull the head off if I can't figure it out by the weekend.

 

Could be a crack between the 2 cylinders possibly?

 

Woah...you are going a little deep there....
A leak down test may keep you from pulling heads and that would give you a chance to look at all the plugs in the process.

 

Are the plug wires in the correct order. Crossed over wires is my guess
Have you looked in the spark plug hole to see if the valves are opening? If it was one cylinder then I would say broken lobe on the cam or bad lifter and inlet valve not opening sufficent to let in enough fuel to make power.

 

If the compression is right with the other 6, how could you have a gasket issue? You would have low compression if it's crossing over. You would also have low compression if you have a stuck open valves.
Does the exhaust smell rich? Are the two plugs wet? Can you hear a miss in the tail pipe?

 

When you had the head off, did you roll the motor over to check if the valves were opening and closing as they should? Hook up a vacuum gauge and see if the reading is steady.

 

Plugs ?
Cap ?
Vacuum leak in intake ?
Try a leak down test before tearing the head off

 

Maybe, those 2 plugs wont fire well under compression. Switch them around to see if the problem follows. Its a long shot but I have seen this once or twice.. Mitch...

 

My first thought too. first and last cylinders in the firing order crossed. Remember what is needed for combustion, compression(check), fuel(check)ignition (check), and at the right time.

 

Fuel, fire, and compression. You have compression.

 

2358 are on one side of the intake with 2&3 close to the carb. Are you missing a bit of rag?
If it was vacuum leak it would effect all 4 cylinders on that runner which is the middle 2 on the passengers side and front and rear on the drivers side.

 

Are the intake valves sticking closed? That would give you a good compression test result but keep it from firing.

 

You said the plugs”look” ok, that doesn’t mean they are firing properly. Swap plugs with different cylinders and see what happens. Then plug wires

 

I'd have a good look under the dist. cap. There may something there that is causing this.

 

Lets see those two plugs. Lippy

 

I've seen plugs fire fine out of the hole but no go in the hole. Lippy

 

Thanks for the comments guys I'm heading to work right now. But won't be able to work on it till later. I'll get you some pictures of the plugs and I did swap the plugs around just to make sure. Going to grab a new distributor cap as this is the one that came with the car. I'll keep you up to date

 

A year or so back my flathead started missing. I too had a cold cylinder as indicated by a thermal IR scan. I pulled the wire from the cap and checked resistance between the cap end and the Rajah terminal on the offensive cylinder. It was open. I removed the Rajah terminal to find that the carbon core had disintegrated inside the cloth casing about an inch. I trimmed the wire, stripped, re-installed the Rajah end and it has been perfect since.

Test your wires before you pull a head.

 

One of the things I do when testing for a "dead hole" is pull the coil wire, pull the plug, put your thumb over the sparkplug hole and have someone crank the engine over, if everything is working well compression should push your thumb off the hole, if not and it passes a compression test, stuck intake valve.

 

I replace the two plug wires with two known good ones and had the same results, also put it inline spark tester that you can see the spark between the wire and the plug and it's sparking. Perhaps the valves are sticking I will check later on

 

Not if only exhaust valves not opening . You still would have compression, just no explosion . I do NOT think it is this .
Stick a vacuum gauge on it , see if its a steady vacuum. Note the vacuum with the plug wires on and off .
I’m leaning on a vacuum leak , either on the intake gasket flange or in the manifold .

 

Can you get to the valve cover, remove it and see what te valves are doing? I don’t think the compression would be anywhere near the other holes if a valve was stuck though, maybe someone will explain it further?
I like the carbon tracking in the cap theory, you could be seeing spark, but not when near TDC on those holes, ie not at the correct time to ignite.

 

If the valves were sticking your compression would be off in those holes.
within 10 lbs high to low is usually considered good for a street engine with some miles on it.

2 and 3 aren't side by side on the cap and that kind of makes one wonder on the cap thing but I would still study that cap carefully. Carbon tracks or cracks or ??.


Worn distributor that isn't providing the spark that you need even though it has what looks like a spark at the right time?
Vacuum leak (s) on the runners feeding 2 and 3?

Agreed, a leak down test isn't a bad idea along with hooking a vacuum gauge to the engine to see what it says. The vacuum gauge usually gives pretty straight answers on older engines.

 

I’m getting my flatheads mixed up, been futzing with my L6 to long I guess!

 

I actually put a vacuum gauge on the thing last week and it was Rock Steady at 21 in which is really good so maybe no stuck valves

 

Leaking inlet manifold/carb base gasket, points gap/worn distributor cam lobe, wire in the dist hitting the rotor every turn, cracked cap???

 

Lots of heads (not sure of stockers) let you see the edge of the valves through the plug hole. Shine a light in there as your friend turns it over.