2 dead cylinders

[ditchtiger]

Cylinders

 

[deadeye]

The confusion here is what cylinders are we discussing?
Firing order
1(5)3(6)24

actual numbers front to back
Or 1234(5)(6)

The second is why some have said head gasket.
the first is what I believe you are saying and is where the frustration is.

Did you pre-oil the motor and make sure the lifters are pumping up? If you have a bad cylinder and it shares the same intake runner as another this may cause vacuum turbulence and interfere with proper intake. Did the intake gasket fit the motor and the intake? Sometimes if the intake or head have been swapped they don't always match exactly. I did an older swap from a 270 to a 302 (GMC) and the 302 had a high pro head so I had to get a proper gasket,the listed one did not work.

 

[Hop Sing]

The confusion here is what cylinders are we discussing?
Firing order
1(5)3(6)24

actual numbers front to back
Or 1234(5)(6)

The second is why some have said head gasket.
the first is what I believe you are saying and is where the frustration is.

Did you pre-oil the motor and make sure the lifters are pumping up? If you have a bad cylinder and it shares the same intake runner as another this may cause vacuum turbulence and interfere with proper intake. Did the intake gasket fit the motor and the intake? Sometimes if the intake or head have been swapped they don't always match exactly. I did an older swap from a 270 to a 302 (GMC) and the 302 had a high pro head so I had to get a proper gasket,the listed one did not work.

So if you are correct and we are talking the second scenario, the thing that would jump out at me, given there is compression, spark, and presumably fuel, is there is something wonky about the distributor cap. IOW, maybe a crack or some reason #5 and #6 are not firing.

Just an idea.

Edit: or it could be a totally bizarre thing that the two plug wires somehow got switched.

No offense to the OP on any of this guessing as you certainly seem to know your way around. Gremlins do happen though!

 

[SUPER X]

How can a cam with one worn out lobe cause 2 cylinders to not work? 1977 straight 6 Chevy 250. #5 exhaust lobe out. 125psi on all Chambers, to 150 on second stroke. 5 and 6 are dead.

You have 2 problems , I would guess a bad valve or damaged piston or rings .The bad cam still will allow it to have compression . I have the same engine but older mine is 160 on all 6

 

[SUPER X]

You have 2 problems , I would guess a bad valve or damaged piston or rings .The bad cam still will allow it to have compression . I have the same engine but older mine is 160 on all 6

P.s. those ones that had the intake casted as apart of the head where known to crack

The valve adgustment might be to tight that will also damage a cam lobe

 

[notazombie]

Here's an answer to your original question and I'll try to keep it as short as possible. When you lose an exhaust lobe the air can enter a cylinder but can't go out the intended exit. That excess compression can find it's way back out the intake valve and disrupt the intake flow/mixture. You are now introducing an unwanted 150psi where a 20inHg should be. At idle or cruise it may manifest as a misfire on any cylinder(s) that shares that intake runner. Under acceleration it may pop back through the intake.
I've had this happen on a couple 327s.

 

[ditchtiger]

Here's an answer to your original question and I'll try to keep it as short as possible. When you lose an exhaust lobe the air can enter a cylinder but can't go out the intended exit. That excess compression can find it's way back out the intake valve and disrupt the intake flow/mixture. You are now introducing an unwanted 150psi where a 20inHg should be. At idle or cruise it may manifest as a misfire on any cylinder(s) that shares that intake runner. Under acceleration it may pop back through the intake.
I've had this happen on a couple 327s.

This is what I was thinking. The air was getting fowled in the intake.

 

[ditchtiger]

P.s. those ones that had the intake casted as apart of the head where known to crack

Had that head originally.

 

[notazombie]

It makes sense once you've seen it before. In essence, since the exhaust valve doesn't open, it becomes a second compression stroke. As the piston passes TDC the intake valve opens and releases all that cylinder pressure right back into the intake manifold. Really messes with the air/fuel and ends up affecting other cylinders.

 

[ditchtiger]

And to add to my woes. Spent 15-20 minutes trying to get the new distributer to seat all the way down. Never did get it.
Had the CARQUEST guys looking for several days to figure this out. In 1977 there were 3 different distributers used. All HEI.
2 used coil over the cap, and one, mine, uses a remote HEI coil. Same coil but different drive gear on the shaft. And the gears can not be swapped. Just my luck to get the odd ball.

 

[notazombie]

What's wrong with the old one?

 

[ditchtiger]

It makes sense once you've seen it before. In essence, since the exhaust valve doesn't open, it becomes a second compression stroke. As the piston passes TDC the intake valve opens and releases all that cylinder pressure right back into the intake manifold. Really messes with the air/fuel and ends up affecting other cylinders.

Really messes with your mind trying to figure the common link to the 2 cylinders. I understood the second compression stroke. This is the only thing that made sense to me. But I had never seen this before. Was a real head scratcher for sure.

 

[ditchtiger]

What's wrong with the old one?

Old one what?

 

[notazombie]

Distributor

 

[ditchtiger]

It was 40 years old.

 

[ZigZagZeke]

Here's an answer to your original question and I'll try to keep it as short as possible. When you lose an exhaust lobe the air can enter a cylinder but can't go out the intended exit. That excess compression can find it's way back out the intake valve and disrupt the intake flow/mixture. You are now introducing an unwanted 150psi where a 20inHg should be. At idle or cruise it may manifest as a misfire on any cylinder(s) that shares that intake runner. Under acceleration it may pop back through the intake.
I've had this happen on a couple 327s.

Exactly what I was thinking.

 

[ZigZagZeke]

And to add to my woes. Spent 15-20 minutes trying to get the new distributer to seat all the way down. Never did get it.
Had the CARQUEST guys looking for several days to figure this out. In 1977 there were 3 different distributers used. All HEI.
2 used coil over the cap, and one, mine, uses a remote HEI coil. Same coil but different drive gear on the shaft. And the gears can not be swapped. Just my luck to get the odd ball.

If you're going to buy a new cam and a new distributor anyway you can switch over to a less oddball distributor/cam drive gear combo.

 

[SUPER X]

If you're going to buy a new cam and a new distributor anyway you can switch over to a less oddball distributor/cam drive gear combo.

The factory hei will plug right in and you would just run 12 volts to it

 

[ditchtiger]

The factory hei will plug right in and you would just run 12 volts to it

It is factory HEI. But an odd ball. 3 distributors used that year. 2 were coil oil the cap. And one with remote coil mounted on the block, which is what I have. Same coil. Just in a different place. But the distributors in that year are "NOT" interchangeable. NOT, NOT EVER.
Splines on the drive gear are different. And don't suggest switching the gear on the dist. shaft. my first thought. lengths are not the same.

 

[ditchtiger]

If you're going to buy a new cam and a new distributor anyway you can switch over to a less oddball distributor/cam drive gear combo.

Been there tried that, drove CARQUEST nuts trying to figure why this did not work.