71 400 Not firing on #4 and #6 cylinder
 

Gurus,

Asking for a friend:
Swapping out 66 389 with mild 71 400. Problem he cant get #4 and #6 cylinders to fire. The following have been done:
New cap and rotor (HEI)
New plug wires, new plugs, swapped plugs around. Fires spark when wire is pulled off of plug..
Valves opening and closing, (hydraulic) adjustment checked. Timing checked, firing order checked 10 times.
What has been done is 66 389 timing cover and balancer has been put on the 400 but this should not keep 4,6 from firing. All cylinders at 120 psi.
We are STUMPED HERE!
HELP!!

terminals in cap appear normal? try swapping plugs see it issue moves or is the plugs themselves?

When did this start, before all these parts where changed?

While you where last driving it?

Do the compression test again with either the plug for 4 or 6 out.
I thing you might have a blown head gasket between those two cylinders.

We are considering an extreme intake gasket leak! Not creating enough mixture to fire... Will investigate today and will reply to all.
Thanks,

We are considering an extreme intake gasket leak! Not creating enough mixture to fire... Will investigate today and will reply to all.
Thanks,

I would think it would be closer to a head gasket instead of intake gasket?
Large spread on intake from 4 to 6 plus exhaust crossover, while head gasket is between cylinders.

I'd also think that big of a intake gasket leak would be real noticeable?
But would have to take intake off either way, so good luck.

I would do a cylinder leakdown test on number 4 and 6 cylinder. What does the vacuum gauge look like? is the needle steady, or oscillating?

Intake gasket failure will do that. It's the last thing to check after you have verified fuel, ignition and compression readings. 120PSI is pretty low I would add. Did you compare those numbers to the other cylinders that are firing OK?

Make sure to use the Felpro blue intake gaskets if you end up doing that repair. The black ones are garbage and many of the aftermarket intake gaskets sold for these engines aren't that great.......

I had a similar problem recently and it turned out to be a bad vacuum leak in one cylinder. It was quite a mystery to figure out because the vacuum leak will create a lean mixture, and even though you have compression, the plug fires and you have fuel, the lean mixture will not allow to cylinder to fire.

I do recall your posting about that issue Tim, but he’s got two cylinders acting up ( 4 & 6 ) and those intake ports are near 6 inches apart!

I guess it’s possible they each have a there own separate vacuum leak, but to me that’s a bit of a stretch.

I guess we will find out when he reports back.

I agree it's unlikely that it's a vacuum leak on two cylinders that are not side by side but can't rule it out because it seems he has pretty much eliminated everything else. I did a lot of head scratching before I finally removed the intake manifold to find my issue. I suppose if the intake manifold was somehow warped bad enough it could happen. One thing he could try is getting a map gas torch and applying the gas around the intake manifold where the gasket contacts the heads and if the gas gets sucked in it may cause the cylinder to fire. Of course in my case it didn't work because of where the leak was.

Guys,

We are still baffled. Ran it and sprayer B-12 Chemtool around all intake areas and no change. Checked everything except the leak down test. Now that seems like it could be the problem. Its a new build from a local machine shop. They put the heads on it, my buddy did the rest and put it in the car and dis a break in. It ran the break in but not good.. The vacuum was low and erratic. He adjusted the lifters while running and that smoothened thing s out to 15-16 inches of vacuum. Still idles rough and took a temperature reading on the exhaust ports of 4-6 and its 60 degrees cooler than the other side. So they are not firing. So the only thing that makes any sense is the compression is leaking from 4-6 (head gasket).. He will do the leak down over the next couple of days and that's where I'm putting my money.. Ill keep you guys posted.. UGH....

Thanks,

He does not need waste effort doing a leak test , at least on4 and 6.
Just remove both plugs and then do a compression test on either one, if the head gasket is bad you will have very low compression show up.

very first post stated that all cylinders are at 120

Since its a new build, maybe try and re-torque the head bolts. It may save you alot of work.

BTDT!
If it is failed head gasket and retorquing seals it up the fix will be only temporary.

After the next cool down and heat up session it will fail for good.

What do the plugs on those cylinders look like? No combustion would have them gas soaked. Partial combustion would be carboned and raw gas contaminated.

Check the lobes and valvetrain adjustment on those cylinders.

Check the leak down and identify leaks

Were heads checked for cracks?

Could the engine be really out of tune and low compression together just runs rough?

What camshaft?

One time I diagnosed a no-fire condition in a 250 chevy 6. It had even compression in the cylinders and spark but would not run. Plugs were gas fouled. Disassembly found a cylinder head cracked between exhaust seats.

And it just so happens that 4 & 6 are where most of the cracking takes place in a Pontiac head due to the high heat of the paired Exh ports.

Any chance #1 and #7 are weak? If so, adjust your idle mixture screws and/or repair the carburetor. 1, 4, 6, and 7 are fed from the same plane of a dual-plane intake manifold, from the same carb primary throttle plate/venturi.

Any vacuum ports on the runners to #4 and #6?

As said--120 psi compression is pretty weak. Not weak enough to kill a cylinder or two, but overall not good. Depends somewhat on altitude, cranking speed, and whether or not you remembered to open the throttle some. Are you sure your compression gauge is accurate?

Low compression because the cam timing is wonky? Did they degree the cam, or just slap it in "dot to dot" and hope for the best?

Does it have adjustable valvetrain?
Also, are the heads the 71 400 heads? (7K3?)

I would try loosening the rocker nuts a turn on those cylinders and see what it does?

Possibly the valves too tight on those cyls. (or all maybe)

:confused: