[erlindsey]

Hi guess, I need some help. Ive got a 1972 400 engine. Its been bored 40 over. Has the 8 eyebrow pistons with 670 heads. My problem is #3 and #5 cylinders arent firing at all. Ive changed the plug wires and plugs. Neither worked. What else should I be looking for? Any Idea's?

[David Jones]

Are 3 and 5 getting spark?

[erlindsey]

yes they are

[facn8me]

Sounds like you need a compression test and pull valve covers and look for flat cam. Rockers no move cylinder no work.

[Schurkey]

Originally Posted by David Jones

Are 3 and 5 getting spark at the plug gap?

Fixed it for you. Fouled or cracked plugs may have spark at the end of the plug wire, but nothing jumps the gap to start combustion.

I'd be looking at #2 and #8. If 2 and 8 are weak, that could indicate poor idle-mixture screw adjustment on that plane of the intake manifold--or a vacuum leak into that plane.

WHAT IS CRANKING COMPRESSION on 3 and 5?

[erlindsey]

I'll check the compression when i get home later this evening. I had the valve covers off. Rocker are working up and dn same as others.

[erlindsey]

145psi on #3 and 115 on #5

[Pontirag]

I dont know what it is about 670 heads but they take a unique piston . suposedly the wrong piston will smack the valve or something but they are not like other 400 heads in terms of piston compatability

[screamingchief]

Originally Posted by Pontirag

I dont know what it is about 670 heads but they take a unique piston . suposedly the wrong piston will smack the valve or something but they are not like other 400 heads in terms of piston compatability

That ^^^^ is complete & utter non-sense,,,sorry...

Besides he clearly said that he has the early/late 8 valve relief pistons,and those have both the early & later valve relief locations in them,so that clearly is'nt the problem here.

As far as the compression tests go,IMHO at the minimum you need to at least do one whole side of the engine to get useable results,you're doing that to determine what constitutes high & low cylinder pressures,as it is now,sure we know that one cylinder is lower than the other,but is #3 lower than the rest of the cylinders as well,and #5 even lower than #3 is,or is #3 psi close to the other cylinders thus making only #5 "low"?

If the other cylinders (1 & 7) on that side all read like 180 psi,than we would know that #3 is also "low" relatively speaking.

And two adjacent "low" cylinders can often indicate a head gasket problem.

Which just so happens to be fairly common on pontiacs between cylinders 3 & 5 and 2 & 4 due to the two exhaust ports being adjacent to each other.

The fewer cylinders you test in a compresion test,the less accurate that test is overall,and it's that much less that test tells you in terms of diagnostic information,as compression tests rely on comparing stronger cylinders against the weaker cylinders,which gives one some insight on the health of the weaker cylinders.

If one is only wanting to test a single cylinder or two for diagnostic purposes,then a leakdown test is the far better test to use,as that test can also tell you where the pressure is escaping the cylinder.

Anyhow,keep at it,you'll figure it out.

BTW,are you using the OE bottle-neck non-adjustable studs,or are you using an adjustable valvetrain on this?

Bret P.

[Sirrotica]

I dont know what it is about 670 heads but they take a unique piston . suposedly the wrong piston will smack the valve or something but they are not like other 400 heads in terms of piston compatability

Originally Posted by screamingchief

That ^^^^ is complete & utter non-sense,,,sorry...

[erlindsey]

Will get all the numbers when i get back to the house today. Should I remove all the plugs before doing the test. How do I perform a leak down test?

[screamingchief]

Originally Posted by erlindsey

Should I remove all the plugs before doing the test.

Yes,and if at all possible it's best to do it with the engine warm as well,and the throttle blades should be held open @ WOT too.

Originally Posted by erlindsey

How do I perform a leak down test?

To begin with,you would need access to a leakdown tester and an air compressor.

Then it's mostly the same as doing a compression test,though instead of looking at what pressure the cylinder itself is making while cranking over,you instead pump in a set amount of pressure (example: pump in 100 psi with the cylinder held @ TDC) and see how much pressure the cylinder itself will hold,then the gauge on the leakdown tester wiill read how much pressure the cylinder is holding,and that will tell you the percentage of leakage for that cylinder.

And as the cylinder is pressurized via the external air source,you can then listen for where the cylinder leakage is occuring,and that'll tell you if the problem is due to bad valves or rings,or possibly even a headgasket.

HTH

Bret P.

[erlindsey]

I took the 1, 3, 5, 7 plugs out and tested the compression. I didnt warm the car up but here are the numbers. #1 was 165psi #3 was 155psi #5 was 155psi and #7 was 160 psi. I checked each one a couple of times because of the different numbers i got the other day. I didnt have a chance to check the other side. I'll try and get those tomorrow.

[Half-Inch Stud]

Originally Posted by erlindsey

I took the 1, 3, 5, 7 plugs out and tested the compression. I didnt warm the car up but here are the numbers. #1 was 165psi #3 was 155psi #5 was 155psi and #7 was 160 psi. I checked each one a couple of times because of the different numbers i got the other day. I didnt have a chance to check the other side. I'll try and get those tomorrow.

Those CYLs are fine.

Perhaps ohm out the plugs for any shorted plugs( rare but happens).

[Richie Hoffman]

Valve could be to tight check adjustment

[steve25]

You`ve got compression, but that only means the rings are ok and the valves are sealing, you can still have bad cam lobes or a blown head gasket between 3 and 5.
Pull #5 plug and with the valve cover off view #3s rockers as you have someone crank the motor over with the coil unhooked, if when the valves on #3 are cloesed you feel compression coming out of #5, then you have a blown head gasket to one degree or another.