67’ Firebird w/ a 71’ 455ci, dead cylinder question

JollyRoger

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Hello everyone!

I recently acquired one of my lifelong dream cars, a 67’ Firebird. The car itself has seen a lot of changes to its drivetrain and one of the changes was one of the previous owners swapping in a 455 from a 71’ Firebird. Now, when I bought the car from the last owner, it had been sitting for 20 years in storage (barn find for him). He didn’t do too much to it but a few replacement parts on the engine to get it running (distributor, alternator, fuel pump, belts, battery, Edelbrock 1411 750cfm, nothing more except possibly spark plugs).

What I have done to it in the 48 hours since buying:

Timing was way off, set factory initial timing to 12 degrees retarded timing (did not set full advance for full total timing yet.)

Basic tuning of the carburetor - set fuel mix and idle and adjusted throttle linkage.

Checked dwell angle, but did not adjust since we had some grounding issues.

Next, performed a compression test to see health of the motor, the read outs below are what we measured on a semi-warm motor ( warmed it up, but took about an hour break to go buy more tools we needed before performing the test):

Cyl 1: 120
Cyl 2: 108
Cyl 3: 118
Cyl 4: 120
Cyl 5: 130
Cyl 6: 0
Cyl 7: 125
Cyl 8: 88

So, dead cyl 6. We troubleshooted what we could, rockers seemed to open the valves fine (but we are amateurs and could have missed something).

Obviously bad rings were our first thoughts, but we wanted to consider other variables like a blown head gasket near both cyl 6 and 8 given the low psi of 8 and it’s location to 6.

Tomorrow we will be performing a leak down test to see if it could also be bad valves. However, I wanted to get the perspectives of everyone else on this issue.
 

melsg5

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to start with you have to adjust dwell before doing the timing.
could be just terminology but the timing is 12 BTDC which means it is advanced not retartded.
concerning the cylinder and a head gasket, they have coolant hydrocarbon test kits which will help in determining if the head gasket is blown.
cylinder number 8 is also too low and number 2 is low. The rule is no more than 10% variation between high and low so high being 130 nothing less than 117 is acceptable. I think both heads have to come off. Did you try a wet test to see if the rings are weak?
 

JollyRoger

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to start with you have to adjust dwell before doing the timing.
could be just terminology but the timing is 12 BTDC which means it is advanced not retartded.
concerning the cylinder and a head gasket, they have coolant hydrocarbon test kits which will help in determining if the head gasket is blown.
cylinder number 8 is also too low and number 2 is low. The rule is no more than 10% variation between high and low so high being 130 nothing less than 117 is acceptable. I think both heads have to come off. Did you try a wet test to see if the rings are weak?
melsg5,

Thanks for the reply. We didn’t realize the dwell timing was something to do beforehand (still learning), will do that after I fix the dead cylinder for sure. And yeah I get the terminology mixed up, it was initial timing at 12 advance.

I’ve never heard of a coolant hydrocarbon test kit, I’ll look into that, but oil shows no signs of milk and the radiator fluid looked fine as well (of course that’s never the most accurate). Also, we never ran the engine more than 10 min, so I don’t know if it would overhear, but at this point I don’t want to test for that taletell sign.

As far as a wet test, we did not, may look into at that as well. We may pull both heads to be sure. I’m debating just rebuilding. No clue on mileage of the engine but when we pulled the valve covers off, it looked brand new (no sludge, flaking, etc.) the push rods even still had a bit of marking paint on them from when it was assembled. Possibly a rebuilt motor st some point?

If I can’t get the answers I need when we starting pulling it apart, I probably will go ahead with the rebuild. Thanks again for the advice!
 

melsg5

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strange that the valve train area would look so clean yet you got poor readings on the compression test. I assume you changed the oil but I wonder if it has some stuck lifters? Try adding a 1/2 quart of Marvel Mystery Oil to the engine oil and let it run for 15 to 20 minutes. I assume the engine is idling rough with those compression numbers so see if the idle improves.
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JollyRoger

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strange that the valve train area would look so clean yet you got poor readings on the compression test. I assume you changed the oil but I wonder if it has some stuck lifters? Try adding a 1/2 quart of Marvel Mystery Oil to the engine oil and let it run for 15 to 20 minutes. I assume the engine is idling rough with those compression numbers so see if the idle improves.
Thanks I’ll try that first!
 

JollyRoger

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Update:

Well as usual with any project I start, I learned everything after the fact.

Car is a 400, the block was from a full-size Pontiac, not a Firebird. Exhaust valve on number 6 was bent. The other low numbers were due to some major carbon built up on the valve seats. Refreshed the head (cleaned, lapped valves, replaced bent one, etc).

Car is almost assembled but I had a bunch of “while you’re in there” moments and bought a bunch of parts to replace.

Engine was definitely rebuilt (.050 over), which is why it looked so damn clean. The bent valve was from the keeper failing (hopefully not an oiling issue or something and just a bad part). My logic is that they threw on two different heads and didn’t clean them up first which would explain why the valves looked so used while the cylinders/Pistons looked damn near brand new (also the bore being .050 over was a good clue (this car is a real hodge podge).

Any who, I’ll post an update once the rest of the parts get here and I get this motor assembled and running. Thanks for everyone’s advice!
 

melsg5

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you checked the heads to make sure they are level?
what is the engine code that helped you determine it's origin?
 
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