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Old 09-07-2004, 10:10 AM
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Vacuum leak? Blown Head Gasket?

Engine Description:
  • 350 4bolt Nickel Block - 75 vintage
  • B&M (Holley) 144 Blower - 8lbs boost
  • Orig. heads
  • Holley 80573 manifold referenced power valve carb

This engine is all of a sudden acting like it has a vacuum leak. Symptoms:
  • Runs rough at startup
  • Runs pretty smooth after warmed up
  • Occasional 'bog down' when give gas at low RPMs
  • #2 and #4 plugs are burning chocolate on top, and white on bottom. This is based on slow driving in city and long idle in garage (not normal method for reading plugs)
  • Same plug burn, to lesser degree, on # 1 and #3
  • Other 4 plugs burn black like I would expect when reading in this manner

I have sprayed water all around the intake, and heads, to try to test for a vacuum leak. I don't see any indication of that.

I am about to decide that I have both head gaskets blown between the front 2 cylindars. I hate to go to the trouble of changing head gaskets if that's not it, but I'm about to give that a go.

I may try to see if I can rig up my compression tester to put pressure in a cylinder and see if that tells me anything. However, I'm of a mind that by the time I mess with all that, be sure valves are closed for the test, etc. I can be well on teh way to gasket changes anyway.

What do you think? Anyone seen this type of plug burn symptoms?

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Old 09-07-2004, 11:12 AM
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As a moderator here............let me be the first to WELCOME you here.

Do you have a compression tester?
Do you have a vacuum gauge?

I would check the compression ..................after I put a vacuum gauge on it.

A blown power valve on the Holley can cause the issues you have......

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Old 09-07-2004, 11:47 AM
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I appreciate the welcome. Glad I found the site.

I do have a compression guage, but it must be faulty. The needle goes up with compression, and right back down. Has for quite a while. So, hmmm, I guess I don't have one

I have a vacuum guage, but not sure what I would do with that to answer my problem.

I changed out the power valve just this weekend. Didn't make any difference.

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