For the last week or so I've been chasing a intermittent misfire on my new 390, so today I did a compression test, here's what I found:

#1: 160 #2: 165 #3: 150 #4: 170 #5: 180 #6: 175 #7: 170 #8: 180

Is it time to pull a head off, or am I worrying too much?

Sam

 

Probably you are worrying too much. Intermittent misfire sounds like igntion problems to me. Sparkplug wires are a likely place to start looking. Run engine at night with hood up and look for spark arcing and/or blue corona around wires. But really, it could be any part of the ignition system such as points, dist cap, rotor button, spark-plugs,etc.

 

Yeah, you're worrying about too much.
X2 on ignition. If compression was the problem, the miss wouldn't be intermittent.
First, if your truck (is it the '73?) has points check the gap. It should be .017". As the distributor cam spins, opening and closing the points, the point rubbing block slowly wears. This does two things, first the point gap closes up. This alone can cause a miss. Second as the point gap closes up the points open later retarding the timing.
If you replace the points be sure to put a small amount of dist. cam grease on the cam and rubbing block or the points rubbing block will wear rapidly.
I'd do a complete tune up using quality parts. Replace the spark plugs, plug wires, distributor cap, rotor, points, and condenser.

 

Sorry guys, I should have added some more info: It's also got some other ugly symptoms, lots of blow by, it's also eating spark plugs like candy (the motor has 500 miles on it and it has had 6 sets of plugs) One set seems to last about a week before they would crack and short out. The entire ignition is new, pertronix ignitor in the dist, etc.

Sam

 

Ouch. Yep, lots of problems. Still though, trying to be optomistic:

Cracked Sparkplugs may be due to igntion timing too far advanced or if it's always the same plug or plugs, look for crossover. This can occur inside the dist cap, or across ignition wires (keep #7 and #8 as far apart as you can)

Blow-by. Before condeming the engine, check your PCV for proper functioning. By the way, what are the symptoms that cause you to suspect high blow-by?

 

The point gap of .017 is only a starting point. For them to be adjusted correctly, you must use a dwell meter. If you're not, then you're leaving something on the table.

 

There's no way a spark plug ceramic is going to crack and shortout on it's own. If they're cracked, I'd look at your installation method as the culprit. Use a spark plug socket with a rubber insert and hold the socket straight with one hand while exerting pressure with the other while tightening it. It's not hard to crack the ceramic portion while tightening them.

 

Yeah, that's the only time I like the GM style distributor.

 

It's definitely not me cracking the plugs, the last set I must've torqued to 4 lbs, and they still cracked. I suspect the carb was running incredibly lean (plugs came out whiter than when they went in) I re-jetted several times (last set that went in were #90s) and it helped, but then it leaned right back out. Could something have clogged up the main jet passages inside that carb? It's the only thing I can think of.

By the way, the whole PCV is brand new, and when I pulled out the breather yesterday it blew out quite a bit of smoke. Some of the gaskets on the motor are starting to leak too.

Sam

 

You haven't said what carb you've got, so I cannot comment on the jetting. I suppose a lean condition could cause detonation and thereby crack the ceramic portion on the cylinder side of the plug (you also didn't say what part of the ceramic cracked) The excessive blowby could be a result of those three cylinders with lower pressure. maybe the rings failed to seat, or they're installed incorrectly ?

 

The carb is the Holley 500 2 barrel.

I'm pretty sure the rings were installed correctly, and It had at 170-180 lbs of compression a few weeks ago. The plugs would crack on the outside and the inside.

Sam

 

Have you checked your fuel delivery, pressure/volume? Usually when you jet richer chasing a lean condition, and it runs good for a bit then leans out, you're just running out of fuel and going lean, and if anything, the bigger jets make it worse, not better. I assume the floats are set right, good genuine Holley needles etc? There are some off-brand "kits" out there with really cheesy needles that don't flow very well

 

#90 jets in a 500 2bbl running lean? Sounds like either fuel delivery issue as mentioned, or a major vacuum leak.
Those jets should have it bordering on blowing black smoke, like a diesel.....

 

Based on all this new information, I would suggest you perform a leak-down test before you go too much farther.

When my balancer slipped and I had the timing set too far advance (as I didn't know it based on the damper markings), I broke #2 ring, oil ring, and the psiton lands between the two, on one piston. Compression stayed 195 in all cylinders, but a leak-down test showed a massive leak in the affected cylinder (compared to the others, it leaked down in 1/2 the time).

The leak-down test convinced me to do a tear-down. Seeing how you are breaking spark-plugs and the compression test results represent a significant recent change, not one due to thousands of miles, a tear-down might be in the near future. A leak-down test may help tell you more.

 

If you have detonation bad enough to crack the spark plug ceramics, then I'd hate to think of what damage has been done to those pistons with the low compresssion. At best, you're looking at broken rings, at worst, broken ring lands on the pistons.