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My son's '65 is running like crap. It was backfiring through the carb and missing badly. After a tune up, it ran fine for about 2 months and is starting to do it again (about 30k miles on a rebuilt 289). Last night I did a compression check and found 130psi in all cylinders ecxept 3. The no. 3 cylinder had 140 and 7 and 8 both had 120. I'm thinking blown gasket between those 2 cylinders. There's no oil in the coolant, and no coolant in the oil. The number 7 plug was very white, like it was running really, really lean. The engine is pulling a very steady 20 lbs. of vaccum at idle. Everything else (timing, advance, vaccum leaks, etc.) all check out ok. I'm planning to pull the heads tommorrow, have the machine shop take a hard look at them and replace with a new gaskets, and will probably replace the cam while we're at it to see if we can get rid of some of the lifter noise this engine's always had.

Oh, and 70sportsroof, if you see this, the '69 coupe blew a heater core today, so I now have you beat ... I have 3 out of 4 Mustangs now parked. *LOL*
 

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Shouldn't blown head gasket be seen as excessive vibrations at vacuum gauge needle at all rpms.
 

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Wish I had your energy John....making me pull heads on any engine is one of the hardest things you'd likely ever do..*G*

What kind of ignition system are you running and where does your PBB (if so equipped) and PCV plumb into the intake?

Do some more diagnostics before removing the heads.

What firing order cam? If early 289/302, try removing the #8 plug wire and see what happens...

Get back to us...I may not be around (just got back from vacation) but some of the other engine guys likely know where I'm headed...

Good luck!
 

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Did you check the plug wires? I've heard of crossfiring before. What about checking the distributor for wobbly shaft? I wouldn't pull the heads if the vacuum is good. I'm chasing a bad cam and cracked head now. I would much rather be troubleshooting a backfire problem.
 
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With only 10-PSI difference between cylinders, you have other problems than what you think ! That's Nothing !!
 

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One sure way to tell if the head gasket is blown between the 2 cylinders is to pull the plugs in the adjacent cylinders and do another compression test in the "culprit". If the compression doesn't drop then you are not swapping pressure between them.
 
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Sounds like ignition to me. Those slightly off numbers on the compression test could be out or adjustment valves. If someone else did the rebuild before you got the motor or car who knows what they did and if it was right. I'd suspect an old timing chain was reinstalled, and it needs replacing.
 

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Just a silly question. Mine was doing similar thing about two days ago. Turned out
to be a bad dist cap. Have you tried that and or the rotor? G/L
 

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Discussion Starter #9
The motor is the 302 I had rebuilt for my '66 when I first put it together, then we yanked it with about 25k miles on it and dropped it in my son's '65, so I could build a better one for me /forums/images/icons/wink.gif. The ignition system is a stock FOMOCO distributor with a Pertronix Ignitor and Pertronix FlameThrower coil. Distributor, Ignitor and coil were all bought new when we rebuilt the engine. I'm runnin a .040 gap on the plugs. Firing order is 289/302 order.

PCV is routed to the PCV port at the base of the Edelbrock 1406 carb. PBB hooks to center of manifold to the rear of the engine on the Edelbrock Performer manifold.

I did find as I was going through everything, that the ground wire for the Pertronix Ignitor wasn't attached well. It basically came right off of the "eye" that was bolted down to the breaker plate. Since I fixed that, it hasn't back fired, but has a serious miss under load. Maybe I'll just replace all the plugs and dist. cap again (plugs look pretty bad this time), re-adjust the valves and see how she runs before yanking the heads.
 

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I did replace the plug wires when going through it last night. This may have helped, as it doesn't seem to backfire any more ... just has a bad miss, but that could just be plugs and cap now. Distributor was new when the motor was built, but that's a good suggestion ... I'll check it out.
 

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Discussion Starter #14
I kept thinking ignition, and thought I had it when I found the ground wire for the Pertronix Ignitor was barely making contact, but still have a miss. The engine was built for me by an excellent machine shop here in Sacramento, and has new timing gears on it. I may have just roasted the plugs and caps too much by an eratic ignition from the loose ground wire, and now a complete tune-up is in order. I'm going to go that route first, anyway.
 

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We did about 2 months ago. It ran fine for 2 months and then the problem came back, which is why I started looking for other problems causing it. I haven't changed the cap/rotor and plugs this go around, but now will, having found that bad ground wire on the Pertronix Ignitor.
 

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Miss could be ignition-related or could be cross-ignition between 7 and 8 if combustion gases from 7 are leaking into 8 since it's next in the firing order....

I think you're on the right track; continue testing......is the PPB hooked to the manifold plenum or to a runner? (my Cobra intake has a port on the #8 runner IIRC). Is the miss random or consistent?

Replace and verify function of plugs and re-test. Read plugs after testing and post results. Note any changes in engine operation at all levels, from idle to WOT.

Good luck!
 

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I've got a 289 with 1500 miles on it with Pertronix II and Flamethrower II. I thought it would be cool to gap the plugs to .045. My car ran terrible, missing, vibrations, rich smell, hard starting, etc, etc, etc...

I messed with the carb, wires, timing, etc with no improvement. I took the plugs out and regapped them back to .035 and it purrs like a kitten now. Not one symptom of what I had before.

This also came on slowly as well. It ran decent after I gapped to .045, then slowly came on so it didn't dawn on me right away that it might be the plug gap. It runs like a completely different car now.

You might try regapping the plugs to stock gap.

Let us know what you find. Thanks.
 
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