Why? Detonation. One question is what does the pistons look like? The valves look very clean. Did you clean them up? I do not see any marks from the plug electrode hitting between piston & head.

I do not think compression is too high or had too much timming. Either motor got too hot and/or leaned out. If any parts of the plugs, parts of pistons, or valves look like you took cutting torch to them then not enough fuel is the problem.

You may have so-so fuel pressure at idle but what about during a good long run? Do you still have the anti-siphon fitting in fuel tank?

That's gona leave a mark---- WOW that valve got hot. You caught it just in time. That engine was running lean. The poping tru the carb was from the fuel being ignited by the red hot exhaust valve before the intake was even closed.

I agree, detonation. Fatten her up a bit and back off the timing when you get it back together. I had this exact problem several years ago. Would actually fire the electrode out of the threaded metal part. Scared the chit out of me twice before I did something about it. Thought I may have just gotten a bad plug the first time. Not. That motor is now an anchor.

Be prepared for at least one (sounds like maybe two or three) new pistons. And of course several valves too.

Jim, What type of intake are you running? All the middle cylinders are cooked and the end ones arn't. Does your other motor have all the same parts?
JB

PS, Have someone good check out your exhaust valve seat. Heat (which you had) can cause it to loosen and break and or fall out later and cause piston damage!
I have a set of 420 Merc heads for sale with all the parts.

John B, Funny you should ask about the intake. They are the ones that came original on the Hawk engines. They are the Edelbrock C454 intakes made for 4500 dominators, with spacer/adapter to 4150 carbs, (previous owner made swap) and the spacers from the intake to the Tall Deck block.

I have not made the switch yet, but just received the new World Products / Dart Tall deck to 4150 carb with the Brass clad water jackets.

Do you think that may be leaning out the inside cylinders? I would think it would be the opposite. I have more pics to post coming up..

Instead of posting the pics here and clogging up the system, I posted these on my site of the heads, pistons, old and new intakes, and spark plugs. I'm tired so I hope the pics are self explaining.

click below
Heads and Pistons

As soon as I saw first pictures it brought back old memories. Of course my motor was a twin turbo motor, & I think I was running 15 lbs of boost at time. I would say not enough fuel. The plugs are result of detonation. The motor I had melted down looked like someone had taken cutting torch between cyls. on both head & block.

I am surprised the pistons do not have any parts missing. I always seemed to lose the corner where valve relief was. From pictures can not tell but area cyls. scuffed up?

What kind of scuff marks would I expect to find?

If you look in the cyld. & do not see any scuff marks (lines/scraches in bore going from top to bottom of bore) you are probably OK. When the motor is in detonation it is wanting to make the pistons go down in bores crooked, thus scuffing the sides up. I would say if pistons look good & bores look good you are very lucky. If you do not see the hash marks in bores I would rehone & rering pistons. Easier to do now than after you get it back together again.

I do have some vertical scratches. Guess I'm gonna have to look at those more closely.

Thanks