86 apache 22

I am in the process of rebuilding my port engine. Gen VI 502 carb motor.
Things are going well except I would like to be certain that I don't re-create the same issue that caused the need for the build.

The last cylinder on the port side had a melted piston. It was on the very edge as to where the ring was visible and a hot spot in the center of the same piston. All other holes looked fine.
Besides the obvious such as fuel pressure, the carb(which I've purchased a new 750 Dbl pmpr), is there something specific that I should pay attention too?
Everyone on here has been a great help, your thought are appreciated. Thanks,
Alex

Griff

Your new 750 carb is too small. You should have gotten an 800 at least.

stevesxm

the failure you describe is from detonation. several things cause it, all well known...

too much timing
too little timing
too lean
vacuum leak at the manifold
bad gas
steam pocket in that cylinderhead causing localized overheat

and any combination of the above to various degrees. some of the above i would expect would show symptoms in all the cylinders with that one the one that failed first. others would be specific to that cylinder.

you are the one there looking at the hardware and you need to very specifically id what went wrong before you make the same mistake again... but of course you know that which is why you asked the question... my point is that i don't think it is going to be some mysterious space alien reason requiring a lot of expert insight. it was something very basic that you simply have to look at your parts and try to reassess the scenario when it failed to tell you what went wrong.

and while griff is right that that carb is small for max power, you should be able to get it rich enough without any problem such that it wouldn't cause this problem.

raeburn

This is what happened to me a couple years ago. Two things fixed the lean/detonation problem that caused this.First the carburetor was flow tested (it was a C&J Holley 850 - nice carb but it didn't flow evenly to all cylinders with my Dart intake (#7 was way too lean) - it took a crazy combination of jet sizes to get an even balance of fuel to all cylinders) I never would have figured this out on my own without having the engine flow/dyno tested.
Second issue was fuel supply. The stock 3/8 hoses were okay, but the anti-syphon valve at the fuel tank was sticking and causing intermittent fuel starvation at full throttle. I never would have discovered this if I didn't decide to pump out the fuel in my tank while the engine was getting rebuilt. At first the fuel was pumping out full stream and then it would go down to just a trickle. I would shut the pump off, try again and same thing!. After replacing the anti-syphon valve with a $0.98 brass fitting, the fuel was pumping out at 120 gallons per hour no problem.
Check your fuel flow, and definately have your engine dyno tested before putting it back in your boat - was the best money I ever spent!!

86 apache 22

Thats unbelieveable!!! That looks exactly like my engine. Same cylinder, looks like my piston. I will take all those precautions, Thanks.

raeburn

#7 piston is almost always the first to go (big blocks seem to always run leaner/hotter in #7). I would definately remove your anti-syphon valve - especially if your boat is more than a few years old (mine was a 1989). Dyno-tuning can reveal so much more than just how much horsepower your making!! Just make sure you do your homework and find a reputable engine builder/tuner who knows what they are doing!