This engine is #3 in the life of the boat, and probably has about 30 hours on it. The second engine I put in the boat lasted about 11 years with the stock carb settings and the tau ceti flame arrestors. That engine lost a cylinder shortly after a head pipe leak, so I assumed it was from water ingestion that time. Since it had over 100 hours, I replaced the whole thing instead of doing a top end. Since this one has relatively low hours, I decided to fix it. It may turn out that I have a front seal leaking at higher rpm but passing a pressure test on the bench. I don't know yet, but I'm going to try a new piston one more time. I'd hate to do a whole engine to find out that wasn't the problem.
I might have found what has burning the piston. I unhooked the carburetor/fuel strainer fuel line from the fuel strainer end and spliced in a one foot long piece of clear line. That revealed a small consistent stream of air bubbles. Since the strainer was hanging by the lines, I turned it upside down a few times to make sure any residual air inside the strainer got carried downstream. The air bubbles kept coming. So I then took out the fuel selector valve and pressure tested it out of the boat. It stood the pressure test and the soapy water test. Then I spliced a clear line into the "on" line coming from the tank, and plugged the "reserve" line. I hooked that line to the fuel strainer. There were no bubbles. Then I spliced in the fuel selector valve into the clear line. No bubbles. So, I decided to splice in a short clear line between the carb return outlet to the return line to see what was coming out. At first, there was alot of air coming out of the carbs and going towards the tank. Then just a few bubbles would hang out at the high point of the line near the return nipple of the carb, but they remained stationary. So, I took it out for a ride on the lake and after some riding even the bubbles in the return line disappeared.
Before the MAG piston melted down a couple of months ago, I was getting an occasional slight surge while cruising between 6000-6300 rpms. The surge cleared up at full throttle though. So I rebuilt the fuel pump and went through the carbs. I didn't see any visible leaks in the lines and the fuel strainer was clean. So, I did a top end thinking the problem was head gasket. I didn't see anything wrong in the carbs, so I thought since the head gasket had leaked a few months before, that was the cause of the melt down. When I had replaced the head gasket, I felt very slight wear in that cylinder and decided to run it because the compression was still good.
So, the new top end made it through break in with no issues. Once I started getting into that 6000-6300 rpm range, it started that little surge again. So, I'm thinking head gasket causing water ingestion. This time, the compression dropped.
Since I caught it early, this second time I just honed it out and did the one MAG piston. This time the dry head gasket failed the pressure test on the bench. So I copper coated it and it passed the pressure test.
So I run it through the break in again, and everything is good. First day out after break in, I notice an occasional slight surge at cruising speed. This time its so slight, I can't even tell if it was a surge or just my thumb pressure varying the throttle. Then it started getting a lean hesitation at take off. It also had that muffled popping sound in the exhaust under load and under 4000 rpms. So, I back the low speed adjusters out to 2&3/4 to get rid of the lean hesitation. That got rid of most of the popping too, but not all of it. At this setting, the plugs would show wet with fuel after idling for a while. So I increase the jet sizes, but that doesn't get rid of the occasional popping. So I was thinking since the ski ran good all these years with the smaller jets, its probably not carb settings.
So, finally I hook up the clear fuel lines and see the bubbles. The surging and lean condition is consistent with bubbles in the lines, so I'm hoping I found the culprit. That MAG piston does have the beginning of a melt through depression in the center, so its time to replace it again. I'm willing to install another piston and try again now I that discovered there were bubbles in the lines.
Oh, so the line between the selector and the strainer got clipped 1/2" on each end. The ends looked a little dry. That must have been the source. I will post up whether or not the next piston holds up. It might be a couple weeks from now.
An EGT would have been nice through all this. Now I'm also thinking maybe a detonation sensor with an audible alarm.