Melted a piston?
 

Well, after getting the carb/idle problem worked out on my friends boat, I'm afraid he melted a piston.
Engine: 502, intercooler, B&M 250 blower, single dominator, 6psi boost, 30 degrees total advance.
He ran it out of gas (incorrect guage) at nearly WOT. The engine started misfiring. Compression check showe 0 psi on cylinders 1 & 2. The guage didn't even flicker while cranking. Remaining cylinders were at 135psi.
Anyone ever burned a valve or blown a head gasket wher the compression guage didn't even move while cranking?

Re: Melted a piston?
 

Might be lucky & just messed up some exhaust valves. If you inject compressed air in the cylinder you will hear where it is excaping. I bet he messed up an exhaust valve at least.

Re: Melted a piston?
 

I just did a compression test yesterday and was reading 0 lbs on a cylinder. I popped the head off and had a bent exhaust valve. The compression guage did not move at all.

Re: Melted a piston?
 

Gary,

this one showed no compression at all.

Re: Melted a piston?
 

Check The Valves ,ive Tuliped Many Exhaust Valves When A Carb Has Gone Lean Under Load,ran Like Hell Before It Stopped

Re: Melted a piston?
 

Check The Valves ,ive Tuliped Many Valves When A Carb Has Gone Lean Under Load,ran Like Hell Before It Stopped,

Re: Melted a piston?
 

I did that to a 7.4 MPI at WOT. I have one piece of what's left of the #1 piston. Exhaust valve started sticking when it appearently went lean and "POP" and that was all she wrote as soon as the piston hit it. Left a gapping whole in the cylinder where the oil and water exchanged places as she was locked tight. RIP 7.4 MPI. :)

Re: Melted a piston?
 

Good news is that it's not locked up and will still run.
Didn't have my leakdown tester, but that's the plan. Hoping for the best preparing for the worst.

Re: Melted a piston?
 

Gary- For what you're describing, you may not need a leakdown tester. If you have an old plug shell that you can weld an air hose nipple to, just pump those cylinders up when they're at TDC. Simplest thing first. Pull those valve covers and drop the tip of a mechanics stethoscope down the past the cam into the bottom of the block. The scopes with a rubber hose work best for this. If nothing abnormal, then drop the exhaust manifolds and listen through the exhaust ports.If you haven't done this before, which I doubt, you may want to calibrate yourself on a good cylinder. Relatively speaking, you'll hear a pretty good 'whoosh' sound even on a good cylinder down in the block as air rushes past the rings. Usually intakes leak nothing. Due to the environment exhausts run in, you usually will hear a little leakage from a good exhaust valve.Let us know how you make out

Re: Melted a piston?
 

Bad news. Air was whisling through the breathers when hooked to #1 cylinder. Expensive weekend.

Re: Melted a piston?
 

Sorry to hear that Gary. I always try to be optimistic. In this case, at least the thing didn't come apart. I'd concentrate on making sure that I found out exactly what caused the problem. Good luck! Lou

Re: Melted a piston?
 

Gary,


this is one thread from my Good Friday holed piston. http://www.offshoreonly.com/forums/s...gine+tech+guys

Near the end you will see the cause of just one lean cylinder.

Dont think this would apply in a blower setup, but vacuum leak seems like a possibility.

Re: Melted a piston?
 

Fortunately it's not my boat or money paying for it. Belongs to a friend, but he's not too mechanically inclined so I do most of his wrenching.
BTW, the problem was due to lack of fuel pressure. He ran it out of gas at WOT! Leaned out on the 1,2,&7 pistons, melting them. Inaccurate fuel guage, he said. Very expensive lesson for a boat he only ran 2 hrs.

Re: Melted a piston?
 

I have an adjustable low fuel pressure switch that u can get from a speed shop that deals in NOS. Normally open switch that I have set for 4.5lbs add into the Mercruiser alarm system. I also added an auto style backup beeper to the mercruiser buzzer so that I can hear it at speed. Cheap insurance.

Re: Melted a piston?
 

Good idea. I can get 5psi marine hobbs switches for about $10 and wire to the alarm.

Re: Melted a piston?
 

[QUOTE=Gary Anderson]
BTW, the problem was due to lack of fuel pressure. He ran it out of gas at WOT! QUOTE]

That was yesterday :rolleyes: I have to keep reminding myself to go back and read original post, easy to forget the facts.

Re: Melted a piston?
 

Might want to check open and close pressure on the bench, problem with the cheaper ones is that they don't consistently do what they are supposed to. The NOS switches are more expensive but very consistent with a small range between open and close pressures.

Re: Melted a piston?
 

Just found the installation sheet
Nitrous Oxide Systems
part #
15660 adj 4-7psi
15750 5psi
15680 adj 14-24 psi
15685 adj 25-50 psi

Normally open switches, about $37 jegs etc.

1/8" pipe thread

Re: Melted a piston?
 

Thanks,
I would have more confidence in that over a $10 sierra part.
Gary

Re: Melted a piston?
 

Gary, I can feel the pain
Ijust did the same thing to my newly Procharger 454Mag.
The strange this was that I didn't lose a whole lot of power.
Just pulled my piston out this AM. I burned a hole thru #5 exhaust valve, tuliped what was left, cracked exhaust seat, started burning thru the pistion and scored the bore.
I'm gonna end up lucky because I can get away with just honing the cylinder.
Just sent my carb to Dean, so that will be the majority of this lesson $$$.

Mo