A Diesel Guy Dealing with a STUPID GAS ENGINE!

[CharlieB]

Re: What Causes A Burnt Valve?

Re: What Causes A Burnt Valve?

You still need to resolve the weak spark issue,

Post #2 has pointed out the coil should have TWO wires on the + side. One coming from the starter solenoid to provide a full 12 V during cranking, the other should be a resistor wire, measuring @ 6 V running.

Failure of the solenoid contacts will cause a weak cranking spark.

 

[Howard Sterndrive]

Re: What Causes A Burnt Valve?

Re: What Causes A Burnt Valve?

Yeah, I didn't think so.

I sure wish I knew what the hell I did to cause this to happen. Did everything by the book... Very frustrating.

my guess would be incorrect valve adjustments, or a lifter decided to pump up a bit after running with full oil pressure.
I always use a compression gauge setting marine engines. The half turn 3/4 turn stuff is fine for cars, but the continuous load on a boat engine gets me verifying the valves are right in the middle of the adjustment range. That way if a lifter decides to pump up a tiny bit more it isn't going to burn up a valve.
Crank starter with valve covers off. watch compression. tighten until the compression starts to drop (valve hanging open), then loosen it until it rattles- then split the difference.

 

[wi4x4man]

Re: What Causes A Burnt Valve?

Re: What Causes A Burnt Valve?

Well, the funny thing is, is that I followed the book to a T. But, you are right! The way that they describe valve adjustment in the book is complete crap. So, I did exactly what you said and used the compression gauge. I ran it, and checked everything again. I even checked it other times as well. For the first 20 hours, absolutely no problems. I mean, there were none at all.

This afternoon I got the engine running again, even though I wasn't developing much compression on #5. Out of the water without a load, she ran pretty well actually. So, I splashed her thinking that perhaps by some stroke of luck it might clear itself. Yeah, wishful thinking I guess. It ran and propelled the boat, but she definitely ran bad with a load. I pulled the valve cover and did some running changes on that valve, but nothing helped either way so I brought her back home. You could hear the high frequency hissing coming from the air intake.

Oh well, it looks like I will be able to pull the head without having to pull the motor.

 

[Howard Sterndrive]

Re: What Causes A Burnt Valve?

Re: What Causes A Burnt Valve?

a poor valve guide and seal in the head can cause combustion gases to leak past the valve. That can superheat the valve spring and weaken it. then the valve hangs. The carbon you mention can cause the valves to stick too. Need a good working thermostat in the engine to keep the heads nice n toasty.

 

[wi4x4man]

Re: What Causes A Burnt Valve?

Re: What Causes A Burnt Valve?

I did the whole hot water test with the t-stat, and it opens right where it is supposed to open at. The water pump flow is great too...

Hopefully in an hour the head will be off and I can see first hand what the heck happened. Maybe I will post a picture or two.

 

[sschefer]

Re: What Causes A Burnt Valve?

Re: What Causes A Burnt Valve?

Orange spark can be as simple as a bad ground to the block. That hissing back through the carb is probably a bad valve seat or weak valve spring or a combination of the two caused by a bad guide. Intake valves rarely burn.

 

[wi4x4man]

Re: What Causes A Burnt Valve?

Re: What Causes A Burnt Valve?

Well, I got the head off and here is what I found.

1) No damage or marks to the piston or the valve at all, so at least it didn't whack the top of the piston due to bad adjustment.

2) Everything was a very flat gray color (except for the walls which shined) but the suspect valve had a different color to it... It was gray, but shiny in a way. Definitely not the same color as the other 5 valves on the head.

3) Although I cannot see it, it "appears" to be seating all of the way on the valve seat. But who knows, it doesn't take much of a warp to give a tiny bit of gap to appear.

4) On the dead cylinder, the side wall of the cylinder has a stripe down the side about 5/8" wide or so. I definitely cannot feel any difference going across the border, and it is just as smooth as the rest of the wall. It is in the same spot that the wrist pin sits. None of the other cylinders have this "stripe" down the side. It didn't wipe off either.

5) All cylinder walls were just as smooth as could be.

So, I am wondering why was that valve shiny, but the exhaust valve was the same color as the rest. Could it have just overheated and baked causing the carbon to crystalize? Kind of grasping at straws here.

For that stripe, I certainly hope that doesn't mean that something is going on with the piston itself. When I did the leak down test, air was definitely coming out of the carb and not the breather holes on the valve covers. It was analogus to a cool gas-smelling breeze coming right out through the carb. The only time air wouldn't come out was when the exhaust valve would open, otherwise it didn't matter where the engine was sitting, it came out. I am hoping that this stripe is not due to bad rings or something wrong with the wrist pin itself. In other words, a problem down the road.

All in all it took about 3 hours to tear everything down while it was still in the boat. I guess that is not a bad amount of time to get a head off. Beats pulling the engine to do it.