Burnt valve?

I bought a 74 commando in bad shape and rebuilt it about 15 years ago. The past couple of years it got harder and harder to start, so I replaced things like plugs, manifold gaskets, coils, and boyer box. Last year, I never got it to start. So I finally got around to pulling the head, and this is what I found:
https://drive.google.com/drive/folders/ ... nl4N3dCa0k

In addition to looking like the inside of a coal mine, It looks like one of the exhaust valves has a couple of flat spots. Definitely not sealing; liquid will flow through. So my questions are:
1) is that what a burnt valve looks like? (sorry it's hard to see in the pictures)
2) would that contribute to the hard starting?
3) if it is a burnt valve, is that generally a symptom of another problem? I know at one point I had a cracked manifold gasket, which led me to run it rich for some time. Would something like that contribute to a burnt valve?

Thanks for any advice!

Looks like the ex valve is shot. I would guess the guides are probably worn out too. Better measure
the jugs for wear too.
Refresh time for the top end. If it was me, Id pull the engine and strip it to check the bottom
end as well. Just that Id rather do the whole thing and remove any worry.

Yeah that's a burnt valve alright on the left side of the picture (timing side) likely from too much coke build up getting between the valve and its seat also likely due to intake valve seals being shot (both sides were really coked up). I am not one to completely rebuild a bike when I encounter a problem but to usually address the problem if all else seems well. Depending on the valve seat's condition you may only need a new valve and seals. Others here will "tear me a new one" for not suggesting a more thorough rebuild. If you know your bike and know there are no other existing problems, then why not?
Lance

Pour some fuel in the combustion chambers with the head upside down. If it leaks into any of the ports before it evaporates, those valves are not seating. Once you've removed the valve springs, if the valves wiggle around when they're off the seat, the guides are probably shot, too. Once the guides go, the valves bounce around when they hit the seats and the seats get pummeled. For best results, new valves and guides and replace any seats that can't be recut to spec. If the seats have to be recut, as they often do with new valves and guides, check the assembled heights of the valve springs. You may have to shim the springs back to spec height. And don't forget to check the head gasket surface for truth (flatness).

Years ago when I was racing my Triton. I had a small piece of a cast iron valve guide break off and ding an exhaust valve seat. I felt the motor go slightly off-song while I was racing however it took me ages to find the problem. When I found it after pulling the valves out of the head, I noticed a small mark also on the piston under where the exhaust valve would be.

seventyfour said:

. So my questions are:
1) is that what a burnt valve looks like? (sorry it's hard to see in the pictures)
2) would that contribute to the hard starting?
3) if it is a burnt valve, is that generally a symptom of another problem? I know at one point I had a cracked manifold gasket, which led me to run it rich for some time. Would something like that contribute to a burnt valve?

Thanks for any advice!

Click to expand...

Answers:

1) Yes
2) That gap under the valve is like a compression release, which is considered to be a starting enhancement. Hard starting is likely due to something else.
3) Possibly, carbon build up under the valve could hold it cracked slightly, letting hot gasses seep past the seat. Other cause could be insufficient clearance in the tappets.

Slick

acotrel said:

Years ago when I was racing my Triton. I had a small piece of a cast iron valve guide break off and ding an exhaust valve seat. I felt the motor go slightly off-song while I was racing however it took me ages to find the problem. When I found it after pulling the valves out of the head, I noticed a small mark also on the piston under where the exhaust valve would be.

Click to expand...

That's one reason racers favour bronze guides.