Burned Valves

[James]

What is required for troubleshooting (not a complete list)(Like Ken said without seeing it we need a lot more information):
What prompt you to check the compression (rough idle)?
Did you do a compression leak down check (different than a compression check) on the cylinders to verified it was the intake valves and not the rings?
Did you check the valve clearances before disassembly?
Did you take a picture before removing the valves from the head?
Did you take a picture of the valves before cleaning?
Did you take a picture of the valve seats before cleaning?
The last time the engine was running, was there a lot of vapor coming from the crankcase breather vent?

Being it is the intake valve, was there a lot of carbon on the valve stem/head and the intake valve pocket/seat?

The way the valve look (I'm guessing here) worn in one spot, I suspect carbon on the seat had worn the valve face off. If the carbon was on the valve seats then it could help seal the valve until it broke away. Then the engine could have been running rough at idle.

What I suspect:
If there was a lot of carbon on the valve, guides are worn out (the o'ring inside the valve retainer don't stop oil vapor from entering through the valve guide, even with a shroud on the spring). With worn guide I suggest a bronze sleeve be installed (In my opinion knurling the guide is at best a temporary fix) on both intake and exhaust guides. I would also have all guides machine for positive seals.

Both heads should get the same repair work.

It look like you might have enough valve margin to grind the valve to new condition.

There is a possibility the valve job will not fix the low compression, if the rings are worn out (this is where a compression leak down test would help).

[CMCamp]

I have attached a picture of that valve before it was cleaned. Should have sent it along with the cleaned shot. Thanks for the feedback.

[AZKen]

"Listen son, Now that there is some bad car...I say listen son, that there is some bad carbon"

[duallyjams]

With two cylinders with bad intake valves, was it popping back through the carb ? that's a definite sign of a burnt valve or was it just running ruff.

[CMCamp]

I bought this truck back in August and started a detailed restoration in October. It has under 15K miles on it, so I wasn't expecting major issues with the engine. What I was chasing was significant black smoke on startup. I thought it might be my getting use to the choke setting on startup, so I kept backing it off until it was very hard to start. Then I ran a compression test and found very low compression in cylinders 2 & 4 plus moderately low in 6. I decided to pull the heads. The first thing I saw was a very dirty intake manifold, lots of carbon buildup. The intake valve in cylinder 1 was damaged beyond repair. I decided to replace the intake valves in 2, 4 and 6. I worked with Jude of Obsolete Automotive Wholesale in VA. He was a tremendous help in identifying and supplying the correct valves. The heads are back on the engine now, but I haven't run a compression check yet; waiting on some other parts. I'll post an update when I have the new numbers. Should be later this week.

[sclor]

Fwiw, seems awfully strange burning intake valves. With cool air gas mixture running through intake, my experience has been that intakes never burn unless they were improperly adjusted and couldn't close all the way. I can't see any other reason for intakes to burn and exhaust valves to hold up.
Regards
Steve
New Orleans.

[CMCamp]

Well, the results are in, compression test is complete. First, a recap of readings before we worked on the heads.

Left bank (psi):
Cyl 1 - 135
Cyl 3 - 135
Cyl 5 - 130
Right bank (psi):
Cyl 2 - 47
Cyl 4 - 47
Cyl 6 - 115

Now the readings I took yesterday.

Left bank (psi):
Cyl 1 - 137
Cyl 3 - 135
Cyl 5 - 125
Right bank (psi):
Cyl 2 - 135
Cyl 4 - 135
Cyl 6 - 132

Looking good, but curious that we lost some compression in Cyl 5.

I'm left scratching my head on how these valves were damaged. I haven't owned the rig long enough to know its history. The best theory I've seen is the one suggested by sclor. Perhaps at some point someone incorrectly set the valve clearances, allowing the inlet valves on 2, 4 and 6 to remain slightly open. They must have gone back in and corrected the clearances, but the damage was done.

Thanks to all for the back and forth on this. Now on to figuring out how to get the exhaust system replaced. Muffler shops up here don't want to bend pipe anymore.

[James]

Originally Posted by CMCamp

Looking good, but curious that we lost some compression in Cyl 5.

To locate a leak in the combustion chamber you need to do a compression leak down check. With equipment similar to this:
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1