Broken or burned intake valve

I suspect I have a broken, stuck or burned intake valve. I’ve done a compression test and have a 0 reading at cylinder 8 all others are within specs. I have pulled the rods and their straight with clear oil passages, the rockers look fine. With the engine running all rockers have consistent movement except for cylinder 8 intake (less travel). There is also blow by through the carb.

I will do a leak down test next and would like to ask if there is anything else I should be testing or performing before I pull the heads. Thanks in advance for your help.

 

That had crossed my mind, but I don’t have a dial indicator is there another way of testing other then sight?

Edit: The engine developed the systems while driving up a hill in third gear at 3000 RPM. The engine has 140,000 miles.

 

x2

 

Thanks for the replies so close to the holidays.

I introduced air into cylinder 8 and it escapes through the carb as expected. I then removed the valve cover to take another look at the rods, rockers and springs and found the intake (cyl 8 ) rod wasn’t lifting. I pulled the rod and it was bent.

I had previously taken the rod out to inspect it and all was good and I reinstalled it. I was careful to make sure the rod was centered in the plunger cap as I did with the others. Although I did notice that this particular rod didn’t sit in as well as the others.

Anyways, the bent rod is just another problem not the original problem.

 

Update: I wanted to measure cam lift so I removed the rockers and swapped in a push rod I borrowed from a 360 I had sitting in the corner of the garage. As mentioned earlier I didn’t have access to a dial indicator so I used a slide caliper and made the best of it. I measured the intake push rod lift at cylinder 8 and 6 both at ¼ lift (FSM 0.260 - 0.270).

I tapped the valves with a mallet and then did a compression test and the reading went from 0 (before tapping) to 60. I did this two more times tapping and testing and each time the readings went up from 80 then to 95 in cylinder 8.

The valve stem heights appeared to look even. I introduced air into the cylinder while tapping the intake valve then I put the rockers back on. I performed another compression test with a reading of 110 added a couple squirts of oil it then read 115.

The bent push rod appears to be my doing, I thought I was being careful while installing it but this particular one as mentioned in an earlier post didn’t seems to sit well.

The two intake cam lobes I measured seem to be wearing evenly although under FSM specs. The compression at cylinder 8 is still low (110) and there still is a pop through the carb while turning it over.

Compression reading 1) 170, 3) 160, 5) 165, 7) 155, 2) 165, 4) 155, 6) 150, 8 ) 110

Side note: I have no previous mechanical or maintenance history of this engine.