Every 1000 miles, the truck won't idle. Compression is low in 3,4,5,6 cylinders. Adjust lash and runs good for another 1000 miles or so. Any suggestions?

 

What are your compression numbers? I'd suspect the rockers aren't holding a setting, or the valves are really burned. Any chance you are adjusting them incorrectly, i.e., not at TDC? Kind of suspicious that two are good and 4 are not.

 

I bought the engine which was was represented to having around 2000 miles on a complete rebuild. After 500 miles the camshaft broke in two places between 3&4&5 cylinders. Took to machine shop. They did anot her rebuild and set the valve lash cold. Once running and up to running temp checked lash, had to reset exhaust valves(to tight). After 500 miles, engine would not idle when at op. temp. Checked compression, 3,4,5 had 70 psi. Reset valves warmed engine to operating temp, all had 125+ psi. Drove 1000 miles, engine would die at idle, reset valves, all 125 psi, after warm up, same as before. Did all again, this time 6 has dropped compression like 3,4,5. I

 

Sounds like the valve adjusters may be to loose .
How much effort does it take to turn them , I assume they don't have a locking nut ?

 

The adjusters turn hard.

 

I'm thinking his valve adjusters do have locking nuts. This is how my engine's valve adjusters are set up:

 

Just to throw something out here that I have found interesting about adjusting valve lash, I have an old 289 whit iron heads and a mechanical camshaft, I adjust the valves a little loose when cold and i achieve .019 when its hot.

I have basically the same motor with Aluminum heads and found that I need to adjust the valves a little tight when cold and I achieve .019 when its hot. Reason being is to allow for expansion of the aluminum heads. I took me a long time to get this thru my head when I first noticed it and it seemed like everyone I talked with was already aware, I guess I missed that Memo.

 

The only way the vales could be getting tighter after running a while is if the valve seats are receding. (been there, done that, got the shirt...) Does the engine have hardened valve seats?

 

The machine shop said they put hardened seats in. Since the original camshaft broke I asked if they checked cam bearings bore alignment, and they said that it wasn't necessary.

 

Am I correct in assuming that when your compression drops after 1000 miles, you are finding .000" clearance Hot? Are the valves actually being held open? When they get that way, is the Cold compression also low, and clearances small?

 

Cold compression is low after 1000 miles.

 

did the seats bottom out eventually?

 

Once there's low enough pressure on the seat, they probably would. With properly done hardened seats and a fresh valve job, my clearances hadn't changed at all when I checked them after a year and 4000 miles.

It sounds like something is wrong on yours, either the valves or the seats. Might be worth pulling the head to see.

 

I pulled the head and plan to take it to the machine shop. Glad to get your comments, gives me hope that the problem can be cured. Still have concerns about possible prombles where the camshaft broke.

 

I have never heard of a cam breaking in two places, that sounds incredible, frankly. Crazy valve pressures, incorrectly installed bearings, I don't know.

This engine rebuild sounds fishy.