The engine misses although with a timing light the ignition never misses? Backfiring out the exhaust although the timing stays within spec? I would agree with mrfixit that this has to be a valve issue. Sounds like an exhaust valve isn't seating- as mrfixit suggests I'd check the follower clearance, if that's within spec I'd check all the valve springs. If all looks ok there I'd suspect a faulty valve seat.

I think the next step may be to look under the valve cover. It just has adjustable cam follower rocker arms, not hydraulic, so having valve noise seems strange. Check the valve springs closely and see if the follower clearance is about.010-.012".

I placed a pressure gauge at regulator and pressure stays steady through brake up. When the engine starts breaking up it misses and pops out the exhaust and the valve train chatters.

Are you sure it is not starving for fuel due to filter, hose, or tank connector?

I would do a compression and leak down test on the cylinders and see if you have a valve issue,vacuum will also tell you if you have a valve problem either ones hanging up or burnt???

I'll have to give this one some thought. Just so I'm clear:

Complete tune up has been done
Fuel system has been replaced
Removing exhaust has no effect
No pressure build up in hyd system

When engine slows down- no missing present- slows down like throttle being closed

At lower RPM it seem to take a little longer but, does the same. On the exaust system, I removed the pipe at the manifold and it did the same. Their is no load on the hydraulic. I messed with it a little today and I noticed that even at an idle now the engine will miss a little and idle smooth again over and over just about 5 seconds apart and the cycle is getting shorter at high speed, its now about 15 to 17 seconds. What ever it is its getting worse.

Does it do this rhythmic cycle only at full throttle (up against the governor)? You mention 25 seconds at full throttle- if it does do this cycle at lower rpms, is the amount of time for it to start this cycle the same? Looking outside the box: could be hyd system, trans or even the muffler (I've seen them collapse internally & cause backpressure)- just some thoughts

Tried two different distributors that worked fine in the other trucks. I have watched the timing mark as it brakes up. At full speed it advances to about 8 degrees and as it brakes up it slowly drops back to about 2 degrees, never stops firing or skips a flash. I have talked to our local engine rebuild and he said he can't think of anything internally that would cause this.

I've never seen this on a Yale- only on Toyotas, but you never know- try adjusting the ignition timing when the engine starts to break up- could be a ignitor going bad.

On Toyotas I've found that when the ignition electronics start to go bad it affects the ignition timing. Usually it's during the starting cycle that the issue comes to light- unit won't start- extremely advancing the timing while cranking will get the engine to start- then retarding the timing to normal specs after starting keeps the engine running good.

Just a thought- the chatter could be from the ignition timing changing on it's own- try checking timing with a timing light while the issue is happening- keep me posted

Governor was replaced. no change. Governor was removed and ran engine without it, no change. carburetor was replaced. no change. lp fuel lock off replaced. no change. lp regulator replaced. no change. Their is a lot of chatter from the valve train when the engine starts to brake up.

Sounds like the pneumatic engine governor under the mixer (carb) is going bad. If you dead head the tilt cyl's, putting the engine under load & accelerate the engine- do the symptoms change? If the engine rpm raises & then plateaus & stays there, replace the governor