Food for thought!

I am in the process of installing a new header on my 89 4.0. WIth the intake off, I have thought of removing the head entirly and lapping the valves. THis engine has 150K on it, figure it couldn't hurt. HAs anyone gone this far, what results have you had. Thinking of decking the head to get a nice fresh flat surface for the new H. gasket.

IMO lapping valves will not do anything for you and would be a waste of effort.

Lapping is intended primarily to provide a perfect valve seal after the valves and seats have been ground or machined. Think of it as a substitute for 'breaking in' a valve job. If a valve does not seat properly after grinding, lapping valves will show the problem and, if minor enough, fix it.
This saves the risk of running an engine with a leaky valve and possibly burning it.

With that much mileage, your valves are either sealing well or are causing problems due to worn valve guides, or are burned or worn. Lapping will do nothing in any case.

IMO it would be very good to remove the head, inspect the ridge or wear on the cylinder wall, and get the head rebuilt. This includes removing valves, grinding seats, valves, removing deposits and new valve guides.

Ok, well I pulled the head and all "looked" ok. the only noticable thing was that the center (intake side) head bolt had allot of rust colored crud on it and the intake valves on 3-6 were cream/white color, exhaust were light tan but not as dark as 1&2 cylinders. The cylinder walls were glassy but still could see a good bit of cross-hatch pattern.
I'm going to replace the valve seals and clean up the valves, now just to find a good machine shop to deck the head.

Not sure if it will matter but I found on the first 2 cylinders that the intake valve were pitted at the seat an awful lot. The exhaust were in descent condition( didn't require much lapping). Got 2 done in about 1.5 hours, 4 more to go. As far as the guides go the valves have no slop side to side just a little carbon build up on the stem.

Well, here is the result. I lapped the valves and replaced the valve seals. Cleaned the ports ( carb cleaner and brush), and did the same to the manifold. Installed a new Pace setter manifold w/ Dynomax cat back system. IT RUNS GREAT!!!! I would have to disagree with Magic Rat, it was NOT a waste of time. Any one can do this, very simple thing to do, and its worth it. Just watch the torque specs on # 11 head bolt. I also used Napa manifold and extreme duty H-gasket.