Found out today from a previous owner that he paid $900.00 for overhaul 'bout three years ago and it missed like this from day one. Mechanic would do nothing about it.

Gonna pull th' valvecover & look at th' valve train - wonder if th' guy got th' valve timing off? Any more thoughts(I put in new plugs, wires, cap, etc.)?

Thnx, Jim

My guess is that the valve lash is tight on those cylinders.
Pay close attention to the exhaust valve lash, try loosing the exhaust valves to .021 hot to see if that helps.
DD

Thanks, Doc. I will pull th' cover tomorrow. Is that somethin' that might not show on a compression check?

Tight exhaust valve(s) may not show-up on a static compression check on a cold engine. If the valves are anywhere between .002 and .019 too tight they will close to give compression but the engine will run rough and miss when idling hot.

Again, I bet this engine will run nice and smooth once the valves are adjusted correctly.
Watch closely on which rockers are the intakes (lashed at .010) and which are the exhaust. (lashed at .020)
DD

I have found such kind of idle problems, too. I did the valve adjustment, but it didnt help me anything. I have done the adjustment annually, and everytime the exhaust side has gone to 0,012"...0,016". Seems to me that the exhaust valves are going to sink to the head ('76 head casting, for unleaded). I quess the hardened surface has gone when the last valve machining was done, and now there is only the base cast iron to valve.

If the compression test seems noticeable difference between cylinders, I'll take the head off to the shop for hardened seats and new valves.

Because of my 1:3,55 gears the engine is running 3000-up almost all the time on the highway. I quess this is the reason for so short time between failures in the valve section. This head have now 25'000 miles after last opening. In my calculations the exhaust valves are now about 0,04" deeper in the head compared when they saw the dayligth last time!

JKKo

'66 Dart with 225 Super Six, D-Duals.

They were not only 0,04" deep, they were doubled that. The head was really ugly with loose liners etc.

There is now coming 12 new valves made from Ford 300 in line six ones. New bronze liners with Chevy style oil seals, not anymore with umbrellas for it. To avoid the exhaus side damage with unleaded the set of six hardened seats will be used. It is a professional shop specialized for cylinder heads only. The whole thing will be finished with multi-angle valve job.

Lets see how big is the difference with the speed. The cost of the repair is... ... a lot.

JKKo

'66 Dart, 225 Super Six, D-Duals

JKKo, tell me more about the different seals/seats you are using. Do you have any pictures? The stock replacment seals I use always seem to ride up the valve stems.