Valve clatter and exhaust leak noise with new head

Replaced a head on a 5.0L ford HO. Original head was cracked. Also removed other head, had the springs checked, new valve stem seals and milled flat (.006 removed). Reassembled with new exhaust manifolds and elbows. On the side with the new head, was experiencing what sounds like a valve clatter and exhaust leak. Futher inspection found no leak and compression check was good. Rough idle up to about 1200rpm then smooths out but sound is still there. Installed new lifters and same problem. Checked rocker arms for lash and torgue thinking I perhaps had a lifter not pumping up. Rocker arms were tight and all lifters pumped up...no lash and all looked good.

Running out of ideas. The new head was a Sierra after market from Marineengine. Came all gooped with an anti-corrosion grease. Appeared it may have been in storage for some time. Cleaned it up and installed. The side with the old head is whisper quiet. Wondering if I have a new head with a faulty valve or spring. Or mayber it is gummed up and a valve or two are partially sticking. Maybe just needs run for a few hours....but a bit nervous to do that.

Late night research may have paid off. Think I made an amateur mistake and it matches with where the exhaust leak seems to be coming from. Looking at exploded head diagrams and looking at the old cracked head I notice two what look like threaded inserts on each end of the head. I thought it was just an insert into a blind hole and since I did not absolutely need them, I assembled the head without the bolts installed. Now believe they are what are called thermistor holes that need plugged because they access the exhaust. Does not seem to be OMC equivalent of these insert still available, but I'll bet the auto parts store has aftermarket plug kits. Might this also be causing the rough idle?

I have wire tied the spark plug wires together and found some information that this can cause induction in adjacent wires and result in premature firing on cyclinders, also causing a knock. Will readjust and see if it helps. Adjust carb, check timing....may be getting there.

RTFM.

good shootin, Cameron

some cheap plastic spacers work well to keep the plug wires neat without touching, and there's a trick for weaving them spaced with tiewraps too.

have you done a vacuum test on the engine

Have not done a vacuum check but installed the thermactor plugs/inserts today. Mistakenly called them thermistor in orginal post. For an OEM replacement marine head I would expect them to come pre-installed and perhaps they were supposed to. Only one was not installed; I believe one was mistakenly overlooked. Quieted it down immensely. No valve or exhaust leak noise at all. And bonus, the low RPM roughness smoothed out which I was not expecting. Engine running wonderfully. Will be taking out this weekend for a good cruise.

Thanks for tip on the wire routing.

Incidently, the thermactor threaded inserts were unavailable at any autoparts store if they even knew what you were talking about. They are available from JEGS online. I found them at local machine shop that local marine mechanics like to use. They immediately knew what I was talking about and had a box of them for $2.00 each. Suspect most machine shops will have them stocked.