Rings or more?

Hello I'm not new just back after a hiatus. My 67 i6 runs great on new plugs but soon foils out. I sent the head off to machine shop got the works. After some of my own porting and polishing. Re installed and same symptoms. I have just bean cleaning the spark plugs every 2 trips. But I'm ready to fix it. During the head r and r the piston walls all looked good. I'm not sure if I want to do a full rebuild or a proper fix. And any improvements I can make. Staying with factory equipment (visible). Holly 1100,manifold.exe

that's a tuff call without seeing some pictures of the cylinder walls or using a bore taper gauge or seeing the pistion to cylinder wall clearance measurements. My best guess from your description of the oil fowled plugs though is that your Rings are worn out. Generally though if the walls are decent you could cut the ridge out, use a ball hone and install a new set of Moly rings, Rod and main bearings for the basics. Add to that as needed a new quality timing chain set, instal and degree in a nice modern grind cam with fresh lifters, a new oil pump and good set of gaskets, seals, and new freeze plugs along with a thorough cleaning of the short block. That should give you a good low budget rebuild, many places still offer a budget Re Ring kit like NAPA that will give you most of those parts except for the cam, lifters, oil pump, and timing chain set some the places will also add some of those other parts to their kits to also save you some money. Good luck on your rebuild

"...runs great on new plugs but soon foils out…"
frm oil, A/F improper mix, rings?
Try the easy 1st (cheeper 1st).

Hi, do a compression test. First dry and next after squirting oil into the cylinder. Are you sure you have the correct spark plugs, and the ignition system is working perfectly? I would not use modern platinum plugs. Just old style like motorcraft bsf82c, or hotter. Good luck

Howdy John:

What plug heat range are you using? What brand? What gap?

I Prefer to check the easy stuff first. Keep it coming.

Adios, David

John, different ring designs call for different honing grits. If you go with plasma-moly make sure the cylinder walls are slick as possible, 500 grit with plasma honing by your machine shop. Moly rings are not sensitive like the plasma-moly.
Cast rings will seat ok with 380 on up, they break in fast.
Have your machine shop determine if boring is necessary.

Is it just one plug or all six? If it's all then it could be caused by a rich condition along with cold plugs. If its just a few; maybe the machinist left off some intake valve seals. When checking the compression, if you squirt oil into the cylinders to check the rings, the oil will fill the dished pistons and raise the compression ratio; giving a false reading. Maybe hook an air hose to the suspect cylinder and listen thru the oil fill.