Diagnosis Needed

Posted by Owen_Dyneto On 2010/3/27 17:50:11

I've been scratching my head over a problem with my old 22nd series Custom 8 which I sold some years ago and myself and the current owner would appreciate any ideas.

First a little background. It was a very high mileage car and while I owned it I did some engine work. Cylinder wall taper was negligible but pistons were worn, so I did a light hone, replaced pistons, rings, wrist pins and bushings, mains and rods (crank was not removed), replaced all exhaust valves and guides, just ground the intakes. Cam was removed, polished, and the lifter bases were polished. Existing springs and lifters were retained, all this was in the 80s and the car has been a sterling runner for perhaps 20,000 miles, up until the current problem. Since my ownereship the current owner has used it very sparingly, perhaps 3000 miles in 14 years.

The car has developed an intermittent miss which usually starts a short time after the engine has been started. While the car idles, the miss will come and go, though rarely is it absent for more than part of a minute. The owner has had the carb done by Daytona, and I rebushed and throughly overhauled the distributor. Plugs, points, condensor, rotor, cap, wires and plugs are new. Timing is correct. There is no vacuum leak at the Electromatic or elsewhere that we can determine.

What gets me is how a miss can turn itself on and off. Manifold vacumm is about 19 when its running right, and fluctuates rapidly between perhaps 12 and 16 or so when it's missing. I haven't taken a compression yet, and no indications of head gasket problems. I'm just beginning to think about one or more weakened or broken valve springs but I'd sure appreciate your ideas. I've probably forgotten some important details but will try to add them as ideas come up.

Thanks for your time and thoughts.

Adding a few details. Car makes no smoke of any kind at any time. When idling normally, if you short the plugs sequentially you get the normal response, i.e. a bit of a stumble on each. When it's missing the first 2 cylinders seem to be nonperforming. At the time of the engine work the manifolds were removed and surface planed and gaskets refreshed. No indication of manifold gasket issues.