My grandad is a winner of a few CTCI Concourse gold medallions and knows a thing or two about the original birds. Problem is, he's being a bit pig headed and he's getting up there in the years, so I'm looking for some advice to save him some time, I hope. He's 85, by the way. Still sanding by hand and turning the torque wrenches. He's restoring a survivor but we're not going full Concours on this one. The owner, younger brother of the original owner, wants to install a McCulloch supercharger and replace the automatic with a standard. That's a good deal of tuning to begin with, but first my grandad got the engine diesel-flushed which brought out a lot of grimy sludge and cleaned up under the valve covers considerably! Also opened a main seal leak, it would seem. After adding new oil he started the car to let it idle and warm when he noticed it didn't sound exactly right. I haven't been doing these old cars long enough to notice, but he pinpointed a misfire in one cylinder. When he removes that plug wire, cold or warm, it makes very little difference. Kinda like that cylinder is firing 25% of the time and removing the plug wire only makes that much difference. It's the last one on the driver side. Number 8. The first thing he thought was that the rings were stuck. So I did a compression check and ALL cylinders were within ONE PSI of each other. Vacuum gauge reads 18.5 but ticks to 17.5 each time the miss happens. Points are trash and replaced with some Hall effect deal. Dwell is within spec. Rev'ing the motor seems to clear it up. Putting it in Drive makes it buck more. It gets worse as it heats up but stays with that cylinder. So now he's pretty sure he's got a dirty or leaky valve. He claims he can lap it while it's still in the head and I'm certain he has done this before, but can we get a second opinion? Is it possible to get such good and equal compression readings if the tappets aren't set just right? In a leak-down test, the ONLY sound comes from the oil cap. Nothing through the exhaust or carb. How can you check the valve guide without removing the head? Would a guide cause an intermittent miss? Would a guide show up in a compression reading? Thanks. I hope to save the ole man some time because he's dreading leaning over the frame to do the job since he's the only one with the skill to use his method on a customers car!
Sounds like a bad valve to me. Probably not burnt but maybe needing a little attention. I would look at the valve which is what your grandpa is going to do and decide if it needs touched on the valve grind machine or just lapping.
He knows his chyit, that's for sure. So I'm curious why a leaky valve would show no differently than the others in a compression check. He's going to lap it without taking it off the motor. Stick some compound on the valve through the plug hole at TDC then grind and clean.
It builds the gauge up to full compression by about the third pump and it's equal to the other cylinders. Can both of these issues be missed in a compression check? That's the question here. I know my grandad knows what he's doing but I'm confused about the readings and want to get more educated.
I probably wouldn't do it that way. But I'm not doing it. A weak valve will cause a miss, a weak valve is not like a burnt valve sometimes they only leak under more pressure then your compression check allows for and even at that if the valve is burnt pretty bad a leak down test is the only way to know for sure. A lot of times when an engine sets the valves that are open (most people don't relieve the vale train) will build up a little surface rust from the moisture in the air, sometimes they will even get a pit or two. Not so much of a problem with stainless valves but these old engines seldom have stainless valves. Anyway a quick lapping with usually clean them up just fine.
That's great to hear. So compression would normally leak at HIGHER RPM than the test but lower than running down the road at part throttle. I know what you mean about the light rust. I've seen how valves can look when left to the elements. He was an aviation mechanic for the navy at one point and said it was always easiest to just lap the valve in the cylinder than to take the chance you crack a bolt or break something else and have a useless engine when it was needed most. He's been doing auto work for 60 years and somehow this method has worked. I guess the only thing that makes me wince is the compound possibly getting in the chamber but that hasn't been a problem yet. How about the worn valve guide, as a last resort before he sits hunched over this thing all afternoon? How do I test a guide?
Regarding the ignition, we changed the wires and the whole distributor and plugs, plus the coil. What else is there in that regard? This was after finding the misfire.
The motor runs like a dream now. The old man has a pretty solid track record of making me look foolish... Thanks all
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