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I've got a '52 Patrician, and I've spent the last several months trying to chase down a slight miss at idle and have come up several possible causes, but have now rectified them and still am experiencing the issue. It's minor, and it doesn't miss noticeably under acceleration or at higher engine speeds, but you can still sense it at moderate engine speeds if you pay very close attention.

Here's what I've checked/changed:
-Spark plugs (Tried 2 sets, one gap'd at .025, the other at .028, the larger gap seems to idle better)
-Replaced points, condenser, cap, and rotor
-Distributor checked (Tested good on a Sun distributor test machine, set points on machine to exact specs)
-Checked for vacuum leaks (very small one at #1 cylinder intake manifold connection, tightened manifold bolts)
-Replaced vacuum advance (diaphragm was leaking)
-Compression: 125 - 127 psi on all cylinders
-Timing: Checked 15 times, always dead-on at 6? btdc
-Vacuum Test: 16 psi (I suspect a leak, but from where? I've tested vacuum at the manifold port with the power brake booster disconnected, at the fuel pump on both sides, and it's the same low number everywhere. Ran acetylene everywhere and only found a very small leak at the #1 cylinder as noted above.

Could it be the carburetor? I'm reticent to pull it off and rebuild it since it has no other symptoms of issues (the car starts immediately hot or cold, there are no flat spots on acceleration, etc.)

What would the symptom be if there was some cross firing because of the way plug cables are bunched together in that housing going to dist. I know crossfiring was an issue on V8's and a bracket with a wider separation was introduced to move the cables.

I've been thinking it could be something to do with the cables, is there a way to test for that (other than by getting new cables)?

You didn't say that you had adjusted the idle mixture screws at the base of the carburetor - if you haven't done that it could very likely be the issue though I suspect there may be more to it as your manifold vacuum of 16 inches (not psi) seems especially low for an engine with such good compression, I'd expect the manifold vacuum to be more like 19 or 20 inches. So you might try temporarily blocking off all the vacuum outlets right at the source, and then try adjusting the idle mixture and re-evaluating.

Yes, I should have mentioned I did a full tune-up (minus cleaning the fuel bowl since the fuel pump was just replaced a year ago).

In addition to what I mentioned already, I:
-Set idle at 375 rpm in high with the parking brake set, per specs
-Tightened manifold bolts to 27 ft. lbs.
-Tightened carburetor flange
-Adjusted idle using a vacuum gauge to obtain smooth idle - the highest and smoothest it gets registers 16 in. Hg. on the vacuum gauge. And sometimes it's very smooth
-Re-torqued cylinder head bolts (I did this two months ago right before I painted the block)

Things I haven't done because I don't have the proper tools, don't know how to do them safely, or have wanted to eliminate other things first:
-Cylinder balance test
-Exhaust gas analysis
-Remove valve cover and inspect valve springs (no reason to suspect broken springs given that I would have trouble at higher RPM as well)
-Rebuild carburetor (I have a spare I want to test but it needs to be rebuilt first, i.e., I don't want to rebuild the one that I'm using now since I know it to be functional -- I'd rather have a known functional and a fresh rebuild to test with in case I bungle the rebuild).

"It's minor, and it doesn't miss noticeably under acceleration or at higher engine speeds, but you can still sense it at moderate engine speeds if you pay very close attention. "





Hi DrewLA, all the specs you've mentioned, and set the engine at were factory settings on a new car 60 years ago. Due to modern variables, eg: gas, age of engine systems, carb, etc., it would appear that there isn't much left to account for a miss that you are focusing on. If the sp wires are cross firing look under the hood at night, if you see sparking you found it. If you're really intent on finding a leaking wire run your hands over the cables as well as looking. . Try changing your timing to 4 BTDC. Sometimes you need to find what works on older systems, the parameters "set to factory specs" sometimes are used as a guide only to get the car started and subsequent adjustments plus or minus are needed to get the car to run as you like it. Maybe it's the nature of the beast or maybe not. Since it doesn't seem to cause any significant problems, any of your future options will be trial and error. Sometimes it's worth letting things be until other symptoms appear. Hope this helps.

Drew,
You didn't post the mileage on the engine although in this case I would say it doesn't matter with that kind of compression numbers. In park or neutral, is the miss obvious at the tail pipe and is it steady? does it decrease as RPM's increase?

In the past when I've experienced a slight miss at idle and I've pretty much eliminated the usual suspects including air/fuel mixture, I simply pull out a pair of insulated plug wire pliers and start pulling off one plug wire at the time, observe how the engine reacts, then reinstall the wire and move on to the next one. In your case the miss should be more obvious on the cylinder that's already slightly missing to begin with. Once you've determined which cylinder is the likely culprit then you can start determining what's causing it. Try swapping the plug wire for that cylinder with one of the others and see if the miss relocates accordingly. There could be carbon build up under one of the valves or maybe a broken or weak valve spring but at least you know which cylinder to focus on.

Gator,

That's a great suggestion, sir. Now to get a pair of insulated spark plug pullers!

To answer your questions, this engine has 40,447 miles on it from new. The first 30,000 miles occurred between 1952 and 1958, and I'm not sure how long it sat between being driven, but it gets driven 100-200 miles a week at highway speeds when it's cooperating now.

In Park, the miss is obvious at the tailpipe, and it's steady. I would definitely say it decreases as RPMs increase -- at 1,000 RPM or so, it's gone.

Quote:

Another way to put this is that when (if) you find a culprit, the engine RPM will be affected LESS by removing that wire, since it's missing sporadically. What you should hear is a steadier idle, yet somewhat lower RPM. Easier to detect on a dead miss, when nothing at all changes after you disconnect the wire, but still doable on an intermittent miss. Just watch out for "pyschosomatic misses" or even worse, phantom "cures."

Have you tried loosening the distributor so you can move it by hand as the engine is running? You might not have a miss, but a very slight timing issue in which the engine time is advanced enough to "skip" a beat. If you you advance the spark slightly by moving the distributor, and the "miss" becomes more prominent, then you probably have it timed about right for economy. If you absolutely must have perfectly smooth idle, retard the timing a bit until it smoothens out. Your RPM will slow some, too, so adjusting base idle speed up a bit will be required.

Thanks for the clarification. I need to get those insulated spark plug wire pliers (hopefully I'll encounter them in my rounds) before I proceed with the wire removal test, so I'm doing some other tests as well.

On the subject of timing, I've checked the timing with my 6-volt timing light, and I'm dead on. I had to adjust and re-time the distributor when I put it back in after my friend tested it on his Sun distributor test machine. I'll advance a few degrees and see if the miss becomes more prominent.

I've been told that since PREMIUM gas in 1952 was the equivalent of 87 octane (r+m/2) we could run a few more degrees of advance than the factory spec'd on today's regular gas and still be fine.