Has this happened to you??

[Bill Robinson]

Two weeks ago, my engine started having a random miss, most noticeable on a hard pull. Compression test showed 50 to 60psi. I poured Marvel Mystery Oil in each cylinder thinking that after sitting up most of the winter, my valves may be sluggish and not closing "on time", due to fuel deposits on the valve stems, even though the gas had been drained between drives. This has happened to me before, and MMO solved the problem.
After sitting all night, I cranked the car and it ran fine, and after a 20-mile run, I checked the compression- all 4 cylinders registered mid 70's (Z head, alum pistons, ss valves, 280 cam, etc).
Last weekend, second day on a tour, at roughly 80-mile mark, I started noticing the random miss coming back. When I got home, I checked compression and it's back in the 50's on all 4 cyls.
Any ideas?

[TXGOAT2]

Is your valve lash set too tight? Is it possible your oil pipe is clogged? I assume you did not run old gas through the engine. If you have a downdraft carburetor, could gasoline have reached the crankcase and diluted the oil?

[Bill Robinson]

My question is- What single issue could cause the compression to drop 20lbs on all 4 cylinders?

The valve lash has not been checked since the rebuild, but it seems to me that if lash is the problem, the miss would be constant. My miss is random- not constant.
I have 3 oil tubes. There was no lint or trash in the transmission oil screen.
The oil was changed after the 20-mile test run- fearing the oil may have been diluted by the Marvel Mystery Oil.
I'm running an NH carb, with a carb heater installed.
Also, the spark plugs were wire-brushed at the time of the oil change. Then checked upon return from the tour. Clean, no Soot.

[Norman Kling]

I'm not sure what compression is normal for a z head, but around 50 is about right for a standard head. It is possible that something in the cylinders could have caused the high reading right after using the mystery oil. Could your timing gear have slipped one tooth off?
Most likely causes of intermittent missing is an ignition problem. If you are running coils, it could be a coil in need of tune up or a dirty timer. It could even be a bad connection in the coil box. If you use a distributor, it could be the point gap or dwell length. It could also be carbon traces in the distributor cap. Bad spark plugs can also be a cause. Under pull the compression is highest and a dirty of miss gapped spark plug can cause a misfire.
Anyway, keep trying. eventually you will find the cause and be able to fix it.
Norm

[TXGOAT2]

It's possible your high compression pressure readings were a result of "wet" cylinders". The engine's normal hot readings may be the lower one. If they are close to the same from cylinder to cylinder, the compression is not likely to be the cause of the miss. Wire brushing spark plugs is not a good idea. The insulators at the cylinder end of the plugs are not glazed, and they can pick up metal off a wire brush, which can degrade their insulating properties. I'd try sandblasting the plugs, or try a different set of plugs. (Don't sandblast the external part of the insulator, which is glazed) Experiment: buy a cheap new spark plug and a new wire brush. Brush the business end of the new plug with the new wire brush. Note the grayish deposits on the new white insulator wherever the wire brush bristles have rubbed across it. That grayish material is metal from the brush.

[got10carz]

I completely agree with Norm. Coil or dirty timer.

[kmatt2]

Your misfire happens after a hard pull or a longer run. Your compression falls off after a long run. You have changed the oil thinking that the Marvel Oil may have diluted the engine oil, and that is good. What did the drain oil look like, it is possible that over winter the old oil was diluted by water or even gas. You said that you wire brushed the spark plugs to clean them up. It is possible you have embedded some bits of wire in the plug causing a misfire, try changing the plugs. Once my T started misfiring after about 50 miles of driving but got better after cooling off. It turned out that some of the adjustable lifter lock nuts had come loose and the valve lash was all over the place. If the problem continues I would check the valve lash, but first check all the wiring to the timer, ( clean out inside ), and the coils.

[Bill Robinson]

I found the miss! (I think). Last week when I changed the oil, I cleaned and lubed the timer. having run out of the timer lube that is sold by the vendors, I used a little "red & sticky". Bad call on my part. It seems that the sticky strings tend to a ground the contact randomly which causes a random miss.

Today's ride to lunch will show me more.

Wire brush- the brush that I used is a stainless-steel toothbrush that I've been using for years. It cleans well, teeth don't break off because the pressure is light, unlike a wire brush attached to a grinder that tends to imbed bristles in my arm .

[TXGOAT2]

I would not use any wire brush on spark plugs. A Ford-type timer can use Marvel Mystery Oil. Some non-Ford timers use a special lube. New Day timers use no lube at all.


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[Charlie B in N.J.]

Completely agree with Txgoat on wet cyls increasing compression as a wet comp test will do the same occasionally. Also agree with Norm. timer or failing coil. Comes & goes so it's not permanent like a skipped tooth or some other non-reversable mechanical condition.

[Altair]

I had developed a miss similar to what you are describing and it was nothing to do with compression nor any lubrication issues, the issue was inside the timer. The paper spacers between the contacts were slightly compressed and barely noticeable, but it was enough to cause the roller skip or jump over the next contact and cause a random miss usually a higher speeds. To look at the timer it was hard to tell, it looked OK, dry and clean, but was skipping at higher speeds.

[J1MGOLDEN]

I put a cup of that Marvel Mystery Oil in my engine.

That made the bands so slick that the brake became totally non-functional.

Fortunately, it was noticed when no one was stopped in front of me.

The Low Band was not very good then, but got me started.