Compression test results (I think I am doing something wrong)

Was the same fault there before you changed the cylinder head? If so, that would point to piston problems on cylinder No4.

Are you able to do a compression test on the engine? If there's piston/ring trouble on the 4th cylinder that could cause a misfire because of poor compression.

The other thing that springs to mind is that there's something amiss with the valve clearances on cylinder No 4.

I'm not familiar with your car - does it have an overhead camshaft? Unfortunately when refitting an overhead camshaft after a rebuild, some owners have managed to bend a valve. This usually happens because the engine has been turned during disassembly/rebuild and the timing marks have not been aligned when refitting the camshaft.

This results in a valve in contact with a piston and when the camshaft is fitted and torqued down the valve becomes bent.

I think your problem may lie in either a bent valve or incorrect valve clearances.

Are you sure it's actually a plug?
Are you getting oil on the plug?
The plug may be firing but you may be losing compression which gives the symptoms of a misfire. As you've changed the plug, lead and coil this points to to a problem elsewhere.

Your first course of action should be to get a mobile mechanic carry out a diagnostic check for you.

To put your mind at rest, do a 2nd compression test using a light oil squirted into the bores:

Doing a Compression Test
Warm the engine. Remove all the spark plugs.
Get a pencil and piece of paper to note down the readings.
Put the compression tester into the No1 cylinder and crank the engine for 10 seconds.
Note down the compression reading.
Repeat process for all cylinders.

Here's an illustration of what you may expect on a 4 cylinder engine:
Cylinder 1 2 3 4
psi 125 122 120 124

125 is the uppermost figure for that engine. Here, this engine is fine. There's a slight variation in psi figures, but that's perfectly normal.

Here's the same engine:
Cylinder 1 2 3 4
psi 110 112 114 112

The psi reading is down. However, as all the figures are pretty much equal it doesn't indicate head /gasket problems. It may point towards worn pistons or burnt valves.

Now consider these psi readings:
Cylinder 1 2 3 4
psi 125 84 86 124

There's more than a 10% drop - a difference - between cylinder 1 and 2, and cylinders 3 and 4.
The compression readings for cylinders 2 and 3 is down; low. There's something clearly wrong.

It suggests that there is a defective head gasket between cylinders 2 and 3 or a crack in the cylinder head. The rising piston compresses the gases which escape into the adjoining cylinder via the defective gasket or cracked head.

However, it could also mean that there is a problem with the valves (burnt/not seating properly) or perhaps piston / ring problems.

A burnt valve - it's usually the exhaust valve as they bear the brunt of the combustion - can cause a reduction in engine power simply because the combustion process isn't occurring properly. Compressed air/fuel gets squeezed out of that cylinder because of the damaged valve . There even may be a misfire - a surge as the car runs.

Worn or broken piston rings allow compressed gases to leak past into the crankcase. A compressed crankcase can force oil out of the dipstick tube. The pressure in the crankcase will leak to atmosphere anywhere it can find an outlet.

The 2nd Compression Test
The second compression test is known as a 'wet test'. The first compression test was the dry test because no oil was added to the bores. The second 'wet' test can give an indication of whether it is the rings or valves at fault (though bear in mind rings and valves do not cause overheating or water in the coolant symptoms).

Spray a liberal amount of light penetrating oil into each cylinder - aim for the cylinder walls, not the centre of the piston. You want the oil to run down the cylinder wall and around the piston to form a seal.

Place a rag over each spark plug hole and spin the engine to eject the surplus oil.

Then carry out a full compression test noting down the results.

Here's the previous results with the 2nd compression readings added:
Cylinder 1 2 3 4
psi 125 84 86 124
Wet 128 112 110 126

The readings have increased. This because the oil sprayed into the bores has formed a temporary seal around the piston, thereby enabling the compression to be raised. It also indicates that the bores/rings are worn on cylinder 2 and 3 - the 'oil seal' has increased the readings but is still low in comparison with cylinders 1 and 2.

This could also indicate that in addition to worn rings there is also burnt valves. Oil cannot form a seal around a valve. A worn or split valve will cause a low compression reading and misfiring symptoms.
Compressions readings should be taken in conjunction with other symptoms. It will help you identify the problem:

Low compression readings between two adjoining cylinders point towards a head gasket/head fault if your vehicle has shown signs of coolant loss, coolant in the oil system - mayonnaise, overheating, rough running and lack of power.

If those symptoms are not present it points towards burnt valves/piston rings. A worn engine may be difficult to start and pressurise the crankcase, but it doesn't cause overheating problems.

Sorry to say I would tend to agree with the mechanic at this point. Too bad all of that didn't get sorted out before spending big bucks on the head gasket repair. The cylinder with 90 psi is probably the reason for the rough running-makes me wonder if the mechanic used due diligence. Was the head checked for straightness of the head to block mating surface? It doesn't take heat very long to warp an aluminum head, and subsequent head gasket failure. Or it could be an exhaust valve not seating well, or a worn cam exhaust lobe on that cylinder. The bottom end has not "now failed", probably pretty worn piston rings; it is just questionable that the low cylinder is from the bottom end or the top end. A leakdown test of that cylinder would pinpoint the problem-I think I would suggest you get that test done-it is more precise than a compression test. It may help you decide-the leakdown test definitely will find the reason for the low compression in that cylinder.
Even so, if you got that one cylinder back to snuff, you would still have an engine with 130 psi as top compression-that is a bit on the low end, probably below the minimum compression that Mazda would recommend. It would still run good, but you might notice a loss of power, compared to an engine with 150 psi or more.

If it runs that well at higher revs, you could just drive it as is-what harm could it do? If the engine craps out, it was just meant to be! It's your call to have this engine rebuilt (again), swap in a better engine, or just run it into the ground till the engine finally gives up. But I would do the leak down test just to know what is up in that cylinder.
Good luck, Vern. Post back with what happens.
Put a vacuum gauge on a manifold vacuum tap and check the vacuum reading; from the hard brakes, it probably has low vacuum, but whether from a loose vacuum line or the engine can't make good vacuum with that low cylinder, I don't know.

Interpreting the Results
of the Engine Compression Test.The idea behind the Engine Compression Test (in the case of a Misfire Condition or Rough Idle Condition) is to find out if any one engine cylinder is not contributing 100% of its power to the overall engine output. If only one cylinder has lower than normal compression, your GM 4.3L (or 5.0L, 5.7L) equipped vehicle will run rough or cause a Misfire Code to set and turn on the Check Engine Light.
So then, to wrap things up, the individual cylinder compression readings of each engine cylinder can not vary more than 15%... and this is how you can find out:

1. 1Grab a calculator and multiply the highest compression reading that you recorded by .15.
   
   So, let's say that cylinder #4 gave you the highest reading of 170 PSI. Well 170 X .15 gives you 26 (25.5 rounded off).
   
2. 2Now, the next step is to subtract 26 from 170... which gives us 144 PSI.
   
3. 3So then, 144 PSI is the lowest possible compression reading that any one of the rest of the engine cylinders can have. Any compression reading below this.. and that engine cylinder will misfire..
   

well first off you need to start by checking the compression on all 8 cylinders!!!! to rule out internal engine failure as oppose to jumped timing. its very rare that timing chains will jump especially a double roller chain. get a compression gauge take the plugs out and get a reading on all the cylinders. you should be no more then 10-20psi difference from cylinder to cylinder. but on the chevy motors if there is a problem they usually show well on a compression test! good luck post back up the results then we can go from there.

If your losing compression then I suggest doing a compression test and then following it up with a wet compression test to determine whether the problem is in the valves or pistons.
Wow one tune up in 122k miles jerking not really an ignition thing more like a fuel thing. If Compression test results are good then do a full tune up even replace coil packs or distributor ( which ever is applicable) and replace fuel filter.
These vehicles are not known for major engine problems (blown engines bad valves etc).
Big with transmission problems if its a 6 cylinder not the 4 cylinders.

bent or worn, the first thing to do is a compression test, the test results will give a better idea of whats wrong, if compression is low then you could have a bad valve on worn rinds,next is to put air into cylinder with valves closed, listen for air,all plugs removed,if it blows out of next cylinder then its a headgasket, if it blows out throtle body or exhaust then its a valve, if it blows out oil cap oil dip stick then its rings,but do compression test on all cylinders first, if compression is good then you have an electrical problem, hope this helps.

well start with a compression test to make sure it isn't a mechanical problem. If compression test results are not what you were hoping for the a cylinder leakdown test will be necessary to find out if you have burnt valves or excessive blowby from piston ringsb that are not sealing correctly due to being broken collapsed or scored cylinders., or head gasket failure. If compression test is good the you need to determine if your injectors are the problem or coil pack, spark plugs or wires. More info would be great if you want a better answer. Like how do you know its only hitting on two cylinders ? How many cylinders does it have to start with ? Model year , little things like that.

have an 02 misfiring on no7 cyl was catalytic converter

Sounds like it needs spark plugs and wires, and a fuel system treatment.