Engine diagnostics

Engine Diagnostics
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Someone provided a great lead-in to the subject -
I have a 1968 1641cc engine that I just rebuilt.  My mechanic said to get a centrifugal advance (009) distributor, which I did.  During the fire-up the engine would barely run -- it had a bad miss on one side. I found the #4 cylinder is not firing (no heat on the exhaust manifold).  All others seem to be working fine.
I checked all the wires all are in the correct order and appear in good condition -- I even checked the resistance of the wires and they are the same.  Common sense tells me I've got gas, I've got fire -- what is wrong with this cylinder?  I even pulled the valve cover and all of the valves seem to be going up and down okay.
Rob responded with an excellent treatise -
Okay - let's start at the top and work through it.
Your tests are good so far (valves working, leads give similar ohms...)
Which cylinder is what -
front of car


    3  1

    4  2


rear of car
Firing order -
1-4-3-2


Distributor -
front of car


    3 &nbsp2

      o

    4 &nbsp1


rear of car
(The "o" is the centre wire to the coil.)
The '68 engine was an H series single-port 1500cc engine.  Do you still have the single-port heads, or have you converted it to twin port?
The reason I ask is that if the single-port heads have a loose manifold attachment (air leak), BOTH cylinders on that side will misfire, but with the twin port heads, it's just possible for a manifold air leak to one cylinder only.  The twin port (TP) manifolds do need some attention to make sure they bed down correctly on the head - I've had a problem there in the past myself - thought I'd snugged the manifold down right but both cylinders that side were misfiring, and it was the manifold not quite sitting flat.
After checking that, check the plugs - swap plugs if you don't have a spare handy.  Does the misfire move with the plug?  I've had two Bosch plugs bad out-of-the-box, and have changed to NGK B5HS (or B5ES for the long thread heads).  These are excellent plugs, very reliable so far.
If the misfire doesn't move with the plug, then get a spare plug and connect the #4 lead to it and lie it down touching the engine case, and start the engine - any spark? (easier to see at night or in the shade).  If you have a spark then the plug in the head is suspect.  (You aren't using "never seize" or similar on the plug threads I hope - that stuff can cause problems in the hot-running VW heads.)
No spark?  Then try another lead (completely swap leads if necessary).  If the misfire moves with the lead, the lead is toast.  If the misfire stays at #4, then maybe the distributor cap is toast. Make sure all wires are seated properly, the cap is clean, both inside and out, and check for hairline cracks.  Have a look at the posts inside the cap - do they all show similar burn marks where the spark jumps from the rotor arm to the post?  Maybe there is too much gap on the #4 post (bad cap?).
I presume you've correctly gapped the points?  Points just barely opening may fail to open for one cylinder.
Test all cylinders for compression (5-6 rotations of the engine is usually enough -- 2-3 seconds cranking).  They should all be more than 100 psi (usually about 115-125 psi for a 1600cc engine), and less than 10 psi between best cylinder and worst cylinder.  If one cylinder has low pressure, it will not be working right at idle, but will still provide some power at higher rpm so you would not notice it then.  If the compression is lower than 100 psi, then the engine needs rebuilding.
Low compression usually means either a burned exhaust valve, or worn out piston rings.  You can test for this.  Squirt about 10 ml (10cc) engine oil in the spark plug hole of the bad cylinder (try and squirt it in a circle around the cylinder using a syringe), and do a compression test again immediately.  If the pressure is better, then the rings are badly worn (the oil seals them a small amount). If the pressure is not changed, then the exhaust valve is burned.  Engine oil will help seal leaky rings a little, but won't help a burned valve, so this test usually works quite well.  If this test gives you results, then you will know if the cylinder heads need to be fixed, or if the pistons and cylinders need to be replaced.
For fun you could then wait a while and then do that cylinder again.  If the compression has dropped at all, that confirms that the oil was filling gaps and has now drained away, so the rings in that cylinder have died.
If one valve is burned, ALL exhaust valves must be replaced. The exhaust valve is the hottest part of the VW engine (they glow red hot when you work the engine hard), and if one is damaged the others will soon follow, so you always replace all the exhaust valves together.  (The inlet valves don't work nearly so hard - you don't have to worry about them).