think I have a misfire

[English]

Digging deeper into my performance issues, I'm having another look at my timing. Turns out the mechanic who did the emissions test had turned my distributor back quite a bit to get it through. Timing was about 2*atdc!
Bringing it forward, I've set it to 12*btdc to get it in the right ball park, but it has an obvious misfire. I've also check the vac and mech advance, both seem to be operating properly.
Tried taking it for a spin with the timing set to 12*, and although it has a very rough idle, once it's going it actually feels stronger.
So... If I have a misfire, would retarding the timing somehow disguise it?
This morning I changed the ht leads, rotor arm and dizzy cap, none of which has made a difference. 

[enaberif]

Take a timing light and attach it to all the spark plug leads and watch your timing light on the balancer.

The light should hold steady and not move. If you have a misfire you will see the balancer shift.

[English]

1 and 6 look good
2 and 3 look bad
The rest I'm not sure on, I'm peering at different angles and from some points they look good but from other angles they look bad. My timing light only advances to 90*. I'll get some timing tape and try again. Going to borrow a remote thermometer from a friend too.
At 12* it runs awful (even if it feels stronger). Just set it to 6*btdc, getting better, it's not so bad when accelerating but jerky at cruise. Have to retard it to 2*btdc before it's comfortable to drive. So frustrating!

[zieg85]

This morning I changed the ht leads, rotor arm and dizzy cap, none of which has made a difference.

By leads do you mean spark plug wires?  If yes then spark plugs is next.

[English]

Yeah sorry, over here they're called HT leads!

Spark plugs done already too.

Clutching at straws here- I see I have a 5 pin module in the distributor, are they prone to cause trouble? I know they were only around for a very short time. Wondering if a random retardation would cause my symptons

[English]

Okay finally found something useful. First of all - timing lights are much easier to use when it's dark !
Secondly - tried the timing light on each cylinder ,most of them are dead steady, but cylinder 5 is hunting (best way to describe it) and cylinder 7 has a slight wobble.
Only acts like that at idle, when I take the revs up both 5 and 7 are steady.
What does this indicate ? Valves?
This is with timing set to 6*
Will try again tomorrow with the timing set to 12*. It runs rougher that way - but why?

[enaberif]

Okay finally found something useful. First of all - timing lights are much easier to use when it's dark !
Secondly - tried the timing light on each cylinder ,most of them are dead steady, but cylinder 5 is hunting (best way to describe it) and cylinder 7 has a slight wobble.
Only acts like that at idle, when I take the revs up both 5 and 7 are steady.
What does this indicate ? Valves?
This is with timing set to 6*
Will try again tomorrow with the timing set to 12*. It runs rougher that way - but why?

Definitely start with the valves and ensure that they are where they should be.

[English]

Okaaaaaaay... Been watching some youtube videos to see how my vacuum gauge should behave, and I'm coming to the conclusion that I have both a sticky valve (I'm guessing on cylinder 5, from what I see with the timing light) and worn valve guides. The needle has a rapid vibration, with a sporadic drop.

Question- how does altering the base timing affect this? When I set it to 12* the misfire is quite bad, when it's at 2* it goes away.

[enaberif]

Okaaaaaaay... Been watching some youtube videos to see how my vacuum gauge should behave, and I'm coming to the conclusion that I have both a sticky valve (I'm guessing on cylinder 5, from what I see with the timing light) and worn valve guides. The needle has a rapid vibration, with a sporadic drop.

Question- how does altering the base timing affect this? When I set it to 12* the misfire is quite bad, when it's at 2* it goes away.

Altering your timing affects your idle. But advancing your distributor you will increase your rpm which then you will hear the issue. Retarding the distributor will slow down your idle which is when the issue goes away.

If you think you have a sticky valve see if you can find someone who has a leak down tester.

[English]

It's not just at idle though, it's as the revs pick up. But goes away once the revs are high.
When the timing's retarded it's not a problem (truck just feels underpowered).

Would a compression test be of any use? Not sure how common leakdown tests are over here, hadn't heard of them till I started looking into this problem.

[enaberif]

It's not just at idle though, it's as the revs pick up. But goes away once the revs are high.
When the timing's retarded it's not a problem (truck just feels underpowered).

Would a compression test be of any use? Not sure how common leakdown tests are over here, hadn't heard of them till I started looking into this problem.

Compression test will tell you how much air your cylinders can hold. Based of those values you can get a general idea of the health of the motors internals.

A leak down test will show you if there are any leaks anywhere within the system due to fault gaskets, seals or even bad valves.

Another test you can do is take a compressor with a fitting and pull out a plug and put a few psi into a cylinder to pressure it up and tap the top of the valve stem lightly and listen to the sound and do this across for all of them and see if it sounds the same. If you think #5 is an issue then specifically focus on that and see how it sounds pressurized.

[English]

Cool, I'll look into getting some testing done. Thanks for all your help!

[bd]

Both compression tests and leak down tests pressurize the cylinders one-at-a-time.  A compression test yields pressure development within the cylinder based on piston travel and cylinder sealing - cylinder pressure is developed by cranking the engine; a leak down test relies on an external air pressure source to determine leakage past piston rings, gaskets, and valves.  Both will indicate when there is a problem.  Each has its pros and cons.  Different techs swear by different methods.  If you can't acquire a leak down gauge, use a compression gauge.

Generally speaking, increases in engine RPM mask a misfire, depending on the severity of the miss.

In addition to what's been suggested already, round out the basic tests by verifying the timing chain wear and indexing of the balancer timing mark.

[enaberif]

If the timing chain was bad then you should see more movement of the balancer would you not?

[bd]

Not really.  The timing chain is under tension created by friction in the rotating assembly and valve train.  The cam won't out-accelerate the crankshaft, so the cam and ignition aren't likely to bounce noticeably between retarded and advanced due to a worn chain.  Also, there is a remote possibility that the balancer has begun to slip, throwing the timing mark off.  Nonetheless, I think both of these latter suggestions are junior to checking cylinder condition as you earlier prompted.