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This is a VW Jetta made in 1984 with 1.6L petrol engine (code "EZ"). It starts OK (though if it has been a wile since last start, it takes some cranking to start the engine), but when climbing a hill, the engine starts misfiring and obviously loses power, so much so that if the hill is steep enough it looks like the car may not be a able to make it to the top. When idling (the automatic transmission in "P" or "N" position), then engine still misfires, but rarely (like once every 10 seconds).

Another weird thing is that if the transmission is in "P" or "N" position, the idling speed is ~1500RPM, but if I put it in "D" and apply the brakes, the idle speed drops to ~900RPM (this is what the manual specifies).

Previously the car had problem starting, but I found the reason - bad capacitor and, in turn, worn out contact breaker points. I replaced the points and the cap (didn't the a proper one, so just used a regular 630V 0.2uF capacitor). The spark plugs are new.

I checked for vacuum leaks, found one (the air intake vacuum unit is broken) plugged it (temporarily just plugged the tube), but that made no difference.

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It would be nice if you could get your hands on the appropriate components for the vehicle instead of using whatever you have laying around.

Have you checked the ignition advance to ensure that it's working properly? Also the ignition timing while rotating the motor by hand.

The extended crank after it's been sitting makes me want you to check fuel pressure key on engine off. And also while at idle.

The broken vacuum line that you say is on the vacuum unit, where does this route to? Depending on the production year of the vehicle there are a few differnet vacuum diagrams.

edit> Thinking about this a little more this morning I had another question. The misfire occurs when going uphill. But, when not going uphill, can you get it to misfire simply by accelerating and getting the transmission to down-shift into the passing gear? If it only happens uphill then have when you get a moment have a look into the gastank and make sure that the pickup for the fuel pump is not all the way to the front of the tank. If it is it most likely is causeing the vehicle to starve itself and causing the misfire.

Then when the vehicle is back on level ground the fuel is now air-raided causing the a random misfire every ~10seconds or so.

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    The ignition timing seems to be correct - I checked it with a timing light. Ignition advance also seems to be working - both centrifugal and vacuum operated. The broken vacuum line goes to a deice on the intake (before air filter) which selects cold air or hot air (heated by the exhaust manifold). The fuel pump is mechanical, but the next time I will try to check if there is any fuel in the carburetor. As for proper components - I try, but not everything is available.
    – Pentium100
    Commented Jul 14, 2013 at 2:12
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You may have a bad distributor, coil or spark plug. The symptoms are consistent with most of the reports I've had when the problem turned out to be somewhere between the distributor (or coilpack) and the cylinder.

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When you drive uphill you will be increasing the load on the engine. The spark plugs will need a higher secondry voltage to (KV) to fire. The extra voltage demand will stress the system. Any weakness such as plug lead insulation, rotor arm and cap insulation, coil pack servicibility, spark plug condition will show up by leaking. An Old Hands trick is too have the engine running in a dark place and have someone snap the throttle open. Any external leakage will show up in the dark as blue flashes to earth. In drive the engine carries the load of the transmission, in neutral it does'nt. The load slows the engine speed. Modern vehicles have a device, Idle Control Valve which maintains a set idle speed.

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